Probabilistic Guidance for Hurricane Storm Surge Psurge

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Probabilistic Guidance for Hurricane Storm Surge
(P-surge)

• Arthur Taylor and Bob Glahn
• Meteorological Development Laboratory, National
  Weather Service
• January 22, 2008

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Hurricane Storm Surge Damage
The greatest potential for loss of life related
to a hurricane is from the storm surge.

• Galveston 1900 6,000 to 12,000 deaths
• Okeechobee 1928 more than 2,500 deaths
• Florida Keys, Labor Day 1935 408 deaths
• New England 1938 600 deaths
• Audrey 1957 390 deaths
• Camille 1969 256 deaths
• Hugo 1989 50 deaths
• Opal 1995 9 deaths
• Katrina 2005 more than 1,800 deaths

Aerial Photo overlay of Katrina 2005 storm surge
over Hancock County, Mississippi
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Richelieu Apartments - Before Camille 1969
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Richelieu Apartments - After Camille 1969
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Storm Surge Forecasting

• The Sea, Lake, and Overland Surges from
  Hurricanes (SLOSH) model is the National Weather
  Services (NWS) operational hurricane storm surge
  model.
• The NWS uses composites of its results to predict
  potential storm surge flooding for evacuation
  planning
• The National Hurricane Center (NHC) begins
  operational SLOSH runs 24 hours before forecast
  hurricane landfall
• The operational runs are based on a single NHC
  forecast track and its associated parameters.
• When provided accurate input, SLOSH results are
  within 20 of high water marks.
• Track and intensity prediction errors cause large
  errors in SLOSH forecasts and can overwhelm the
  SLOSH results.

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Hurricane Ivan A Case Study
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Probabilistic Storm Surge Methodology

• Use an ensemble of SLOSH runs to create
  probabilistic storm surge (P-surge)
• Intended to be used operationally so it is based
  on NHCs official advisory.
• P-surges ensemble perturbations are determined
  by statistics of past performance of the
  advisories
• Hurricane forecast errors which impact storm
  surge
• Cross track errors (impacts Location)
• Along track errors (impacts Forward Speed)
• Intensity errors (impacts Pressure)
• Size of the storm errors.

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Katrina Advisory 23
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Varying Katrinas Tracks

• Include 90 of possible cross track error
  (roughly 3 times the size of the cone of error).
• Spacing based on size of the storm

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Varying the Other Parameters

• Size Small (30), Medium (40), Large (30)
• Forward Speed Fast (30), Medium (40), Slow
  (30)
• Intensity Strong (30), Medium (40), Weak (30)
• The weight of a run is cross track weight
  along track weight intensity weight size
  weight

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Is P-surge Statistically Reliable?

• If we forecast a 20 chance of storm surge
  exceeding 5 feet numerous times, then on 20 of
  those times storm surge should exceed 5 feet.
• Create a reliability diagram comparing the ratio
  of occurrence with forecast probability.
• Problem Insufficient observations
• Number of hurricanes making landfall is
  relatively small.
• Observations are made where there has been surge.
  
• 340 observations for storms between 1998-2005

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SLOSH Hindcast

• Used SLOSH hindcast runs for observations.
• NHC used best historical information for input
• Given accurate input, model results are within
  20 of high water marks.
• Advantage
• Uniform observations everywhere, even where
  there is little or no surge.
• Disadvantage
• Same surge model used in analysis as in P-surge.

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Reliability Diagrams for Forecasts gt 5 feet
21589
5010
48hr
36hr
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Probability of gt X feet of Storm Surge

• To calculate the probability of exceeding X feet
• For each cell, add the associated weights of the
  hypothetical storms whose maximum surge values
  are greater than X feet.
• Example
• Five hypothetical storms have weights of 0.1,
  0.2, 0.4, 0.2, and 0.1
• The first two exceeded X feet in a given cell.
• The probability of exceeding X feet in that cell
  is 30 (0.1 0.2 30)

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Probability of gt 5 feet of Storm Surge for
Katrina Adv 23
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Height Exceeded by X percent of the Ensemble of
Storms

• To calculate the height exceeded by X percent of
  the ensemble runs
• For each cell, find the surge value where the
  weights of the surge values which are higher add
  up to a value lt X.
• Example
• 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.
• 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 5 foot value
  represents the next 40.

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Height Exceeded by 10 of the Ensemble for
Katrina Adv 23
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http//www.weather.gov/mdl/psurge

• When is it available?
• Beginning when the NHC issues a hurricane watch
  or warning for the continental US
• As close to the advisory release time as
  possible

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Current Development

• We were experimental in 2007
• The model is running in NCEPs job stream.
• The data are flowing to the National Digital
  Guidance Database (NDGD)
• The data will soon be available to NWS forecast
  offices.
• A decision will be made soon on becoming
  operational in 2008.
• We continue to develop training material.
• We continue to update the error statistics.