noaa’s national weather service: probabilistic storm surge (p-surge) arthur taylor, nicole p....

NOAA’sNOAA’sNational Weather Service:National Weather Service:

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

Arthur Taylor, Nicole P. KurkowskiArthur Taylor, Nicole P. Kurkowski

MDL / OSTMDL / OST

July 26, 2006July 26, 2006

Probabilistic Storm Surge 2006Probabilistic Storm Surge 2006Probabilistic 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 errors.Structure 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 advisory.Can 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 Rmax.forecasting 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 advisory.NHC’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

• 1.645 standard 1.645 standard deviations (sd) to left deviations (sd) to left and right, is equivalent and right, is equivalent to 90% of stormsto 90% of storms

• 0.67 sd to left and 0.67 sd to left and right would be average right would be average errorerror

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

• Calculations are Calculations are done when 34 knot done when 34 knot winds or greater are in winds or greater are in the SLOSH basinthe SLOSH basin


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

To calculate probability of exceeding X feet, we look To calculate probability of exceeding X feet, we look at each cell in each SLOSH run’s envelope. at each cell in each SLOSH run’s envelope. • If that value exceeds X, we add the weight associated with If that value exceeds X, we add the weight associated with

that SLOSH run to the total.that SLOSH run to the total.

• Otherwise we don’t increase the total.Otherwise we don’t increase the total.

• The total weight is considered the probability of exceeding The total weight is considered the 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 storms.percent of storms.

Determine what height to choose so that in any cell Determine what height to choose so that in any cell there is a specified probability of exceeding it.there is a specified probability of exceeding it.

How? How? • For each cell, sort the heights of each SLOSH run.For each cell, sort the heights of each SLOSH run.

• From the tallest height downward, add up the weights From the tallest height downward, add up the weights associated with each SLOSH run until the given probability associated with each SLOSH run until the given probability is exceeded. The height associated with the last weight is exceeded. The height associated with the last weight added is the value for that cell.added is the value for that cell.


Katrina Adv 23: 10% of storms Katrina Adv 23: 10% of storms can exceed this heightcan exceed this height


Is It Reliable?Is It Reliable?(from a statistical perspective)(from a statistical perspective)

If we forecast 20% If we forecast 20% chance of exceeding 5 chance of exceeding 5 feet, does it actually feet, does it actually exceed 5 feet 20% of the exceed 5 feet 20% of the time?time?

Combined: Bonnie98, Combined: Bonnie98, Earl98, Georges98, Earl98, Georges98, Bret99, Floyd99, Lili02, Bret99, Floyd99, Lili02, Claudette, Isabel03, Claudette, Isabel03, Charley04, Frances04, Charley04, Frances04, Gaston04, Ivan04, Gaston04, Ivan04, Jeanne04, Katrina05 Jeanne04, Katrina05

Ratio of occurance for storms forecast to make landfall in 24 hours

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

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0 10 20 30 40 50 60 70 80 90 100

Probability forecast (%) > 5 feet

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


When are our products available?When are our products available?

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

• Available approximately 1 hour after the advisory release Available approximately 1 hour after the advisory release time.time.


ImplicationsImplications

How can you use this? How can you use this?

• Please let us know how you plan to use it.Please let us know how you plan to use it.

Thoughts as to how one might use it:Thoughts as to how one might use it:

• The “probability of storm surge > 5 feet” product can show where it is The “probability of storm surge > 5 feet” product can show where it is likely to flood, and how susceptible an area is to storm surge.likely to flood, and how susceptible an area is to storm surge.

• The “height exceeded by 10% of storms” product can show the extent The “height exceeded by 10% of storms” product can show the extent of the potential flooding, approximating “how bad it might get”.of the potential flooding, approximating “how bad it might get”.

Feedback:Feedback:

• [email protected]@noaa.gov

• http://www.weather.gov/survey/nws-survey.php?codehttp://www.weather.gov/survey/nws-survey.php?code==phssphss


Future DevelopmentFuture Development

• We are continuing to examine whether we need to calibrate We are continuing to examine whether we need to calibrate the probabilities, by looking at reliability diagrams.the probabilities, by looking at reliability diagrams.

• Currently we initialize the sea-surface height to be at 0.7 Currently we initialize the sea-surface height to be at 0.7 feet. The NHC has requested us to initialize it with 2 feet in feet. The NHC has requested us to initialize it with 2 feet in the Gulf of Mexico and 0 feet in the Atlantic Ocean. After the Gulf of Mexico and 0 feet in the Atlantic Ocean. After making the change, we will need to reinvestigate the making the change, we will need to reinvestigate the calibration question.calibration question.

• 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).

noaa’s national weather service: probabilistic storm surge (p-surge) arthur taylor, nicole p....

Documents

storm surge estimate

storm errors

probability of storm

impact storm surge

slosh surge forecasts

current rmax

nhcs official advisory

forecast pressure