Probabilistic Guidance for Hurricane Storm Surge (P-surge) Arthur Taylor and Bob Glahn Meteorological Development Laboratory, National Weather Service

Probabilistic Guidance for Hurricane Storm Probabilistic Guidance for Hurricane Storm Surge (P-surge)Surge (P-surge)

Arthur Taylor and Bob GlahnArthur Taylor and Bob Glahn

Meteorological Development Laboratory, National Weather ServiceMeteorological Development Laboratory, National Weather Service

January 22, 2008January 22, 2008

Probabilistic Storm Surge 2008Probabilistic Storm Surge 2008

Hurricane Storm Surge DamageHurricane Storm Surge Damage

• Galveston 1900 – 6,000 to 12,000 deathsGalveston 1900 – 6,000 to 12,000 deaths• Okeechobee 1928 – more than 2,500 deathsOkeechobee 1928 – more than 2,500 deaths• Florida Keys, Labor Day 1935 – 408 deathsFlorida Keys, Labor Day 1935 – 408 deaths• New England 1938 – 600 deathsNew England 1938 – 600 deaths• Audrey 1957 – 390 deathsAudrey 1957 – 390 deaths• Camille 1969 – 256 deathsCamille 1969 – 256 deaths• Hugo 1989 – 50 deathsHugo 1989 – 50 deaths• Opal 1995 – 9 deathsOpal 1995 – 9 deaths• Katrina 2005 – more than 1,800 deathsKatrina 2005 – more than 1,800 deaths Aerial Photo overlay of

Katrina 2005 storm surge over Hancock County, Mississippi

““The greatest potential for loss of life The greatest potential for loss of life related to a hurricane is from the storm related to a hurricane is from the storm surge.”surge.”

Richelieu Apartments - Before Camille 1969

Richelieu Apartments - After Camille 1969


Storm Surge ForecastingStorm Surge Forecasting

The Sea, Lake, and Overland Surges from Hurricanes The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model is the National Weather Service’s (NWS) (SLOSH) model is the National Weather Service’s (NWS) operational hurricane storm surge model.operational hurricane storm surge model.• The NWS uses composites of its results to predict potential storm surge The NWS uses composites of its results to predict potential storm surge

flooding for evacuation planningflooding for evacuation planning

• The National Hurricane Center (NHC) begins operational SLOSH runs The National Hurricane Center (NHC) begins operational SLOSH runs 24 hours before forecast hurricane landfall24 hours before forecast hurricane landfall

The operational runs are based on a single NHC forecast The operational runs are based on a single NHC forecast track and its associated parameters.track and its associated parameters.• When provided accurate input, SLOSH results are within 20% of high When provided accurate input, SLOSH results are within 20% of high

water marks.water marks.

• Track and intensity prediction errors cause large errors in SLOSH Track and intensity prediction errors cause large errors in SLOSH forecasts and can overwhelm the SLOSH results.forecasts and can overwhelm the SLOSH results.


Hurricane Ivan: A Case StudyHurricane Ivan: A Case Study


Probabilistic Storm Surge Probabilistic Storm Surge MethodologyMethodology

Use an ensemble of SLOSH runs to create Use an ensemble of SLOSH runs to create probabilistic storm surge (P-surge) probabilistic storm surge (P-surge) • Intended to be used operationally so it is based on NHC’s Intended to be used operationally so it is based on NHC’s

official advisory.official advisory.• P-surge’s ensemble perturbations are determined by P-surge’s ensemble perturbations are determined by

statistics of past performance of the advisoriesstatistics of past performance of the advisories

Hurricane forecast errors which impact storm surge:Hurricane forecast errors which impact storm surge:• Cross track errors (impacts Location)Cross track errors (impacts Location)• Along track errors (impacts Forward Speed)Along track errors (impacts Forward Speed)• Intensity errors (impacts Pressure)Intensity errors (impacts Pressure)• Size of the storm errors.Size of the storm errors.


Katrina Advisory 23Katrina Advisory 23


Varying Katrina’s TracksVarying Katrina’s Tracks

• Include 90% of Include 90% of possible cross track possible cross track error (roughly 3 error (roughly 3 times the size of times the size of the cone of error).the cone of error).

• Spacing based on Spacing based on size of the stormsize of the storm


Varying the Other ParametersVarying the Other Parameters

Size: Small (30%), Medium (40%), Large (30%)Size: Small (30%), Medium (40%), Large (30%)

Forward Speed: Fast (30%), Medium (40%), Slow (30%)Forward Speed: Fast (30%), Medium (40%), Slow (30%)

Intensity: Strong (30%), Medium (40%), Weak (30%)Intensity: Strong (30%), Medium (40%), Weak (30%)

The weight of a run is: cross track weight * along track weight * intensity The weight of a run is: cross track weight * along track weight * intensity weight * size weightweight * size weight


Is P-surge Statistically Reliable?Is P-surge Statistically Reliable?

If we forecast a 20% chance of storm surge exceeding If we forecast a 20% chance of storm surge exceeding 5 feet numerous times, then on 20% of those times 5 feet numerous times, then on 20% of those times storm surge should exceed 5 feet.storm surge should exceed 5 feet.• Create a reliability diagram comparing the ratio of Create a reliability diagram comparing the ratio of

occurrence with forecast probability.occurrence with forecast probability.

Problem: Insufficient observationsProblem: Insufficient observations• Number of hurricanes making landfall is relatively small. Number of hurricanes making landfall is relatively small.

• Observations are made where there has been surge. Observations are made where there has been surge.

340 observations for storms between 1998-2005340 observations for storms between 1998-2005


SLOSH HindcastSLOSH Hindcast

Used SLOSH hindcast runs for “observations”.Used SLOSH hindcast runs for “observations”.• NHC used best historical information for inputNHC used best historical information for input

• Given accurate input, model results are within 20% of high Given accurate input, model results are within 20% of high water marks.water marks.

Advantage: Advantage: • Uniform “observations” everywhere, even where there is Uniform “observations” everywhere, even where there is

little or no surge.little or no surge.

Disadvantage: Disadvantage: • Same surge model used in analysis as in P-surge.Same surge model used in analysis as in P-surge.

Reliability Diagrams for Forecasts > 5 feet

48hr

2158912hr

2108517877

26042

25268

2881043648

59354

90598

176494

286559

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100

Probability forecast (%) > 5 feet

Rat

io o

f O

ccu

rren

ce

5010

100

24hr

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.915064

14340

39439

4226564931

57568

71179

125725

246406

380314

0

1

0 10 20 30 40 50 60 70 80 90


Rat

io o

f O

ccu

rren

ce

1205

9107

7488

20908

3281875688

111020

212597

448559

5453960

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100


Rat

io o

f O

ccu

rren

ce

1997

36hr

229

4195

11217

27574

45712

69783

90632

182758

311550

476672

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100


Rat

io o

f O

ccu

rren

ce


Probability of Probability of >> X feet of Storm X feet of Storm SurgeSurge

To calculate the probability of exceeding X feet:To calculate the probability of exceeding X feet:• For each cell, add the associated weights of the hypothetical For each cell, add the associated weights of the hypothetical

storms whose maximum surge values are greater than X storms whose maximum surge values are greater than X feet.feet.

Example: Example: • Five hypothetical storms have weights of 0.1, 0.2, 0.4, 0.2, Five hypothetical storms have weights of 0.1, 0.2, 0.4, 0.2,

and 0.1and 0.1

• The first two exceeded X feet in a given cell.The first two exceeded X feet in a given cell.

• The probability of exceeding X feet in that cell is: 30% (0.1 The probability of exceeding X feet in that cell is: 30% (0.1 + 0.2 = 30%)+ 0.2 = 30%)


Probability of Probability of >> 5 feet of Storm 5 feet of Storm Surge for Katrina Adv 23Surge for Katrina Adv 23


Height Exceeded by X percent of the Height Exceeded by X percent of the Ensemble of StormsEnsemble of Storms

To calculate the height exceeded by X percent of the To calculate the height exceeded by X percent of the ensemble runs:ensemble runs:• For each cell, find the surge value where the weights of the For each cell, find the surge value where the weights of the

surge values which are higher add up to a value surge values which are higher add up to a value << X. X.

Example: Example: • Five hypothetical storms have maximum surge values of 6, Five hypothetical storms have maximum surge values of 6,

5, 4, 3, 2 feet and respective weights of 0.2, 0.4, 0.1, 0.1, 0.2. 5, 4, 3, 2 feet and respective weights of 0.2, 0.4, 0.1, 0.1, 0.2.

• The height exceeded by 60% of the ensemble is 4 feet, since The height exceeded by 60% of the ensemble is 4 feet, since the 6 foot value represents the top 20% of the storms, and the 6 foot value represents the top 20% of the storms, and the 5 foot value represents the next 40%.the 5 foot value represents the next 40%.


Height Exceeded by 10% of the Height Exceeded by 10% of the Ensemble for Katrina Adv 23Ensemble for Katrina Adv 23


http://www.weather.gov/mdl/psurgehttp://www.weather.gov/mdl/psurge

When is it When is it available?available?

• Beginning when Beginning when the NHC issues a the NHC issues a hurricane watch hurricane watch or warning for the or warning for the continental UScontinental US

• As close to the As close to the advisory release advisory release time as possibletime as possible


Current DevelopmentCurrent Development

• We were “experimental” in 2007We were “experimental” in 2007

• The model is running in NCEP’s job stream.The model is running in NCEP’s job stream.

• The data are flowing to the National Digital Guidance The data are flowing to the National Digital Guidance Database (NDGD)Database (NDGD)

• The data will soon be available to NWS forecast offices.The data will soon be available to NWS forecast offices.

• A decision will be made soon on becoming “operational” in A decision will be made soon on becoming “operational” in 2008.2008.

• We continue to develop training material.We continue to develop training material.

• We continue to update the error statistics.We continue to update the error statistics.

Documents

Probabilistic Guidance for Hurricane Storm Surge (P-surge) Arthur Taylor and Bob Glahn Meteorological Development Laboratory, National Weather Service