A CLIMATOLOGY OF THE CHARACTERISTICS OF TROPICAL ... - TDL

A CLIMATOLOGY OF THE CHARACTERISTICS OF TROPICAL CYCLONES IN THE

NORTHEAST PACIFIC DURING THE PERIOD 1966-1980

by

RICHARD ARTHUR ALLARD, B.S.

A THESIS

IN

ATMOSPHERIC SCIENCE

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Accepted

May, 1984

C l ' ^ - ACKNOWLEDGEMENTS

I am indebted to Dr. Richard E. Peterson for his guidance,

support, and understanding throughout the course of this research.

My sincere thanks extend to Dr. Donald Haragan and Dr. Joseph Minor

for their keen insight and helpful suggestions. I can not thank

Wendy Schneider enough for her unending support and encouragement

during this study. I am grateful to Eric Pani for his assistance

with the computer output. I would also like to recognize my parents,

Ernest and Jeannette Allard, for their love and encouragement through

out my college career. Many thanks go to Denise Bentley who typed

the manuscript, and to all my friends who made my stay at Texas Tech

an enjoyable one.

ii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

ABSTRACT v

LIST OF TABLES vi

LIST OF FIGURES vii

I. INTRODUCTION 1

1.1 Historical Background 1

1.2 Data Sources Past and Present 9

II. THE CLIMATOLOGICAL AND SYNOPTIC SETTING 13

2.1 The ITCZ and Tropical

Cyclogenesis 13

2.2 El Nino and the Southern

Oscillation 19

III. RESULTS AND ANALYSIS 23

3.1 NETROPAC Cyclones 1966-1980 23

3.2 Areas of Origin, Intensification,

and Dissipation 42

3.3 Recurving Storms 46

3.3.1 Synoptic Situation 53

3.4 Mean Storm Direction and Speed . . . . 53

3.5 Seedlings and the Initiation

of NETROPAC Cyclones 56 3.6 Atlantic Storms Which Crossed

into the NETROPAC 59

111

IV. LANDFALLING NETROPAC CYCLONES 61

4.1 The Orography of West Mexico 61

4.2 Offshore NETROPAC Cyclones 63

4.3 Landfailing Storms 66

4.4 Storm Surge 72

4.5 Damage and Casualties 74

4.6 Design Basis Storm 77

V. CONCLUSIONS 83

LIST OF REFERENCES 85

APPENDIX 91

IV

ABSTRACT

The Northeast Tropical Pacific (NETROPAC) ranks second only to

the Northwest Tropical Pacific in terms of annual tropical cyclone

occurrence. Prior to the introduction of the weather satellite in the

1960's, cyclone frequencies had been grossly underestimated. During

the past decade, the decline in ship reports has made the satellite

the primary tool for estimates of NETROPAC cyclone location and inten

sity. In this study, NETROPAC cyclone tracks are constructed for the

period 1966-1980. The majority of storms form between 10.0 and 30.0 N

latitude with a mean direction toward 300 with a speed of 12 kt. The

results indicate that three tropical cyclones per year can be expected

in the area bounded by 15.0-17.5°N and 105.0-115.0°W. Most of the

NETROPAC experiences at least one cyclone between 10.0 and 25.0 N per

2.5 by 2.5 degree latitude-longitude grid square. Research indicates

that the poleward position of the intertropical convergence zone, weak

vertical wind shear between 850 and 200 mb, and a sea-surface temper

ature of 26.5 C or greater are necessary for tropical cyclone develop

ment. Recurving NETROPAC cyclones and those which make landfall upon

the west coast of Mexico are studied. The resulting damage patterns

are also investigated.

LIST OF TABLES

Table 3.1 NETROPAC Cyclone Occurrence 1966-1980 24

Table 3.2 NWTROATL Cyclone Occurrence 1966-1980 25

Table 3.3 NETROPAC Cyclone Occurrence 1966-1980

in Bi-weekly Periods 27

Table 3.4 Length of the Hurricane Season 30

Table 3.5 Recurving NETROPAC Cyclones 51

Table 3.6 NETROPAC Cyclone Origins 1969-1979 58

Table 4.1 Tropical Cyclones Passing Within 100/200 nmi of Cities Along the Western Coast of Mexico 64

Table 4.2 Landfailing NETROPAC Cyclones 1966-1980 . . . . 67

Table 4.3 Damage and Fatalities in Mexico associated with Northeast Pacific Tropical Cyclones 1966-1980 75

VI

LIST OF FIGURES

Figure 1.1 Surface and upper air network for western Mexico

Figure 2.1 The position of the equatorial trough during (a) January and (b) August

Figure 2.2 Monthly sea surface temperatures (SST) during May and August

Figure 2.3 Vertical wind shear between 850 mb and 200 mb for (a) January and (b) August . . . .

Figure 3.1 Total NETROPAC cyclone occurrences 1966-1980 using a 15-day running mean . . . .

Figure 3.2 Total occurrence of NETROPAC cyclones (annual) 1966-1980

Figure 3.3 Total occurrence of NETROPAC cyclones for May

Figure 3.4 Total occurrence of NETROPAC cyclones for June

Figure 3.5 Total occurrence of NETROPAC cyclones for July

Figure 3.6 Total occurrence of NETROPAC cyclones for August

Figure 3.7 Total occurrence of NETROPAC cyclones for September

Figure 3.8 Total occurrence of NETROPAC cyclones for October

Figure 3.9 Total occurrence of NETROPAC cyclones for November

Figure 3.10 Average annual frequencies for NETROPAC cyclones based on data from 1966-1980 . . . .

Figure 3.11 Points where tropical disturbances first reached tropical storm intensity . . . .

12

16

17

18

29

32

33

34

35

36

37

38

39

41

43

Vll

Figure 3.12 Points where hurricane intensity was first observed 44

Figure 3.13 Location points where dissipation of NETROPAC cyclones occurred 45

Figure 3.14 Recurving NETROPAC cyclones for May-June 47

Figure 3.15 Recurving NETROPAC cyclones for August-September 48

Figure 3.16 Recurving NETROPAC cyclones for October-November 49

Figure 3.17 Points of recurvature for NETROPAC

cyclones during 1966-1980 52

Figure 3.18 Mean storm movement and direction 55

Figure 3.19 Network used to track seedlings across the Atlantic 57

Figure 4.1 Total occurrence of landfailing

NETROPAC cyclones 70

Figure 4.2 Points of landfall for NETROPAC cyclones . . . 71

Figure 4.3 Storms for which estimated maximum wind speed exceeded 70 kt 80

Figure 4.4 Storms for which maximum wind speed

exceeded 90 kt 81

Figure A.1 NETROPAC cyclone tracks for 1966 92





Figure A. 6 NETROPAC cyclone tracks for 1971 97


Vlll

Figure A.8 NETROPAC cyclone tracks for 1973

Figure A. 9 NETROPAC cyclone tracks for 1974



Figure A.12 NETROPAC cyclone tracks for 1977




99

100

101

102

103

104

105

106

IX

CHAPTER I

INTRODUCTION

Since the mid-nineteenth century, the Northeast Tropical Pacific

(NETROPAC) has slowly become recognized as a major source region for

tropical cyclones. Although NETROPAC cyclone tracks were published as

early as the mid-1850's, it was not until the 1920's that this area

was generally accepted as a region for tropical cyclone activity. At

that time, the best estimates of frequency indicated no more than five

cyclones per year. Several decades later, after a period of weather

satellite observation, it became clear that cyclone frequencies had

been grossly underestimated; by the late 1970's the number of detected

tropical cyclones had increased to a more representative 15 per year.

In a matter of fifty years, the NETROPAC—an area once considered by

many to be barren of tropical cyclones—had become the second-ranking

region in the world for tropical cyclone occurrence. (The Northwest

Tropical Pacific ranks first.)

1.1 Historical Background

While over the years many discounted the existence of tropical

cyclones in the NETROPAC, there were a few observers who realized the

damage potential of such storms. As early as the seventeenth century.

Sir William Dampier had several bouts with rough seas in this region.

Hurd (1929) quotes Dampier from one of his voyages while off the coast

of Guatemala: "...seldom a day passed but we had one or two violent

tornadoes." (Terms such as "hurricanes," "cyclones," "tornadoes,"

etc., in the early literature were used rather interchangeably; the

context reveals the meaning.)

William C. Redfield furthered the knowledge of the scientific

community in the mid-1800's by examining tropical cyclones in both the

Atlantic and Pacific Oceans. Redfield (1856) published the tracks of

13 "progressive gales and hurricanes" which extended from the west

coast of Mexico to 125 W longitude during the period 1842-1855. In

addition, another set of NETROPAC cyclone data was collected by the

Deutsche Seewarte in which 45 cyclones from 1832-1892 were doctimented.

Several regional storm phenomena are most likely of tropical

cyclone nature. A well-known and greatly feared storm along the west

ern coast of Mexico is known as El Cordonazo—the lash of St. Francis

(Kalstrom, 1952). The wind storm received its name because it seemed

to occur near the Feast of St. Francis, October 4. The weather asso

ciated with El Cordonazo includes strong southeast and southwest winds,

heavy rains and flooding along the coast. The residents along the

west coast of Mexico are also familiar with the Chubasco—a heavy

coastal squall accompanied with violent thunder and a vivid display of

lightning. This activity is usually associated with a nearby tropical

cyclone (Court, 1980).

Another meteorological phenomenon associated with NETROPAC

cyclones is the Sonora storm—summer thunderstorms which occur in the

mountains and deserts of lower and southern California (Blake, 1923).

These storms were named after the Mexican state in which they were

believed to originate. Blake found that these storms were most prom-

inent in July and August and could persist for a day to as long as a

week. Commenting on these storms Blake said: "very often highways,

railroads and bridges are washed away, and arroyas are turned into

booming streams, small farms in the river beds are inundated...."

Although he had no solid evidence to support his hypothesis, Blake

speculated that the rains from Sonora storms were linked to tropical

disturbances from Mexico.

Although several published works indicated cyclone activity in

the NETROPAC, the United States Weather Bureau in 1910 still denied

the existence of such storms. A document released by the Hydrographic

Office stated: "The occurrence of tropical storms is confined to the

summer and autumn months of the respective hemispheres and to the

western parts of several oceans" (Court, 1980). There was no refer

ence made to the NETROPAC.

Tropical cyclones in the NETROPAC which occurred between Hawaii

and Mexico were examined by Visher (1922). With the exception of six

cyclones documented by Hurd, he found no reference to cyclone activity

between 1892 and 1915. Combining all available cyclone information,

Visher determined a frequency of two tropical cyclones per year during

the period 1855-1922. September had the greatest frequency with 28%

of all storms, while October was close behind with 25%.

The pioneer in tropical cyclone research in the NETROPAC was

Willis Hurd, a marine climatologist with the United States Weather

Bureau from 1910-1944. Although the groundwork had been laid by Red-

field, Blake, and Visher, Hurd (1929) took an active interest in de

fining the areas of origin, dissipation, and the tracks of these

cyclones. He noted that upon completion of the Panama Canal in 1914,

new shipping channels were opened and the density of weather-related

ship observations dramatically increased.

Hurd postulated that many of the tropical cyclones occurred

within a triangle bounded by Point Eugenia (28°N, 115°W) to 10°N and

125 W, eastward to the coast of Costa Rica. In the 19-year period of

study, 1910-1928, he found a frequency of five cyclones per year, with

34% of those storms attaining hurricane status. Hurd determined that

the hurricane season in the NETROPAC usually lasted about six months,

beginning in early June and ending in November. The month with the

greatest frequency of storms was September in which 40% of all tropi

cal storms occurred.

After a very long interval, Rosendal (1962) examined tropical

cyclones in the NETROPAC for the period 1947-1961. He portrayed

cyclone tracks for the region, and attributed the observed increase in

cyclone frequencies to better aerial reconnaissance coverage since

World War II.

Prior to the 1960's, tropical cyclone detection in the NETROPAC

was primarily based upon radio and ship reports. Many times the only

advance warning of an impending tropical storm or hurricane came from

ships whose routes were off the Baja California coast southeast to the

Panama Canal. As radar and radio communication coverage expanded

following World War II, detection of these tropical storms increased,

though a good number of the storms undoubtedly remained undetected in

regions of sparse shipping activity. The advent of satellites, how

ever, greatly increased the number of cyclones detected around the

globe, especially in the NETROPAC.

Although the NETROPAC has been found to be a major region for

tropical cyclones, study of this area has suffered compared to regions

such as the Northwest Tropical Atlantic. This is demonstrated in part

by a rapid decline in reconnaissance missions flown in the Northeast

Pacific. (There were no flights in 1980 or 1981.) Operationally, a

heavier reliance has been placed on satellite-derived information of

tropical cyclone activity.

During the past decade, investigators of tropical cyclones in the

NETROPAC have gradually incorporated satellite data into their

studies. Serra (1971) studied tropical cyclones which affected the

western coast of Mexico for the period 1921-1969. More than earlier

researchers, he recognized that a tropical storm or hurricane did not

have to cross inland to affect a coastal region. On that premise he

determined the probability of tropical cyclones moving within a speci

fied distance of a particular coastal location; specifically, Serra

computed the probability that at least one tropical storm would come

within a 200 nmi of each of the ten western states of Mexico. He

found that the Baja California peninsula had the highest probability

(.97) of being affected by a tropical storm and a .46 probability that

such a storm would make landfall. Based on his research, Serra pro

posed some generalized storm tracks representative of NETROPAC

cyclones.

A climatology of tropical cyclones in the NETROPAC for the period

1966-1971 was compiled by Hansen (1972). Employing a 5 longitude-

latitude grid, he examined possible factors affecting frequency, for-

mation, duration, intensification and dissipation of NETROPAC cyclones,

He concluded that the NETROPAC had the greatest frequency per unit

area of tropical cyclones in the world. He found the usual range in

speed of NETROPAC cyclones to be 5 to 8 kt with a mean track direction

toward 292°.

Baum and Rasch (1975) prepared digitized tropical cyclone tracks

in the NETROPAC for the 14-year period 1961-1974. In their study,

2.5 degree longitude-latitude boxes were used to study cyclone tracks.

No conclusions however were drawn from this study. The climatology

and forecasting of NETROPAC tropical cyclones was described for the

10-year period 1965-1974 by Renard and Bowman (1976). Utilizing again

a 5 degree longitude-latitude grid, they examined the frequency,

recurvature, duration and speed of movement of NETROPAC cyclones. In

this study, they found the prime area for initiation of NETROPAC trop

ical cyclones to lie between 10 N and 15 N from 150 to 415 nmi off the

west coast of North America. They also discussed the forecasting

errors associated with NETROPAC cyclones. They concluded that the

initiation time of the forecast was related to accuracy. Forecasts

based on 1800 GMT satellite imagery were found to be the best while

the 1200 GMT forecasts were the least accurate. Since most of the

data in their study period consisted of visual satellite imagery, it

seems reasonable to expect the best forecasting accuracy at 1800 GMT

while the 1200 GMT (before sunrise) period would demonstrate the

poorest accuracy.

Eidemiller (1978) studied tropical cyclones affecting the south

western United States and northwestern Mexico for the period 1954-

1976. In his research, he determined the probability of a tropical

cyclone reaching a specified distance from San Diego for any given

month. Eidemiller found that no hurricanes had come within 150 nmi

of San Diego. He concluded that the region of highest tropical

cyclone probability in his study area was 150 nmi southwest of Cabo

San Lucas, Baja California, Mexico. In that region, he found there

was a 90-100% probability that a tropical storm would occur each year,

while the southwestern United States had an annual tropical storm

probability of less than 10%. He noted that tropical storms, or their

remnants, were an important source of rainfall during the late summer

months in Arizona.

Tropical cyclone effects on California for 1906-1978 were com

piled by Court (1980). Tropical cyclone tracks as well as summaries

of damage from rain, wind and waves were provided. In the latter

portion of his study, tropical cyclones affecting California in the

1970's were discussed. Tropical Storm Kathleen was the first cyclone

to hit southern California in 37 years with damage estimated in excess

of $160 million. Doreen crossed the Baja California peninsula in

August of 1977. The remnants of Doreen hit the San Diego and Los

Angeles areas, with several inches of rain reported; damage in San

Diego county alone was estimated at $25 million. In early September

of 1979, the remnants of hurricane Norman affected the Los Angeles

area; although there were some reports of flooding, no substantial

damage was reported. Court concluded that tropical cyclones would

always pose a threat to Southern California in the late summer and

early autumn, but the rainfall could be beneficial to some areas.

8

Most recently, Leftwich and Brown (1981) portrayed tropical cyclone

tracks for the period 1949-1979 in the NETROPAC; little additional

analysis was presented though.

The first satellite image of a NETROPAC cyclone (Tropical Storm

Liza) was recorded by TIROS III on 19 July 1961 (Mull, 1962). In late

August of 1965, the first visual sighting of a NETROPAC cyclone was

made by Major Gordon Cooper aboard the Gemini Mission (Paum, 1966).

Daily satellite coverage began in 1966 with the use of polar orbiters

and infrared imagery. As satellite technology improved in the early

1970's, ship reports grew more sparse. By 1974 only 3% of tropical

cyclone advisories were based on ship-reported locations. In 1975

the SMS-2 (synchronous meteorological satellite) provided the fore

caster with full-disk resolution of 4 mi and sector resolution of 2

mi. Since 1979, GOES-West (geostationary orbital earth satellite)

has provided excellent coverage of NETROPAC cyclones including visual

and infrared imagery at 30 min intervals.

Prior to the satellite era, tropical cyclone frequencies were

grossly underestimated. For the period 1947-1961, Rosendal (1962)

found an average of 7.5 tropical cyclones and 3.2 hurricanes per year

in the NETROPAC. Since then, as satellite coverage has become more

comprehensive, cyclone frequencies have increased to more representa

tive values. Hansen (1972) found a frequency of 9-1 tropical cyclones

and 5.4 hurricanes during the years 1966-1971. (During the period of

the present study, 1966-1980, an average of 14.3 tropical storms and

7.2 hurricanes were found to occur in the NETROPAC.) This dramatic

increase is not believed to be related to changes in large-scale

global circulations; rather, it represents a more accurate view of

cyclone frequencies since the satellite has become operationally

effective.

While these past studies lend insight into the behavior of

NETROPAC cyclones, the data has been based either in part from the

pre-satellite era (when cyclone frequencies were underestimated) or

the time period of satellite-derived data has been too limited. With

the broader data base now available, a climatology of tropical cyclone

activity in the NETROPAC for the period 1966-1980 is justified. Not

only may this provide the basis for future studies of storm behavior

but could be of use to the navigator as well as inhabitants of the

Mexican Coast—those most likely to feel the effects of a NETROPAC

cyclone.

1.2 Data Sources Past and Present

The NETROPAC, as defined for this study, is the area bounded by

the West Coast of North America extending to 140 W and between 5 and

40 N. The North Central Pacific has the same latitude boundaries but

extends from 140 to 180 W. Some of the storm tracks in this present

study extend into the North Central Pacific; however, all statistics

and discussion of results are terminated at 140 W-

The first complete annual listings of NETROPAC cyclones were made

beginning in 1946. Ship observations and aircraft reconnaissance were

used to estimate tropical cyclone intensities and locations. Although

radar observations by ships were of great importance in determining

cyclone characteristics, it was not until the advent of satellites in

10

the 1960's that tropical cyclones could be consistently monitored.

The period 1966-1980 was chosen for this climatology because it best

represented the NETROPAC in terms of accuracy and length of data base.

The primary source of data for this research consisted of trop

ical cyclone track information contained on magnetic tape from the

National Climatic Center in Asheville, North Carolina. The data base

consisted of 12 hour (0000 and 1200 GMT) "best track" positions. A

best track position is determined a. posteriori after a tropical cyclone

has occurred. Satellite imagery as well as reconnaissance data (if

available) are used to make the best estimate of storm location. This

is necessary since many storms do not pass over land areas where

cyclone coordinates would be easier to determine. Other information

contained on the tape includes stage of the storm (tropical depression,

tropical storm or hurricane), movement during the past 12 hours (whole

degrees), maximum wind and average speed of the storm. The tropical

cyclone tracks in this study were smoothed using a SAS cubic-spline

technique (SAS Institute, 1981). This was chosen to represent the

large-scale circulations of the cyclones.

Many of the tropical cyclone characteristics used as data in this

study have been based on interpretation of satellite-imagery. Dvorak

(1975) developed a technique using satellite imagery to estimate

present and future intensities of tropical cyclones. Cloud character

istics are analyzed based on empirical relationships derived from

satellite data. The intensity of the cyclone is based on three param

eters; central features which describe the cloud system center, the

outer boundary features and vertical cloud depth. Although Dvorak's

11

scheme was developed with Atlantic satellite data, it has been used

by the Eastern Pacific Hurricane Warning Center in Redwood City,

California, with a good degree of confidence. (Now that increased

infrared information is routinely available, further technique develop

ment is taking place.)

Supplementary data was obtained from annual summaries contained

in Monthly Weather Review and Mariners Weather Log. In addition to

summaries on reconnaissance activity, damage reports on those storms

making landfall or affecting land areas were included.

The surface and upper air data for Mexico were found to be inade

quate for any type of kinematic or thermodynamic analysis. Surface

reports at 3-hour intervals are available for major cities such as

Acapulco, Mazatlan, La Paz and Guaymas. However, most of the damaging

tropical cyclones made landfall at areas where data were not recorded.

Upper air data are even more sparse with Guadalupe Island (29.0 N,

118.5 W), Guaymas and Mazatlan the only West Mexico stations providing

observations. The next closest upper air station is Chihuahua. Fig

ure 1.1 shows the surface and upper air stations for the western por

tion of Mexico.

12

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CHAPTER II

THE CLIMATOLOGICAL AND SYNOPTIC SETTING

In this study, a tropical disturbance is defined as a distinctly

organized cloud mass with a breadth ranging from 100 to 600 km which

maintains its identity for at least 24 hours. A tropical depression

is a warm-core, cyclonically rotating wind system with maximum winds

less than 34 kt (17 m/sec). A tropical storm has similar requirements

as a depression; however, the maximum sustained winds must be greater

than 34 kt (17 m/sec) but less than 64 kt (33 m/sec). A hurricane

must have maximum sustained winds greater than 64 kt (33 m/sec).

Throughout this study, the terms tropical cyclone and tropical storm

will be used interchangeably.

2.1 The ITCZ and Tropical Cyclogenesis

The hurricane season in the NETROPAC generally runs from late

May to early October. There have been a few years when tropical

cyclone activity has continued into November, but that has been the

exception rather than the rule. (The unusual year 1983 included an

early December NETROPAC cyclone!) During the NETROPAC hurricane sea

son, the intertropical convergence zone (ITCZ) can play an important

role in initiating disturbances which become tropical cyclones. The

ITCZ is defined as a belt of lower pressure where the trade winds of

both hemispheres converge. This can show up on satellite imagery as a

band of cloudiness or "cloud blobs" extending around the globe in a

somewhat discontinuous fashion. The ITCZ or equatorial trough lags

13

14

the march of the sun's zenith position by about two months. Although

there are some daily as well as year-to-year differences in the posi

tion of the equatorial trough, it usually reaches a maximum poleward

position of 15 N in the Northern Hemisphere summer and retreats to

near 5°S for a brief period in the winter (Riehl, 1979). Gray (1968)

describes the equatorial trough as being of two types; the trade wind

equatorial trough and the doidrum equatorial trough. The trade wind

equatorial trough is defined as a line of lowest pressure where the

northeast and southeast trades meet. Along this line, surface winds

are not calm and tropical disturbances do not usually form. The del-

drum equatorial trough, which is found in the NETROPAC during the

summer months, is an area where calm surface winds prevail due to the

turning of the trade winds which have crossed the equator. On the

poleward side of the doidrum equatorial trough, cyclonic wind shear

and strong surface relative vorticity can create the impetus for a

tropical disturbances.

Gray (1968) discusses several factors favorable for tropical

cyclone development. They include:

1. A sea-surface temperature of at least 26.5

C. For a warm-core disturbance to form and be

maintained, warm moist air expanding moist

adiabatically must be considerably warmer than

its environment to a level of at least 12 km.

2. A location removed from the equator by at

least 5 degrees. Near the equator the Coriolis

15

force is too weak to provide vorticity.

3. Weak vertical wind shear between 850 and

200 mb. Gray found that a large vertical wind

shear between 850 and 200 mb ventilated heat

away from a developing disturbance. This inhibits

the concentration of the heat gained from the

latent heat of condensation. Consequently,

significant pressure falls will not occur.

4. Cyclonic wind shear and high relative

vorticity, which usually occur on the pole

ward side of the doidrum equatorial trough.

The positions of the equatorial trough during the months of Jan

uary and August are shown in Figure 2.1. In January, when NETROPAC

cyclones do not occur, the trade wind equatorial trough is displaced

about five degrees north of the equator. However, in August, when the

equatorial trough is displaced further north and assumes a doidrum

character, cyclone frequencies in the NETROPAC are at their highest

levels.

Monthly sea-surface temperatures (SST) for the NETROPAC are

depicted in Figure 2.2. The 80°F (26.7°C) isotherm corresponds with

the threshold temperature considered necessary (since Palm^n, 1969)

for tropical cyclone development. Vertical wind shears between 850

and 200 mb for the months of February and August are shown in Figure

2.3. Again, the wind shear is high in January when tropical cyclone

activity does not occur, while the month of August shows a minimum

16

(a)

(b)

Figure 2.1 The position of the equatorial trough during (a) January and (b) August. Solid line represents the doidrum equatorial trough and the dashed line represents the trade wind equatorial trough. (After Gray, 1968)

17

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18

(a)

(b)

Figure 2.3 Vertical wind shear between 850 mb and 200 mb for (a) January and (b) August. (After Gray, 1968)

19

in wind shear but a maximum in cyclone frequency. (Further aspects of

upper level conditions will be discussed in a later section concerning

cyclone tracks and recurvature.)

Research by Frank (1970-1979) indicates that many of the tropical

cyclones that develop in the NETROPAC originated in the Atlantic

Ocean. Frank defines a seedling as "a discrete system of apparently

organized convection, generally 100 to 300 nmi in diameter, originat

ing in the tropics or subtropics, having non-frontal, migratory char

acter, and having maintained its identity for 24 hours or more."

Using satellite imagery combined with surface and upper air data,

these disturbances have been tracked from as far as the west coast of

Africa. Some of these wave disturbances originated in the intertrop

ical convergence zone (ITCZ) and eventually became embedded in an

easterly flow. These waves generally were found to amplify in regions

of weak vertical wind shear. The seedlings which initiated NETROPAC

tropical cyclones will be discussed in detail in Chapter 3.

2.2 El Nino and the Southern Oscillation

During the past several years the "El Niflo" effect has been cited

by some (Rasmusson and Carpenter, 1982) to explain anomalous precip

itation patterns in the Central and Western Pacific. El Niflo is a

warm ocean current which usually develops off the coast of Peru near

Christmas. However, some researchers (Ramage, 1975) associate El Niflo

with more extreme periods of sea-surface temperature (SST) anomalies

which have occurred infrequently during the past century. This sec

tion will discuss those less frequent El Niflo events.

20

The strength of the southeast trades is an important factor in

determining whether El Nino will occur. Riehl (1979) attributes the

strength of the SE trades to upwelling along the coast of South Amer

ica. Upwelling also occurs along the equator where easterlies are

prevalent, thus a "cold tongue" is found off the coast of Chile and

Peru extending to the Central Pacific. It appears that weakening of

the SE trades can cause a major El Nino event (Ramage, 1975). How

ever, the weakening of the SE trades is not as easily explained.

Rasmusson and Wallace (1983) found a relationship between the onset

of warmer SST in the Pacific and the collapse of the surface anti

cyclone in the southern subtropical Pacific. The resulting change in

surface winds stress could explain the diminished effect of upwelling

along the coast of South America and subsequent warmer SST.

Another phenomenon associated with El Niflo is the Southern Oscil

lation (SO), a circulation pattern for low latitudes in which Walker

(1924) found a relationship between pressure and temperature anomalies

over the Pacific and Indian Oceans. When the pressure is above the

mean in the Pacific, it tends to be below the mean in the Indian Ocean

from Africa to Australia. Rainfall anomalies can be found in areas

where the pressure is lower than the mean. The SO index (Tahiti-

Darwin sea-level pressure difference) has a period of approximately

3.3 years (after Rasmusson and Carpenter, 1982) and appears to be a

good measure of cyclone activity in the NETROPAC. Based upon data by

Winston (1982), negative values of the SO Index in 1969, 1972 and 1977

correspond well with negative anomalies of cyclone frequency in the

NETROPAC. Positive correlation was found for the years 1970, 1974 and

21

1975 when the SO Index was positive and the NETROPAC cyclone frequen

cies were above the 15-year mean of 14.3.

Rasmusson and Carpenter (1982) examined variations in SST and

surface windfields associated with the Southern Oscillation and El

Nino. They looked at six warm episodes during the period 1949-1982.

In months preceding a warm period, the equatorial easterlies were

stronger than normal west of the dateline. They attributed this

anomaly pattern to the position of the South Pacific Convergence Zone

(SPCZ) which was located to the southwest of its normal position.

During the months of October through November prior to El Niflo, equa

torial easterly anomalies were replaced by westerly anomalies. It

was not until April through June that maximum SST anomalies occurred

along the coast of South America and progressed to near 170 W by

year's end. Winston (1982) estimated the lag of SST anomalies between

Peru and 170 W to be three to six months which implies a speed of 0.5

to 1.0 cm/sec. It appears that indications for a major El Nino event

usually occur a few months in advance and the event can last upwards

to a year or more.

During the period of this study, 1966-1980, the only significant

El Nino events occurred in 1969 and 1972. Ramage (1975) found that

for the period of March 1972 to March 1973, a very strong El Niflo

occurred in which SST anomalies of several degrees were common at

some time across much of the Pacific. However, looking at NETROPAC

cyclone frequencies during El Niflo years, there does not appear to be

any positive correlation betvi een SST anomalies and cyclone frequencies,

In 1972 there were 12 tropical cyclones in the NETROPAC, 2 below

22

the 15-year mean of 14.3. (A SST above 26.5 C is considered necessary

but does not guarantee that tropical cyclogenesis will occur.)

Wooster and Guillen (1974) considered 1969 (in which only 10 NETROPAC

cyclones occurred) to be a minor year for El Nino. Most recently,

the 1982-1983 El Nino/SO event was the most intense on record (Ras

musson and Wallace, 1983). During 1982 and 1983 19 tropical cyclones

were observed in the NETROPAC. SST anomalies above 3 K were common

across much of the equatorial Pacific. (Rainfall records were shat

tered in Equador, while drought conditions were common in agricultural

areas in Peru and Bolivia.) Some attribute the usually drier condi

tions in the NETROPAC during El Nino to a more southerly position of

the doidrum equatorial trough in the summer. One must also consider

that the highest SST associated with El Nino in the NETROPAC occur

during the winter months when tropical cyclone activity rarely occurs.

CHAPTER III

RESULTS AND ANALYSIS

This chapter will present the analysis of NETROPAC cyclone data

which was obtained on magnetic tape from the National Climatic Center

in Asheville, N.C. The first section will examine cyclone frequencies

and storm tracks while the latter sections will investigate recurving

cyclones and "seedlings" which initiated NETROPAC cyclones.

3.1 NETROPAC Cyclones 1966-1980

A total of 214 tropical storms formed in the NETROPAC during

1966-1980. Of that total, slightly more than half (108) become hurri

canes. Previous research had indicated much smaller cyclone frequen

cies in the NETROPAC; however, an annual average of 14.3 tropical

storms and 7.2 hurricanes were found to occur during the period of

this study. A monthly breakdown of tropical cyclone occurrence (Table

3.1) shows August with a maximum of almost 4 tropical cyclones

annually. This is followed closely by July (3.4) and September (2.6),

while May and November average less than one tropical cyclone per

year. Hurricane incidence reaches a maximum in August, when an aver

age of 2.47 hurricanes occur. July and September each have a mean of

1.40 hurricanes while there were no hurricanes recorded during this

fifteen-year period for November.

For comparison. Table 3.2 shows the frequency of tropical storm

and hurricane occurrence in the Northwest Tropical Atlantic (NWTROATL)

during the same time period. These data were obtained from annual

23

24

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tropical cyclone surnmaries contained in Mariners Weather Log and

Monthly Weather Review. Compared to the NETROPAC, the NWTROATL had

30% fewer tropical cyclones. Of those storms, 58% became hurricanes

while 51% of the NETROPAC cyclones spawned hurricanes.

Ballenzweig (1959) and others have indicated that tropical

cyclone formation is above normal when the Pacific subtropical high

pressure system is displaced to a more northerly position. He asserts

that this increases the strength of the westerlies whose position is

displaced northward about 5-10 degrees of latitude. During years with

infrequent tropical cyclone activity, they found that the subtropical

high was displaced to lower than normal latitudes.

The most active years for NETROPAC cyclone development were 1968,

1970 and 1978 when 18 tropical cyclones were detected. (More recently

there were 19 in 1982 and 1983.) In August 1968, seven tropical cy

clones formed, almost twice the annual average for the month! Andrews

(1968) attributed this anomaly to a westerly pattern which was strong

er than normal at southern mid-latitudes. The quietest year in this

period was 1977 when only eight tropical cyclones formed. Hurricane

formation was a maximum in 1971 and 1978; the 12-hurricane figure for

these years was well above the 15 year average of 7.2.

Tropical cyclone and hurricane frequencies for the NETROPAC are

displayed for bi-weekly periods in Table 3.3. It should come as no

surprise that the periods 16-31 May and 1-15 Nov have the smallest

frequencies. The interval from 16-31 Aug shows the highest frequency

for the 15-year period in which 38 tropical cyclones occurred. This

corresponds well with the time in which the ITCZ has reached its max-

27

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28

imum poleward position and SST are also at their highest values.

There is a sharp decrease of 50% for the following period from 1-15

Sep in which only 18 cyclones occurred. The decrease in the duration

of the tropical cyclones could have an effect on this decline, as

well as other factors for cyclone development which are maximized in

August.

Figure 3.1 represents a 15-day running mean of daily NETROPAC

occurrence during the period of this study. This multi-modal distri

bution shows a maximum number of cyclones in late August, in which

an average of 14 tropical storms were found to occur. Two secondary

peaks occur in late July and in middle and late September. Tropical

cyclone duration is longest in August (Renard and Bowman, 1976) when

SST are usually the highest and hurricane incidence is a maximum.

Tropical storm incidence (those tropical storms that did not become

hurricanes) is a maximum in July—causing the secondary maximum on

the smoothed frequency curve. Recurving tropical cyclones in Septem

ber could account for the other secondary peak in Figure 3.1. Cy

clones which do not recurve will tend to dissipate more quickly over

the colder ocean waters. However, storms that recurve have an ex

tended life cycle, and thus increase the cyclone frequencies in Sep

tember.

The length of the hurricane season is generally defined by the

dates in which tropical cyclones of at least depression stage are

reached. The hurricane season in the NETROPAC, depicted in Table 3.4,

averaged 148 days during the period 1966-1980. The earliest beginning

date occurred 21 May 1971 while the latest closing date was found on

29

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30

Table 3.4 Length of the Hurricane Season

YEAR

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

AVERAGE

BEGIN

20 JUN

7 JUN

20 JUN

31 MAY

30 MAY

21 MAY

31 MAY

1 JUN

27 MAY

2 JUN

2 JUN

25 MAY

30 MAY

31 MAY

9 JUN

3 JUN

END

19 OCT

3 NOV

28 OCT

12 OCT

8 NOV

31 OCT

15 NOV

9 OCT

24 OCT

7 NOV

29 OCT

23 OCT

24 OCT

18 NOV

29 OCT

29 OCT

# DAYS

121

149

130

134

162

163

168

130

150

158

149

151

147

171

142

148

31

18 November 1979.

NETROPAC cyclone tracks were plotted for each year during the

period of this study. The Appendix (see Figures A1-A15) contains the

tracks of all tropical depressions, tropical storms and hurricanes

which occurred in the NETROPAC from 1966-1980. (Tropical depressions

were not included in the actual statistics of this study.) Although

some storms do recurve toward the west coast of Mexico, most of the

storm tracks have an east-west or southeast-northwest orientation.

After the cyclone tracks were plotted, a 2.5 by 2.5 degree longitude-

latitude grid was overlaid to count cyclone crossings across each

"block." A storm was counted only once regardless of the number of

times it crossed a particular grid square. Figure 3.2 illustrates

the annual frequency distribution of NETROPAC cyclones which were at

least tropical storm intensity when crossing any given block. Very

few tropical cyclones are found equatorward of 10 N and poleward of

30 N. North of 25 N SST are usually too cold to support a warm-core

disturbance. As a consequence, many cyclones dissipate over the

colder ocean waters. Very few cyclones occur south of 6-7 N because

the Coriolis force near the equator is considered too small to pro

vide vorticity. The block bounded by 15.0-17.5°N and 107.5-110°W

represents the greatest frequency of tropical cyclone occurrences at

54. Frequencies drop off markedly poleward of 20 N over the colder

ocean waters.

Figures 3.3 through 3.9 represent the monthly distribution of the

occurrence of NETROPAC cyclones. In May, when cyclones are not

32

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40

common, there is little set pattern to the frequency distribution;

cyclone activity is confined between 10 and 20 N only. During June

and July, when the ITCZ is further north, frequencies increase in

both magnitude and areal extent. The block bounded by 15.0-17.5 N

and 100.^-102.5 W represents a maximum of eight storms during the

month of June, three of which eventually made landfall nearby on the

west coast of Mexico. This June maximum is associated with the posi

tion of the ITCZ, which is still displaced to the south of this

region. As a result, cyclone activity is confined equatorward of

20 N, where SST's are higher. By August, tropical cyclone occurrences

have reached a maximum of 17 between 15.0-17.5 N and 110.0-115.0 W.

Frequencies are relatively high along the west coast of Mexico during

August and September when recurving storms are most prevalent. How

ever by October and especially November, cyclone occurrences have

been reduced to less than 10 as the ITCZ has retreated southward

towards the equator and SST's have decreased.

Average annual frequencies for the NETROPAC are depicted in Fig

ure 3.10. The block bounded by 15.0-17.5°N and 107.5-110.0°W repre

sents a frequency of more than 3.5 tropical cyclones per year. A

large portion of the NETROPAC between 10.0°N and 22.5°N has a fre

quency of at least one tropical cyclone per year. The elliptical

shape of the isopleths indicates a generalized trend toward east-west

cyclone movement.

41

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42

3.2 Areas of Origin, Intensification and Dissipation

The points where tropical disturbances first reached tropical

storm intensity are plotted in Figure 3.11. The area bounded between

95-0-115.0°W and 10.0-20.0°N contains 75% of the 220 total data

points. SST charts for this region indicate that this is indeed in

the region of the 26.5°C isotherm (80°F). During August, when tropi

cal cyclone formation is a maximum, vertical wind shear is a minimum

over this area (see Figure 2.3).

The location points in which hurricane intensity was initially

observed are depicted in Figure 3.12. Note that there are no points

south of 10.0 N or north of 25.0 N. The bulk of the data appear to

be shifted slightly more to the northwest of the points of origin.

The storms which eventually became hurricanes usually travelled a

relatively short distance of a few degrees of latitude before inten

sification occurred, thus indicating the dynamic potential for storm

development in this region.

Figure 3.13 represents the locations where the tropical storms

or hurricanes began to dissipate. While the data demonstrates a good

degree of variability, it is obvious that some storms decayed over

relatively cooler ocean waters; i.e., north of 20 N. A small percen

tage of the tropical cyclones dissipated after making landfall upon

the west coast of Mexico. The orographic features of Mexico coupled

with the cutoff of moisture contributed to their somewhat rapid decay.

(The orographic features of Mexico will be discussed in greater detail

in Chapter 4.) Sadler (1964) mentions strong wind shear between

43

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46

20,000 feet and the tropopause as a major cause for cyclone dissipa

tion as they pass west of the subtropical ridge. Upwelling along the

Baja California west coast, where SST are much cooler than the rest

of the NETROPAC during the hurricane season, is another factor affect

ing dissipation.

3.3 Recurving Storms

A recurving tropical cyclone is defined here as one obtaining an

eastward component of motion. Another stipulation in this research

is that the intensity of the storm be of at least tropical storm stage

when recurvature occurs. (A recurving tropical cyclone is classified

by the month in which it forms.)

Hansen (1972) studied recurving NETROPAC tropical cyclones for

the period 1966-1971. He found that 16% of those storms recurved.

Renard and Bowman (1976) investigated tropical cyclones during the

period 1965-1974 and their results showed that 32% of the cyclones

recurved. Of those storms, 76% recurved during the months of August,

September and October. In this study, 38 out of a total of 214 storms

(18.1%) recurved. This is considerably smaller than the values pre

sented by Renard and Bowman. However, tropical depressions were in

cluded in their data base and this could account for a higher recur

vature rate. Of the recurving tropical cyclones in this study, 37%

made landfall along the western coastline of Mexico.

The tracks of recurving NETROPAC cyclones in Figures 3.14 through

3.16 are divided into three groups: May-June, August-September and

October-November. Of the 52 storms which formed in July, none of

47

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48

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50

them recurved! Storm tracks for July indicate that most of the storms

traversed an east-west path across the NETROPAC; with the subtropical

anticyclone to the north, this apparently blocked any storms from re

curving. Table 3.5 lists the coordinates where NETROPAC cyclones

recurved and the dates for which recurvature occurred. The storm

numbers correspond with those listed with the storm tracks in Figures

3.14-3.16. Figure 3.14 represents recurving NETROPAC cyclones during

the months of May-June. The general track direction is toward the

northwest with a few cyclones making landfall. Note that all these

storms are confined to the east of 115.0 W. This could be due to

relatively cold SST which can cause the cyclones to dissipate after a

short period of time, thus keeping storm track lengths to a minimum.

Although three of the six May storms recurved, September is the pri

mary month for recurvature; 40% recurve. Recurving NETROPAC cyclones

during the months August-September are depicted in Figure 3.15. The

cyclone tracks cover a large area in the Northeast Pacific, occasion

ally extending beyond 130 W. Landfalling storms are more common with

a few of the cyclones crossing over Baja California. By October and

November (Figure 3.16) the mean latitude of recurvature is near 17.5 N

and storm tracks do not extend beyond 117.5 N. In early autumn, as

the subtropical high moves southward, a good number of the storms re

curve and make landfall along the west coast of Mexico.

Figure 3.17 represents the points of recurvature for the 38

recurving NETROPAC cyclones. The boxed area represents one standard

deviation, in which 67% of the data fall. The mean coordinates for

recurvature are 18.5 N and 113.6 W. This is slightly southwest of

Table 3.5 Recurving NETROPAC Cyclones 51

Storm

1 2 3 4 5 6 7 8

Name

Aletta Annette Connie Eileen Annette Dolores Ava Adelle

MAY AND JUNE

Latitude

16.8 11.3 16.0 21.4 13.8 13.6 17.8 16.5

Longitude

104.4 98.5 113.2 106.3 107.6 100.0 110.7 103.9

Date

29 May 1974 6 Jun 1976 15 Jun 1974 29 Jun 1970 5 Jun 1972 15 Jun 1974 28 May 1977 23 Jun 1966

AUGUST AND SEPTEMBER

Storm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Name

Helga Nanette Naomi Irah Eileen Francessca Florence Hyacinth Gwen Lorraine Katrina Lily Monica Monica Olivia Norman Liza Olivia Paul

Latitude

22.5 20.5 21.7 18.3 24.0 16.4 15-8 24.2 10.2 12.3 22.9 23.6 16.4 17.4 23.9 26.6 14.0 15.2 20.3

Longitude

116.3 129.9 107.6 109.7 129.8 131.8 127.8 124.9 125.0 120.9 111.8 122.7 114.9 113.6 114.5 120.5 109.0 94.8 109.5

Date

13 Sep 20 Sep 12 Sep 24 Sep 28 Aug 13 Sep 22 Sep 4 Sep 10 Aug 24 Aug 31 Aug 9 Sep 1 Sep 19 Sep 29 Sep 5 Sep 26 Sep 22 Sep 25 Sep

1966 1967 1968 1973 1966 1966 1977 1972 1976 1974 1967 1967 1971 1967 1971 1978 1976 1978 1978

OCTOBER AND NOVEMBER

Storm

1 2 3 4 5 6 7 8 9 10 11

Name

Olivia Pauline Irah Jennifer Selma Priscilla Joanne Olivia Naomi Heather Ignacio

Latitude

15.4 24.1 17.2 19.7 15.7 20.1 20.7 16.5 17.0 25.9 17.4

Longitude

114.7 111.2 115.6 108.5 111.6 106.3 116.6 111.0 116.0 117.0 108.0

Date

10 2 2 11 3 12 4 23 26 7 28

Oct Oct Oct Oct Nov Oct Oct Oct Oct Oct Oct

1967 1968 1969 1969 1970 1971 1972 1975 1976 1977 1979

52

— o o u

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o o

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53

Hansen's (1972) mean position of 19.3°N and 112.7°W.

3.3.1 Synoptic Situation

The relatively high rate of recurvature during the months of

September and October is associated with the weakening and migration

of the North Pacific subtropical anticyclone. As the high moves

southward during the late summer, the anticyclone presents a barrier

to the westward moving storms, forcing them to move toward the north

and northeast. The appearance of a trough to the west can also help

steer the cyclone toward the northeast. Gray (1968) attributes poor

recurvature years to the presence of a strong subtropical high north

of the storms. This tends to block storms and prevent them from

moving toward the east. Denney (1976) cites cool inflow as a cause

for tropical cyclones to weaken. He suggests that storms passing

west of Cabo San Lucas (22.5 N, 110.0 W) encounter cooler water to

their right, leading to dissipation. The data from this research in

dicates that very few storms recurved just west of Cabo San Lucas.

This can be attributed in part to cooler SST off the west coast of

Baja California. The majority of recurving storms had an inflection

point west of 110 W but at lower latitudes where SST are higher.

3.4 Mean Storm Direction and Speed

Figure 3.18 shows the previous 12-hour movement of NETROPAC

cyclones by 2.5 by 2.5 degree longitude-latitude blocks. This anal

ysis only includes tropical cyclones of tropical storm or hurricane

intensity. The number in the upper right-hand corner of each "block"

represents the number of data points which were found to occur in a

54

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56

particular grid square. The vector mean 12-hr speed is indicated in

the lower left and the vector mean 12-hr direction in whole degrees

is depicted in the lower right-hand corner. At lower latitudes, the

tropical cyclones move in a more westward direction. However, as

latitude increases, the mean direction becomes northwestward until

some squares north of 22.5 N show northeastward motion.

3.5 Seedlings and the Initiation of NETROPAC Cyclones

As stated earlier, research by Frank (1970) has indicated that

many of the NETROPAC cyclones had their origins as disturbances or

"seedlings" from the Atlantic. Some of these seedlings have been

tracked from as far as the west coast of Africa. In addition to the

use of satellite imagery, surface and upper-air data from Dakar,

Barbados and San Andres Island were utilized to track the seedlings

(see Figure 3-19), some of which initiated NETROPAC tropical cyclones

Table 3.6 represents a breakdown of the origins of NETROPAC cyclones

during the period 1969-1979. The data were extracted from annual re

ports on Atlantic tropical disturbances (Frank, 1970-1979). During

this 11-year study, only 20% of all NETROPAC cyclones originated in

the Northeast Pacific. Eighty percent of the seedlings had their

origins from the Atlantic side of Central America. Of that total,

61% formed from African waves while the other 19% formed in the

Caribbean.

57

O z o

d CO

U

<

CO CO o u o CO

CO 00 d

1 3 H

o

4->

z

CTi

CO

u 3 00

i < ^ ' — ^ 3 0 i u

58

YEAR

Table 3.6

NETROPAC Cyclone Origins 1969-1979

AFRICAN WAVES

FORMED IN ATLANTIC

TOTAL FROM ATLANTIC

FORMED IN PACIFIC

TOTAL 93 29 122 30

# NETROPAC STORMS

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

4

10

8

4

6

8

13

13

7

13

7

4

2

5

2

3

4

2

1

1

3

2

8

12

13

6

9

12

15

14

8

16

9

2

5

5

6

3

5

1

0

0

2

1

10

17

18

12

12

17

16

14

8

18

10

152

61% 19% 80% 20% 100%

59

3.6 Atlantic Storms Which Crossed into the NETROPAC

During the period of this research, three Atlantic tropical

cyclones made landfall over Central America and these storms or their

remnants eventually became NETROPAC cyclones. The following section

will discuss these storms and the paths they traversed across Central

America.

Hurricane Irene made landfall in southern Nicaragua (Denney,

1972) near daybreak on 19 September 1971. Reports indicated remnants

of Irene with 30 kt winds near Lake Nicaragua. (This large lake—

7.608 X 10^ km^—could have been a factor in the regeneration of the

storm.) On 20 September, the remnants of Irene were renamed Tropical

Storm Olivia, a NETROPAC cyclone with maximum winds of 50 kt, located

about 50 mi west of Managua, Nicaragua. Olivia eventually attained

hurricane status with sustained winds in excess of 100 kt. However,

Olivia was classified only as a tropical depression when landfall was

made near San Ignacio, Baja California, on 29 September. Little dam

age was reported in Mexico due to the storm.

On the morning of 20 September 1974, the remnants of Caribbean

Hurricane Fifi, which made landfall and wreaked havoc the previous

day upon Honduras, regenerated into NETROPAC Tropical Storm Orlene

(Baum, 1975). Moving northwest, parallel to the coastline of Mexico,

Orlene became a hurricane on 23 September with peak winds near 70 kt

and made landfall near Culiacan, Sinaloa, Mexico. Fifi must be con

sidered a very rare event since the storm passed across western

Guatemala where elevations over 4 km are common.

60

Atlantic Hurricane Greta made landfall near the Belize coast

during the evening of 19 September 1978 (Gunther, 1979). It had been

downgraded to a tropical storm as it passed over northern Guatemala.

Moving west-southwest it was downgraded to a tropical depression less

than 100 miles from the NETROPAC coast. Once over the warm Pacific

waters, the storm intensified and was renamed Tropical Storm Olivia.

After briefly attaining hurricane status on the morning of 22 Septem

ber, Olivia began to dissipate later that day while off the coast of

the Gulf of Tehuantepec.

It is unusual for a tropical cyclone to survive a treak across

the Atlantic and continue across Central America into the NETROPAC.

Although these storms were downgraded after landfall, several factors

could contribute to their regeneration in the NETROPAC. As a general

rule, Caribbean hurricanes usually trace a parabolic path into the

Gulf of Mexico (Anon., 1975), being influenced by the Atlantic sub

tropical anticyclone. However, tropical cyclone activity is not un

known to areas such as Honduras where more than 25 hurricanes and

tropical storms have hit during this century (Anon., 1975). Many

tropical disturbances and cyclones that make landfall on Central

America dissipate after encountering mountainous terrain with eleva

tions generally from 1.5-3.0 km. Storms such as Fifi/Orlene had the

benefit of very warm SST in excess of 29 C and the influence of both

oceans to maintain the moisture and energy required for such a storm

to survive into the NETROPAC.

CHAPTER IV

LANDFALLING NETROPAC CYCLONES

This chapter will examine offshore and landfalling storms which

posed some threat to the west coast of Mexico. The damage caused by

these cyclones due to storm surge and high winds will be discussed.

Finally, the last section will investigate the logistics in the deter

mination of a design basis storm.

4.1 The Orography of West Mexico

The Republic of Mexico, located within the tropics and subtrop

ics, has extensive mountain ranges and a large plateau in the central

portion of the country with a mean elevation of 1.8 km above sea

level. More than 50% of the country has an elevation higher than 1 km

above sea level, with the highest elevation of 5.667 km occurring at

Citlaltepetl (19.4°N, 97.1°W). The Atlantic side of Mexico, facing

the Gulf of Mexico, is influenced by the northeast trades throughout

the year and experiences more precipitation than its counterpart on

the Pacific west coast (Rumney, 1968). The primary orographic feature

in western Mexico is the Sierra Madre Occidental which extends from

Northwest Mexico in the state of Sonora south-southeastward to just

below the Tropic of Cancer in Zacatecas. The continental divide runs

parallel to the Sierra Madre Occidental whose mean elevation exceeds

1.5 km.

Most of the west coast of Mexico (except for Baja California) is

61

62

considered a tropical forest and woodland with elevations generally

less than 1 km. Rainfall from NETROPAC cyclones (especially those

which recurve), or their remnants, can play a major role in the pre

cipitation regime along the Mexican west coast during the late summer

and early fall. Annual rainfall totals along the coast increase from

381 mm at Los Mochis, Sinaloa, to 1401 mm at Acapulco. Most of the

precipitation along this coastal stretch occurs during the five-month

period June-October. Mazatlan, which has an annual mean of 811 mm of

rainfall, receives 91% of its precipitation during this five-month

period with a maximum of 250 mm occurring in September. Acapulco

receives 25% (361 mm) of its mean rainfall in September.

The Baja California peninsula (also called Lower California),

with the exception of the southeast portion, is classified as a low

latitude, west coast desert (Hastings and Turner, 1965). It is part

of the Sonora Desert which also includes the Mexican state of Sonora

and southern Arizona. The prominent orographic feature along Lower

California is a narrow mountain chain which parallels the eastern

coastline from southern California to near the tip of the peninsula.

The highest elevation (3.095 km) is found in northern Baja California

in the Sierra San Pedro Martir range where elevations above 1.5 km

are common throughout the state. The mean annual precipitation for

Baja California is about 150 ram with extremes ranging from 30 mm at

Bataques (about 40 nmi southwest of Yuma, Arizona) to 750 mm at Sierra

de la Laguna which is 25 nmi due south of La Paz. The east coastal

region from east central Baja California southward to Cabo San Lucas,

receives substantially more precipitation than the west coast. Most

63

of the precipitation to the east occurs in the summer and fall and

can be attributed in part to NETROPAC cyclones or their remnants

which occasionally extend their influence into the central portion of

the Gulf of California. Unlike the east coast, northwestern Baja

California experiences its rainfall maximum in January. Mosino-Alem^n

(1974) attributes this precipitation pattern to the southern displace

ment of the Pacific subtropical high in the fall which allows mid-

latitude disturbances to occasionally penetrate further south. He

also states that the stabilizing affect of the cold California current

causes a rainfall minimum in this area during the summer, with some

locales reporting drought conditions for several months in a row.

4.2 Offshore NETROPAC Cyclones

Tropical cyclones or hurricanes which passed within a 100/200 nmi

radius of six selected cities along the West Coast of Mexico are

represented in Table 4.1. These cities include: Guaymas (Sonora),

La Paz (B.C), Los Mochis (Sinaloa), Mazatldn (Sinaloa), Manzanillo

(Colima) and Acapulco (Guerrero). These cities were chosen because

they represent major equidistant coastal population centers in western

Mexico. The 200 nmi limit was chosen because it best represents the

upper limit of the horizontal extent between the eye of the cyclone

and the periphery of the cirrus cloud shield (after Serra, 1971).

Hansen (1972) found that NETROPAC cyclones were smaller than those in

the Atlantic; he determined a distance of 100 nmi between the eye of

the cyclone and its outer cloud shield.

A most active year was 1971 in which four of the six coastal

Table 4.1 64

Tropical Cyclones Passing vithin 100/200 nmi of Cities Along the Western Coast of Mexico

Guaymas Los Mochis La Paz Mazatlan Manzanillo Acapulco

100 nmi 200 nmi

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

TOTAL

15-YR 100 nmi AVERAGE 200 nmi

1 2

2 2

1 1

1 1

1 5

1 1

1 1

1 1

1 2

2 2

2 3

1 2

1 1

1 2

1 1

1 2

1 3

1 2

3 3

2 2

2 5

3 6

2 2

2 4

1 2

1 7

1 1

2 4

3 4

^

—

4 9

27 60

1 1

-

5 10

0 .33 0.67

2 2

1

12 22

0.80 1.47

1

-

6 14

0.40 0 .93

1

3 4

1

1

18 37

1.20 2.47

1 2

1 1

10 33

0.67 2 .20

65

cities had at least three tropical cyclones pass within 200 nmi of

their respective coastlines; during that year 18 NETROPAC cyclones

formed. A very quiet cyclone season occurred the following year in

which there were only 8 cyclones documented; of that total only one

storm passed within the 200 nmi extent of one city—Acapulco. Manzan

illo has the greatest cyclone activity off its coastline with an annual

average of almost 2.5 cyclones passing within 200 nmi of its coast;

this is closely followed by Acapulco (2.2) and La Paz (1.47), while

less than one cyclone passed within 200 nmi of Guaymas, Los Mochis

or Mazatlan. The frequency is smaller when a 100 nmi proximity is

specified; Manzanillo averaged 1.20 per year while values decreased

from 0.80 at La Paz to 0.27 at Guaymas. The area at highest risk

(excluding landfalling storms) appears to be centered near Manzanillo

and extends southeastward toward Acapulco and northwestward to Cabo

Corrientes (20.3 N, 105.4 W). North of Cabo Corrientes, the coastline

juts eastward towards Tepic (21.3 N, 104.5 W) and frequencies of

cyclones passing within 200/100 nmi of the coastline decrease towards

Mazatlan. The frequencies at La Paz seem amplified in that they

include NETROPAC cyclones which occur in both the Pacific and the

Gulf of California; however, recurving NETROPAC cyclones in late summer

and early autumn certainly pose a threat to the southern half of Baja

California.

Only a few NETROPAC cyclones have made landfall across the con

tinental United States during this century, however. Southern (1979)

suggests that a city such as San Diego could be hit by a major tropical

66

cyclone within the next 25-50 years. Although "super-tropical

cyclones" (central pressure < 920 mb, wind speeds > 135 kt) may occur

only once every 100 to 1000 years, southern California should be pre

pared in the event such an unlikely storm occurs.

4.3 Landfalling Storms

A total of 214 NETROPAC cyclones were documented for the period

1966-1980; of that quantity, 30 (14%) made landfall upon the west coast

of Mexico, with four of those cyclones making landfall at more than one

coastal location. This section examines the areas of landfalling storms

and the damage which ensued.

Table 4.2 lists all NETROPAC cyclones which made landfall upon

the west coast of Mexico. Some of the coordinates and times at which

landfall occurred were based on an interpolation of positions given in

the annual summaries published in Mariners Weather Log and Monthly

Weather Review. The estimated wind speed at the time of landfall as

well as maximum winds for the storm are specified. The highest estim

ated wind occurred with Hurricane Madeline in which speeds were in

excess of 125 kt. (In 1982, there were reports that Hurricane Paul's

winds were gusting over 200 ktl) The angle which a landfalling storm

made with the coast is denoted by 0. A value of 90 is assigned to a

landfalling storm moving perpendicular to the coast, while 0=0 indi

cates a storm moving parallel and to the left of the coast. The limited

data in this research indicates that NETROPAC cyclones approach the

coast with an average 0=61. The cyclostrophic assumption was used to

determine Ap (departure of central pressure from the surroundings).

67

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68

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69

The highest Ap was found for Hurricane Madeline with 42 mb.

The distribution of the number of landfalling storms (tropical

depressions excluded) with 2.5 by 2.5 latitude-longitude blocks is

shown in Figure 4.1. The block bounded by 17.5-20.0°N and 102.5-

105.0 W exhibits a maximum number of eight tropical cyclones which

moved inland. This area includes Manzanillo, which averages 2.5 trop

ical cyclones passing within 200 nmi of its coastline annually. The

number of landfalling storms decreases towards the Gulf of California,

with 30% striking Baja California. This is primarily due to recurving

NETROPAC cyclones during August-October.

A scatter diagram of landfalling NETROPAC cyclones is shown in

Figure 4.2. (This represents a 15-year period of data; when a longer

record becomes available, the coastline should demonstrate a greater

range in the distribution of landfalling cyclones.) The primary areas

of landfall can be divided into three major groups: i) from Manzan

illo to just southeast of Acapulco, ii) the eastern coastline of the

Gulf of California extending from Mazatlan northwestward to Guaymas,

and ill) the west coast of Baja California. The first group (Manzan-

illo-Acapulco) experienced 15 landfalling tropical cyclones with 67%

of them occurring in the months of May and June. This can be attri

buted to a more southerly position of the ITCZ in late spring which in

turn suppresses most tropical activity further south than in the summer

and early fall. Recurving storms occurred farther north due to the

position of the subtropical anticyclone—thus accounting for only one

storm striking land in September. Only seven tropical cyclones made

landfall in the second group (Mazatldn-Guajrmas), with six of them

70

(no a

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71

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72

occurring in September-October. Five of those storms had recurved in

the NETROPAC. Twelve NETROPAC cyclones made landfall on Baja Califor

nia (primarily on west coast), with recurving storms accounting for

58% of this total. All of these cyclones occurred during August-

September with 50% striking land in the latter month. Unlike Group i,

which experiences most of its coastal strikes in the early part of the

hurricane season, no landfalling storms occurred during the months of

May-June. This could be due to lower SST and the position of the ITCZ

which is still displaced well to the south at this time of year.

4.4 Storm Surge

One of the most devastating effects of a tropical cyclone can be

the storm surge: a superelevation of sea level due to a combination

of wind-driven water and an uplift influenced by the pressure drop.

Simpson (1981) describes several factors which could increase storm

surge as a landfalling hurricane approaches the open coast. Peak surge

increases with:

1. lower central pressure;

2. increase of the radius of maximum wind to

(but not beyond) 50 km;

3. decreasing slope of the bottom surface

from the beach seaward for a distance equal

to the diameter of the ring of maximum winds;

4. with some increases in the speed of

approach to the coast.

73

Research by Jelesnianski (1972) has shown that surge can be just

as damaging for hurricanes moving alongshore. He found that peak

surges were generally greater for down-the-coast movement (coastline

to the right of storm track) compared to up-the-coast movement. This

can be attributed to the fact that usually the strongest winds are

found in the right-front quadrant of the storm. Since most NETROPAC

cyclones generally move toward the northwest, storm surge should be

more severe for the west coast of Mexico than for the east coast of

North America. He also found that maximum surges for down-the-coast

movement occur with relatively fast moving cyclones (>15 kt) with

tracks close to the shore (within a distance of the RMW). Along the

western coastline only one example of up-the-coast (southeastward)

movement occurred. For the eastern coast of Baja California, storms

moving northwestward in the Gulf of California, are also moving up-

coast; there were four such storms in the 15-year period.

The topography off the coast of West Mexico is relatively deep;

for the most part, depths are well over 0.2 km just 25 nmi offshore.

This is in sharp contrast to the Atlantic side of the U.S. where the

0.2 km depth occurs at least 75 nmi off the coast. Studies by Jeles

nianski have shown that storm surge will be greater where the shoal

depth increases gradually off the coast; as a result the relatively

deep waters off west Mexico give this area some relief against poten

tial surge inundation. There are a few bays and inlets, however, where

the water is shallow for a distance, and which could cause some higher

74

surges. Just south of Los Mochis, the Bay of Topolobampo extends a few

miles inland; this area is prone to flooding as demonstrated by Hurri

cane Paul (1982) whose high winds and flooding caused extensive damage.

Off the west coast of Baja California, the water is relatively shallow

from Magdalena Bay to Ballenas Bay (24.4-26.6°N), however these areas

have few inhabitants.

4.5 Damage and Casualties

One of the major limitations in the study of meteorological

phenomena across western Mexico is the lack of reporting stations in

sparsely populated areas along the coast (as well as inland). As a

result, the intensities of some landfalling NETROPAC cyclones may be

misrepresented. Satellite imagery is the primary tool for intensity

estimates; however, this cannot replace first-hand synoptic observa

tions. Reports regarding storm damage in publications such as Monthly

Weather Review and Mariners Weather Log, for the most part, are based

on newspaper reports of storm damage. Damage and fatalities from

cyclones affecting remote towns and villages may go unreported. Table

4.3, which summarizes the damage and fatalities in Mexico in associa

tion with NETROPAC cyclones, most certainly underestimates the de

struction caused by some of these storms.

The worst NETROPAC cyclone on record (in terms of fatalities)

during the period 1966-1980 was Hurricane Liza (1976) which killed

several hundred people in La Paz. The storm moved onshore about 35 nmi

northwest of Los Mochis with winds of 100 kt. Flooding, as a direct

result of the storm, caused a dam to break in La Paz—killing 435

•H

75

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76

people. Hurricane Bridget (1971) was the worst tropical cyclone to

hit Acapulco in more than 25 years. Although there were no report ad

casualties, 20 boats were sunk and wind and water damage exeeded $40

million (U.S.). In 1975, 20 people lost their lives after three

shrimp boats sank off the coast of Mazatlan as a result of Hurricane

Olivia. More than 50,000 people were evacuated from low-lying areas

and 30,000 were left homeless as 7,000 dwellings were destroyed.

Damage was estimated at $20 million.

There were no reports of damage or casualties from NETROPAC

cyclones from 1977-1980. More recently, 74 people were killed by two

tropical cyclones in 1981. Tropical Storm Lidia brought heavy rains

and flooding between Los Mochis and Culiacan with damage to houses,

crops and highways surpassing $60 million. Hurricane Norma (whose

remnants produced severe flooding in Central Texas) moved onshore

near Mazatlan resulting in $24 million in damage as result of crop and

cattle losses (Gunther, 1982). Eight people were killed with damage

to agriculture and property was reported at $70 million due to Hurri

cane Paul, whose eye passed over Cabo San Lucas (B.C.) and made land

fall just south of Los Mochis on 30 September, 1982 (Gunther, 1983).

Although NETROPAC cyclones primarily strike the west coast of

Mexico, the southwestern United States has been affected by the rem

nants of these storms in recent years. In early September 1970,

moisture associated with Tropical Storm Norma brought heavy rain and

flooding to Arizona—killing 22 people and causing $1 million in

damage. Kathleen (1976) was the first tropical storm to hit Califor

nia since 1939. Five people lost their lives and damage due to flood-

77

ing was estimated at $160 million. Most recently, the remnants of

Hurricane Olivia (1982) brought heavy rains and flooding to much of

California with more than $325 million in agricultural losses alone!

4.6 Design Basis Storm

One of the objectives of this research was to determine the

meteorological parameters for a standard design storm (SDS). This

SDS would be modeled after a hurricane which would hit the coast of

Mexico once every 100 years. The data base utilized in this study,

however, was definitely lacking in pressure and wind data. Further

more, the record for satellite-derived maximum wind speeds spanned

only eight years (1973-1980). As a result, the specification of a

SDS was not justified. The following section will investigate pre

vious research in this area of study and discuss how these results

could be applied to the NETROPAC.

In the mid-1970's, the state of Texas adopted the voluntary im

plementation of minimum building standards based on a Texas design

hurricane which would have a return period of 100 years. The clima-

tological data collected for this study (Ho et^ al, 1975) included

minimum sea level pressure (MSLP) , radius of maximum wind (RMW) , storm

speed and direction. The data base consisted of all hurricanes which

crossed or passed within 100 nmi of the Gulf and eastern coastline of

the U.S. since 1873. An upper limit on the MSLP of 982 mb was placed

on all hurricanes; this was based on the Hyrdometeorological Branch

Model (Myers, 1954) for which the maximum cyclostrophic wind speed is

assumed to be 64 kt, the minimum wind speed required for hurricane

78

classification. The adopted criteria for the Texas design storm in

cluded a MSLP of 903 mb at Port Isabel (24.1°N, 97.2°W) increasing to

937 mb at Port Orange (30.1 N, 93.4°W). The design storm was assumed

to be moving normal to the coastline with a speed of 14 kt with a

RMW of 14 nmi. Storm surges were calculated at 100 nmi intervals

along the coastline with an average tidal surge of 13.5 ft.

The Northwest Tropical Atlantic has a data base whose record

exceeds 100 years (1873-present), therefore the characteristics of

the 100-year Texas design storm seem justified. Such is not the case,

however, for the NETROPAC. Although storm tracks of NETROPAC cyclones

were documented beginning in 1946, it was not until 1954 that a dis

tinction was made between intensities of hurricane versus tropical

storms. Prior to 1965, tropical depressions were not included in the

annual summaries prepared by the Eastern Pacific Hurricane Center

(EPHC) in Redwood City, Ca. (Leftwich and Brown, 1981). Furthermore,

it was not until 1973 that the technique for estimating cyclone inten

sities and maximum wind speeds from satellite imagery (Dvorak, 1973)

was implemented by the EPHC.

A survey of measured minimum central pressures in the NETROPAC

during 1957-1980 has shown that "a good long term data base does not

exist for this region" (Blumel, 1983). This research has demonstrated

that ship reports constituted the primary source of pressure data

prior to 1970. Since then, aerial reconnaissance had become the major

source of MSLP information—with the exception of estimates from sat

ellite imagery. The sharp decrease in ship-reported pressure data was

primarily due to advances in satellite imagery, which helped naviga-

79

tors avoid potentially severe weather and rough seas. In the mid

1970's, however, reconnaissance missions declined due to federal bud

get cuts. It appears that for the foreseeable future, satellite imag

ery will be the primary tool for estimating MSLP and maximum winds of

NETROPAC cyclones.

The circulation features of NETROPAC cyclones based on data ac

quired from reconnaissance missions were examined by Upton (1973). A

total of 225 missions occurred during the seven-year period of study,

1966-1972. Although 84% of the reconnaissance data were not used in

this study (the aircraft was required to be within 100 nmi of the

cyclone center), Upton determined radially averaged profiles of D-

values (departure of height from a standard atmosphere), isotherms

and isotachs at various levels. In greater agreement with Hansen

(1972), he determined that NETROPAC cyclones were smaller in horizon

tal extent than those cyclones in the Northwest Atlantic, but rela

tively intense for their size. This research indicated that NETROPAC

hurricanes had an average maximum wind speed of 84 kt and a RMW of

23.5 nmi, while tropical storms averaged 41 kt and 27.4 nmi, respec

tively.

The lowest reported MSLP in the NETROPAC of 915 mb was recorded

by reconnaissance in August 1973 as Hurricane Ava was 300 nmi south

west of Acapulco and heading on a westward course. Maximum sustained

wind speeds of 137 kt were documented. Several landfalling NETROPAC

cyclones have had wind speeds in excess of 100 kt; however, they

usually decreased dramatically soon after landfall.

Figures 4.3-4.4 further demonstrate the lack of data in the

80

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82

NETROPAC for determining the characteristics of a SDS. Figure 4.3

depicts the recorded observations (1973-1980) of maximum estimated

winds exceeding 70 kt. Similarly, Figure 4.4 shows the recorded ob

servations in which maximum estimated winds exceeded 90 kt. The data

for these figures were extracted from the magnetic tape obtained from

the National Climatic Center in Asheville, N.C. Since 64 kt is the

minimum sustained wind speed for hurricane classification, the data

could suggest that very few hurricanes affect the coastline of western

Mexico. Such is not the case, however, as 43% of the landfalling

tropical cyclones in this study were of hurricane intensity. This

discrepancy can be attributed to the documentation of storm tracks at

12-hr intervals contained on the data tape. Usually, if a storm had

dissipated within 12-hr after landfall, its final coordinates as a

tropical storm or hurricane were not recorded. (This was due to poor

surface observations and a reliance on satellite-estimated cyclone

locations.) As a result, reports published in Monthly Weather Review

and Mariners Weather Log were utilized to determine coordinates and

intensities of those storms which made landfall. Although a majority

of NETROPAC hurricanes do not strike land, there is a need to collect

a data set over a period of many years to give a more accurate view

of some extreme wind speed experienced along the coastline.

CHAPTER V

CONCLUSIONS

The NETROPAC is an area rich in tropical cyclone formation during

the months of May through November. Most of the cyclone activity is

confined between 10.0 and 25.0 N with a general track direction toward

300 degrees and a speed of 12 kt. During the period of this study,

1966-1980, an average of 14.3 tropical storms and 7.2 hurricanes

occurred annually. August displays the highest occurrence with an

average of almost 4 cyclones, while an average of less than one cyclone

occur during May or November. Although the majority of NETROPAC

cyclones dissipate at sea, landfalling storms will always pose a

threat to the west coast of Mexico. Recurving cyclones, which occur

frequently in the last summer and early autumn, account for 47% of

all landfalling storms. It is hoped that the United States and Mexico

will recognize the damage potential of these storms before hundreds

more lives are lost as a result of extreme winds and flooding. An

active reconnaissance program in the NETROPAC as well as a sound sur

face and upper air network in western Mexico would be a valuable tool

to both the forecaster and researchers of NETROPAC cyclones.

During the past 25 years, NETROPAC cyclones have claimed more

than 2000 lives, tens of thousands have been left homeless, and ex

treme winds and flooding have wreaked havoc from Acapulco to Baja Cal

ifornia. An improved public warning program in western Mexico could

aid in the evacuation of areas of impending danger; however, struc-

83

84

tural damage will still be a major concern. Thus it seems appropriate

to determine the characteristics of a design basis storm, one for

which minimum building codes could be established for coastal segments

along west Mexico. Such a storm could be based on a tropical cyclone

which would affect a particular coastal segment once every 50 or 100

years. Although such an undertaking was not feasible during this

research, improvements in satellite technology in the next decade

should provide a better source of qualitative and quantiative data

necessary to determine the characteristics of a design basis storm.

Since pressure data is not routinely available in the NETROPAC, satel

lite imagery could be used to estimate maximum wind speeds and the

NSLP. Atkinson and Holliday (1977) studied 28 years of Pacific

typhoon data and determined the following relationship between maximxim

winds and MSLP

Vm-6.7(1010-Pc) exp (0.644) (1)

where Vm represents the maximum sustained winds (kt) and Pc is the

MSLP (mb). Since Vm can be estimated from satellite imagery, Pc could

be determined from (1). As a longer record of satellite estimated

wind speeds becomes available during the next decade, such a relation

ship could be employed to determine some of the characteristics of a

design basis storm for the west coast of Mexico. Hourly data also

needs to be sought from coastal and inland stations to attempt to

follow the filling of landfalling storms.

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APPENDIX

91

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