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Scott A. Yuknis High impact weather forecasts, climate

assessment and prediction. 14 Boatwright’s Loop Plymouth, MA 02360

Phone/Fax 508.927.4610 Cell: 508.813.3499

ClimateImpact@comcast.net

Climate Impact Company 2017 Tropical Cyclone Season Outlook

Issued: Wednesday, August 2, 2017

Highlight: Forecasting 6 hurricanes, several pointed toward the Gulf

Executive Summary: The most active part of the tropical cyclone season arrives

Aug. 1st and lasts through October. So far 5 tropical storms have developed in the

North Atlantic basin. Climate Impact Company expects 10 storms, 6 hurricanes

and 3 intense hurricanes over the next several months. The character of hurricanes

will be long distance low latitude travelers although 2-3 hurricanes are forecast to

affect the East Coast including a direct hit in North Carolina. Accumulated tropical

cyclone activity index is 119 implying the busiest year since 2012 is ahead.

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Storms Hurricanes Major

Hurricane

ACE

CIC 15 (12) 6 (7) 3 (3) 119 (111)

NOAA 11-17 5-9 2-4 N/A

TSR 17 7 3 116

CSU 15 8 3 135

50-year

history

11.7 6.2 2.4 92.9

30-year

history

13.2 6.7 2.8 108.4

15-year

history

15.1 7.0 3.1 112.9

Table 1: The 2017 North Atlantic tropical cyclone forecast by Climate Impact

Company (previous forecast) and comparison with NOAA, Tropical Storm Risk

U.K. and Colorado State University. The 50-year, 30-year and 15-year

climatology are included.

Climate discussion: After a hot first two months of summer the U.S. thermal

pattern flips cool in the Central U.S. in August and some of that cooler weather

extends into the West-Central and East States. The Climate Impact Company

selected climate forecast analog years (2014, 2006 and 2004) identified the cooler

pattern change defeating soil moisture-driven forecasts and ENSO/SSTA outlooks.

The analog upper air pattern for July is quite similar to July 2017 featuring a

trough in the northeast Pacific and eastern North America while a ridge pattern

stretches across the western half of the U.S. and western North Atlantic (Fig. 1-2).

Therefore we continue with the analog forecast to drive the early August update of

the Climate Impact Company 2017 tropical cyclone season forecast for the North

Atlantic basin.

The analog forecast is based on three primary SSTA regimes: ENSO, PDO and

AMO. Now that the core of the tropical cyclone season has arrived

(AUG/SEP/OCT) the focus is on ENSO and AMO. The ENSO analog forecast is

confident that neutral phase continues through the end of the warm season with

marginal El Nino possible later this year (Fig. 3). ENSO will be classified neutral

for the 2017 tropical cyclone season. The AMO forecast confidently indicates the

warm phase is likely for the 2017 tropical cyclone season (Fig. 4). Neutral ENSO

favors near to above normal tropical cyclone activity and accumulated cyclone

energy (Fig. 5-6) while +AMO favors above normal intensity of tropical cyclones.

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Fig. 1-2: Comparing the July 500 MB anomaly pattern analog years (2014, 2006 and 2004) with

July 2017. There are similarities with the northeast Pacific and eastern North America trough

plus the western U.S. and west-central North Atlantic ridge.

Fig. 3: The ENSO analogs indicate neutral ENSO to borderline El Nino for the tropical cyclone

season. Late year weak El Nino is in question while neutral ENSO for most of tropical cyclone

season is likely.

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Fig. 4: The AMO analogs indicate recovery into the warm phase for tropical cyclone season and

the (analog) agreement is excellent.

Globally, the ocean surface remains near record warmth despite lack of an El Nino.

Prominent to the North America climate pattern is the above normal jet stream

strength in the middle latitudes paralleled in the ocean surface thermal structure by

the rapid transition of anomalous cool to warm SSTA over the northeast/east

Pacific and north-central/central North Atlantic (Fig. 7). Meanwhile subtropical

ridging affecting the western U.S. and western North Atlantic basin has been

fueled by warm SSTA.

Fig. 5-6: Seasonal tropical cyclone activity and accumulated cyclone energy

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A closer look at the North Atlantic basin (Fig. 8) indicates a warmer than normal

regime trending warmer the past 30 days. Most of the anomalous warmth (and

warmer trend) is across the western North Atlantic which promotes strengthening

of tropical systems reaching this zone. The outer North Atlantic is near normal

although cooler through mid-summer. The cool zone near the Cape Verde Islands

indicates trade winds are increasing and up-welling subsurface cooler waters. This

effect is likely spread westward across much of the central North Atlantic tropics

and subtropics.

Fig. 7: Global sea surface temperature anomalies identify primary oceanic features well-

correlated to the upper atmosphere flow. The mean trough positions in the northeast Pacific and

northeast Canada to the north-central North Atlantic are related to the cool SSTA in these zones

while 500 MB ridging in the upper atmosphere well-correlated to the warm SSTA of the warm

Pacific and warm western North Atlantic.

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Fig. 8: The North Atlantic basin is warmer than normal and trending warmer the past 30 days.

The warming is mainly in the western North Atlantic in front of where most tropical cyclones

track. The subtropical and tropical North Atlantic has trended cooler during mid-summer.

Finally, upper ocean heat is critical to tropical cyclone development. Near and

north of the equator in the Atlantic Ocean the top 100 meters of the ocean across

the Atlantic basin is warmer than normal while between 100 and 200 meters cooler

than normal conditions exist (Fig. 9). The subtropics are quite warm at all levels of

the upper ocean. The general warm conditions near the surface should support

stronger than normal tropical cyclones ahead.

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Fig. 9: Upper ocean heat just north of the equatorial region in the Atlantic Ocean is warmer

than normal in a shallow layer to the subtropics while at depth of 100 meters in the deep tropics

the ocean is cooler than normal.

Forecast methodology: The updated seasonal forecast for 2017 uses a simple

weighting of the Climate Impact Company selected analog years for Aug. 1 to

Nov. 30. Previous 2017 activity is added to the analog forecast. The outlook is

expressed in number of events, accumulated tropical cyclone activity and projected

regions for hurricane development and based on the analogs the most likely path of

hurricanes.

The seasonal outlook: The Climate Impact Company 2017 tropical cyclone

season outlook indicates 15 tropical storms, 6 hurricanes and 3 intense hurricanes

(Table 2). The outlook is revised slightly higher due to early season storms

although lowers in hurricane activity. Interestingly, despite only 6 hurricanes a

robust ACE index of 119 is forecast. The seasonal activity is very close to the 15-

year normal and slightly more active than the conventional 30-year climatology

(Table 3).

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Analog year Tropical

cyclone

Hurricane Severe

Hurricane

ACE

2014 7 5 2 ~55

2006 7 4 2 ~65

2004 15 9 6 225

Average 9.7 (10) 6.0 3.3 (3) 115.0

2017 so far 5 0 0 4

Seasonal

forecast

15 6 3 119

Table 2: The Climate Impact Company seasonal activity forecast for the 2017 season is based on

3 analog years and previous 2017 activity.

Tropical

Cyclone

Hurricane Severe

Hurricane

ACE

Forecast 15 6 3 119

Previous

forecast

12 7 3 111

50-year 11.7 6.2 2.4 92.9

30-year 13.2 6.7 2.8 108.4

15-year 15.1 7.0 3.1 112.9

El Nino 10.3 5.4 2.2 77.4

Last year 15 7 3 45

Table 3: The 2017 seasonal forecast is adjusted more active due to early season storms but

expects one less hurricane. The 2017 forecast is very close to the 15-year normal.

Discussion: There are several prominent diagnostics that should play a major role

in the tropical cyclone season activity now that the most active part of the season is

arriving.

1. Tendency for an upper trough in the Central/East-Central U.S.: Since 2005

when a record 28 tropical storms formed in the North Atlantic a tendency for

a weak upper trough over the East-Central U.S. has been present during the

core of tropical cyclone season. Upper westerly shear is produced by the

upper low across the northern Gulf of Mexico and off the Mid-Atlantic coast

acting as a buffer to protect the U.S. mainland from hurricane strikes. This

pattern is likely to return for AUG/SEP/OCT based on the analogs (Fig. 10).

Therefore the risk of a hurricane striking the northern Gulf of Mexico is not

eliminated but lowered. The upper trough should also turn systems

approaching the U.S. East Coast north and northeast away from the coast.

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This pattern is not present all of the time therefore 1 or 2 hurricanes could

sneak into the U.S. Coast during the upcoming peak of season.

2. The analogs also indicate presence of an upper trough near the Azores: The

trough is associated with the cool pool in the north-central North Atlantic.

Implied is above average westerly flow aloft in the central and eastern

subtropical North Atlantic. This pattern likely forces tropical cyclones

forming in the outer tropical North Atlantic to move due west or quickly

shear apart if turning north.

3. The blocking high pressure area: On average a blocking high pressure area is

forecast across eastern Canada and Greenland to the north and northwest of

the warm SSTA pattern in the northwest North Atlantic basin. The concern

is if a tropical cyclone moves far enough north close to the U.S. coast the

steering current around the high pressure area could force a system toward

the Northeast Corridor coastline.

Fig. 10: The upper air pattern projected by the analog years for AUG/SEP/OCT 2017.

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4. Warm SSTA pattern across the North Atlantic: The majority of the North

Atlantic where tropical cyclones form and migrate is warmer than normal.

Additionally, upper ocean heat is above normal. These conditions favor

formation of stronger tropical cyclones if the proper low shear aloft

environment evolves.

5. Madden Julian oscillation: When tropical waves are plentiful and looking

robust the upper level environment is likely absent of upper shear. These

conditions are most likely when the MJO stretches from the tropical North

Atlantic to far western tropical Indian Ocean. So far this season this MJO

signature has not developed. However, based on climatology we should see

2-3 periods of favorable MJO over the next 90 days to produce hurricanes.

The hurricane tracks outlook: The outlook is based on the projected upper air

pattern and averaged seasonal activity of the analog years. Forecast confidence is

highest on southern latitude development areas and hurricane tracks which are

propelled westward by a lower than normal latitude subtropical high pressure area

(Fig. 11). Northeast Mexico to southern Texas is considered a prime target for

these low latitude long-distance travelling hurricanes. Presence of westerly shear

across the southern U.S. as produced by the semi-persistent upper low over the

East-Central U.S. should, on average protect the northern Gulf Coast. However,

the westerly shear pattern is not always present and given the warmer-than-normal

upper ocean across the tropical North Atlantic 1 or 2 hurricanes are likely to slip

into the U.S. coastline including Florida (from the Gulf of Mexico) and into North

Carolina. Although not indicated there is concern that a system moving due north

and just off the East Coast could back westward ad into the mainland guided by

easterly steering south of the eastern Canada/Greenland upper ridge pattern.

Overall 6 hurricanes are forecast and all 6 move into the western half of the North

Atlantic basin with most likely staying south.

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Fig. 11: The Climate Impact Company 2017 seasonal forecast with hurricane generation areas

and prevailing tracks indicated.

Regional forecast discussion: In 2017 so far, a total of 5 tropical storms have

occurred. Climate Impact Company expects an additional 10 tropical storms, 6

hurricanes and 3 intense hurricanes to occur through the next several months.

Based on the analog forecast coupled with specifics of the 2017 atmosphere and

ocean here are the regional risks.

1. Texas: A semi-permanent upper trough over the Missouri Valley implies

hurricane risk to the Texas Coast is below normal. However, given the

likelihood of the subtropical ridge extending westward across the Gulf a low

latitude track of hurricanes across the Yucatan and toward northeast Mexico

should be a popular track this year. Given this scenario Texas will be at risk

on 2 or 3 occasions for a hurricane strike although best estimate is these

systems stay just south of Texas or turn northest toward Florida.

2. North coast of Gulf to western Florida: The Missouri Valley semi-persistent

upper trough indicates increased risk of tropical cyclones moving toward the

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Gulf of Mexico then turning northeastward towrd the Florida west coast.

Forecast confidence is reasonable hurricane will strike the west coast of

Florida this season. Westerly sher in the southern U.S. is above normal

which may protect the northern Gulf states from a direct hit from a

hurricane.

3. Florida to Carolina coast: The concern is that the persistent Missouri Valley

upper trough consolidates or shifts slightly west causing a southerly

steeringflow across the Southeast U.S. Coast to Florida. If so, hurricanes

turn north near the coast and could clip the Carolinas. The outlook indicates

at least 1 hurricane into North Carolina. There’s enough westerly component

of the high level wind most of the time to lower the risk of a hurricane

moving inland the east coast of Florida.

4. Mid-Atlantic to Northeast coast: The outlook indicates north-moving

hurricanes will stay offshore. However, given the blocking ridge pattern

forecast across eastern Canada and Greenland there is risk of a system

traveling far enough north to turn toward the Northeast Coastline.

5. Caribbean: An active year is forecast with several hurricanes likely to

develop or pass through the Caribbean Sea. Given proximity to Cuba,

Hispaniola and Puero Rico all of these island nations are at above normal

risk of hurricane strikes this season.

APPENDIX A

2017 storm names

Arlene Bret Cindy Don Emily

Franklin Gert Harvey Imma Jose

Katia Lee Maria Nate Ophelia

Philippe Rina Sean Tammy Vince

Whitney