Probabilistic Hurricane Storm Surge (P-Surge) Arthur Taylor MDL / OST December 4, 2006

Probabilistic Hurricane Storm Surge Probabilistic Hurricane Storm Surge (P-Surge)(P-Surge)

Arthur TaylorArthur Taylor

MDL / OSTMDL / OST

December 4, 2006December 4, 2006

Probabilistic Storm Surge 2006Probabilistic Storm Surge 2006

IntroductionIntroduction

NHC begins operational SLOSH runs 24 hours NHC begins operational SLOSH runs 24 hours before landfall.before landfall.• Provides a storm surge estimate for non-evacuation Provides a storm surge estimate for non-evacuation

applications.applications.

Problem: Surges are based on a single NHC forecast Problem: Surges are based on a single NHC forecast track and associated parameters. track and associated parameters. • When provided accurate input, SLOSH results are within When provided accurate input, SLOSH results are within

20% of high water marks.20% of high water marks.

• Track and intensity prediction errors are the largest cause Track and intensity prediction errors are the largest cause of errors in SLOSH surge forecasts and can overwhelm the of errors in SLOSH surge forecasts and can overwhelm the SLOSH results.SLOSH results.


Probabilistic Storm Surge Probabilistic Storm Surge MethodologyMethodology

Create an ensemble of SLOSH runs based on NHC’s Create an ensemble of SLOSH runs based on NHC’s official advisory and historic forecast errors.official advisory and historic forecast errors.• Creates a probability of storm surge for this one forecast of Creates a probability of storm surge for this one forecast of

this particular threatening hurricane. Not to be confused this particular threatening hurricane. Not to be confused with FEMA’s 100-year surge levels.with FEMA’s 100-year surge levels.

Which hurricane forecast errors most impact storm Which hurricane forecast errors most impact storm surge?surge?• Cross track error (impacts landfall location)Cross track error (impacts landfall location)• Along track error (impacts the timing of the storm)Along track error (impacts the timing of the storm)• Intensity errorsIntensity errors• Structure of the storm errorsStructure of the storm errors


SLOSH’s Input TrackSLOSH’s Input Track

LocationLocation• Can get from NHC’s advisoryCan get from NHC’s advisory

Forward SpeedForward Speed• Can compute from NHC’s advisoryCan compute from NHC’s advisory

Radius of Maximum Winds (Rmax)Radius of Maximum Winds (Rmax)• Not given in NHC’s advisory due to lack of skill in Not given in NHC’s advisory due to lack of skill in

forecasting changes in Rmaxforecasting changes in Rmax

PressurePressure• Can only get the current value (no forecast values) from Can only get the current value (no forecast values) from

NHC’s advisoryNHC’s advisory


SLOSH’s Rmax and PressureSLOSH’s Rmax and Pressure

Since NHC’s advisory does not provide Rmax, or Since NHC’s advisory does not provide Rmax, or forecast Pressure, we need to compute them.forecast Pressure, we need to compute them.

• The SLOSH parametric wind model relates Rmax, The SLOSH parametric wind model relates Rmax, Pressure, and Maximum Wind Speed (Vmax). Given any Pressure, and Maximum Wind Speed (Vmax). Given any two, the third can be computed.two, the third can be computed.

• Vmax is provided in NHC’s advisory.Vmax is provided in NHC’s advisory.

• Since the current Pressure is provided, one can estimate the Since the current Pressure is provided, one can estimate the current Rmax.current Rmax.

• We assume that Rmax remains constant, then calculate the We assume that Rmax remains constant, then calculate the resulting Pressures. resulting Pressures.


Example: Katrina Advisory 23Example: Katrina Advisory 23


Varying Katrina’s TracksVarying Katrina’s Tracks

• The NHC’s cone The NHC’s cone of error is 50% of of error is 50% of possible cross track possible cross track error. error. • We include 90% We include 90% of possible cross of possible cross track error track error (roughly 3 times (roughly 3 times the size of the cone the size of the cone of error).of error).

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


Varying the Other Parameters:Varying 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%)


Determine Which Basins to RunDetermine Which Basins to Run

We try all SLOSH We try all SLOSH input tracks in all input tracks in all operational basins:operational basins:• For each basin, For each basin,

eliminate tracks which eliminate tracks which never forecast tropical never forecast tropical storm force winds.storm force winds.

• Remove basins where all Remove basins where all the tracks were the tracks were eliminated.eliminated.

• Treat eliminated tracks Treat eliminated tracks as if they generated no as if they generated no surge in a basin.surge in a basin.


Calculate probability of Calculate probability of exceeding X feetexceeding X feet

Look at each cell in Look at each cell in each SLOSH run. each SLOSH run. • If the surge exceeds X, If the surge exceeds X,

add the weight add the weight associated with that associated with that SLOSH run to the total.SLOSH run to the total.

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

• The total weight is the The total weight is the probability of exceeding probability of exceeding X feet.X feet.


Katrina Adv 23: Probability > 5 Katrina Adv 23: Probability > 5 feet of storm surgefeet of storm surge


Calculate height exceeded by X Calculate height exceeded by X percent of ensemble storms.percent of ensemble storms.

Determine the exceedance surge height, for each cell, Determine the exceedance surge height, for each cell, so that only X percent of the ensemble surges exceed so that only X percent of the ensemble surges exceed it. it. • For each cell, sort the heights of each ensemble SLOSH For each cell, sort the heights of each ensemble SLOSH

run.run.

• Starting from the tallest height, sum the weights until the Starting from the tallest height, sum the weights until the sum is equal to the given percentage, X.sum is equal to the given percentage, X.

• The height associated with the last weight added is the The height associated with the last weight added is the exceedance height for that cell.exceedance height for that cell.


Katrina Adv 23: 10% of the Katrina Adv 23: 10% of the ensemble storms exceed this heightensemble storms exceed this height


Where can you access our product?http://www.weather.gov/mdl/psurge

When is it available?When is it available?

• Beginning with the Beginning with the first NHC advisory first NHC advisory forecasting landfall of forecasting landfall of a hurricane in 24 a hurricane in 24 hours.hours.

• Available approx. 1-2 Available approx. 1-2 hours after the hours after the advisory release time.advisory release time.


Customer FeedbackCustomer Feedback

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What is your affiliation?

From: From: 6-1-20066-1-2006 To: To: 11-15-200611-15-2006

Total Number of Responses: Total Number of Responses: 126126


Customer Feedback ScoresCustomer Feedback Scores

What is the perceived technical quality of the storm surge graphics?

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Customer Feedback CommentsCustomer Feedback Comments

“Excellent product should save lives.”

“Nice product. Easy to read.”

“Storm surge is the one area that local forecasters either rarely mention or speak in such generalities that the information is useless. This tool is greatly appreciated by those on us living in the coastal high hazard area.”

“I need to know what my risk of storm surge is where I live - this graphical representation brings an important level of visual realization to this dangerous situation.”

“Extend the product out to 48 hours prior to landfall instead of 24 hours.”

“You only give two ranges of storm surge…10% and > 5 feet. Add additional thresholds to the graphics.”

“Needs ability to zoom in closer to effected areas.”

“Hard to understand the 10 % exceedance graphic.”


Is it statistically Reliable?Is it statistically Reliable?

If we forecast 20% chance of exceeding 5 feet, does it If we forecast 20% chance of exceeding 5 feet, does it actually exceed 5 feet 20% of the time?actually exceed 5 feet 20% of the time?• Step 1: Create forecasts for various projections and Step 1: Create forecasts for various projections and

thresholds for the following storms: Bonnie98, Bret99, thresholds for the following storms: Bonnie98, Bret99, Charley04, Claudette03, Dennis05, Earl98, Floyd99, Charley04, Claudette03, Dennis05, Earl98, Floyd99, Frances04, Georges98, Gaston04, Isabel03, Ivan04, Frances04, Georges98, Gaston04, Isabel03, Ivan04, Jeanne04, Katrina05, Lili02, Wilma05Jeanne04, Katrina05, Lili02, Wilma05

• Step 2: Get a matching analysis of storm surge.Step 2: Get a matching analysis of storm surge.

• Step 3: In each grid cell when we forecast 15-25% Step 3: In each grid cell when we forecast 15-25% probability of exceeding 5 feet, calculate the observed probability of exceeding 5 feet, calculate the observed relative frequency. Repeat for other probability groups, relative frequency. Repeat for other probability groups, threshold values, and forecast projections.threshold values, and forecast projections.

>2 ft Forecasts

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>5 ft Forecasts

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>7 ft Forecasts

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Current DevelopmentCurrent Development

• We were “experimental” in 2006, and plan on becoming We were “experimental” in 2006, and plan on becoming “operational” in 2007.“operational” in 2007.

• We are investigating other methods of verifying the We are investigating other methods of verifying the forecasts.forecasts.

• We are working on adding the data to the NDGD (National We are working on adding the data to the NDGD (National Digital Guidance Database).Digital Guidance Database).

• We are working on delivering the data to AWIPS.We are working on delivering the data to AWIPS.

• We are developing more training material.We are developing more training material.

• Based on the feedback from 2006, we plan to add more Based on the feedback from 2006, we plan to add more “zoom” capability.“zoom” capability.


Future DevelopmentFuture Development

• We would like to We would like to include probability include probability over a time range, over a time range, both incremental and both incremental and cumulative.cumulative.

• We would like to We would like to allow interaction allow interaction with the data in a with the data in a manner similar to the manner similar to the SLOSH Display SLOSH Display program.program.

• We would like to We would like to investigate its investigate its applicability to applicability to Tropical storms.Tropical storms.

Documents

Probabilistic Hurricane Storm Surge (P-Surge) Arthur Taylor MDL / OST December 4, 2006