Probabilistic Hurricane Storm Surge (psurge) NOAA

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Probabilistic Hurricane Storm Surge (psurge)
NOAAs National Weather Service

• Arthur Taylor
• MDL / OST
• April 13, 2006

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Introduction

• NHC begins operational SLOSH runs 24 hours before
  landfall.
• Provides a storm surge estimate for
  non-evacuation applications.
• Problem Surges are based on a single NHC
  forecast track and associated parameters.
• When provided accurate input, SLOSH results are
  within 20 of high water marks.
• Track and intensity prediction errors are the
  largest cause of errors in SLOSH surge forecasts
  and can overwhelm the SLOSH results.

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Example Katrina Advisory 23
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Probabilistic Storm Surge Methodology

• Create an ensemble of SLOSH runs based on NHCs
  official advisory and historic forecast errors.
• Creates a probability of storm surge for this one
  forecast of this particular threatening
  hurricane. Not to be confused with FEMAs
  100-year surge levels.
• Which hurricane forecast errors most impact storm
  surge?
• Cross track error (impacts landfall location)
• Along track error (impacts the timing of the
  storm)
• Intensity errors
• Structure of the storm errors.

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Varying Katrinas Tracks

• The cone of error is 50 of possible cross track
  error.
• 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)

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Calculate probability of exceeding X feet

• Look at each cell in each SLOSH run.
• If the surge exceeds X, add the weight associated
  with that SLOSH run to the total.
• The weight of a run is cross track weight
  along track weight intensity weight size
  weight
• The total weight is the probability of exceeding
  X feet.

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Katrina Adv 23 Probability gt 5 feet of storm
surge
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Calculate height exceeded by X percent of storms.

• Determine what height to choose so that in any
  cell there is a specified probability of
  exceeding it.
• How?
• For each cell, sort the heights of each SLOSH
  run.
• From the tallest height downward, add up the
  weights associated with each SLOSH run until the
  given probability is exceeded. The height
  associated with the last weight added is the
  value for that cell.

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Katrina Adv 23 10 of storms can exceed this
height
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Arlene Adv 10 Probability gt 5 feet of storm surge
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Arlene Adv 10 10 of storms can exceed this
height
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Expected Products in 2006

• What
• Probability of storm surge exceeding 5 feet.
• Height exceeded by 10 of storms.
• Suggestions? (What could you use?)
• Where
• http//www.weather.gov/mdl/psurge
• The National Digital Guidance Database (NDGD).
• When
• Beginning with the first NHC advisory forecasting
  landfall of a hurricane in 48 hours.
• Available approximately 1 hour after the advisory
  release time.

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Is It Reliable?(from a statistical perspective)

• If we forecast 20 chance of exceeding 5 feet,
  does it actually exceed 5 feet 20 of the time?
• Combined Bonnie98, Earl98, Georges98, Bret99,
  Floyd99, Lili02, Claudette, Isabel03, Charley04,
  Frances04, Gaston04, Ivan04, Jeanne04, Katrina05

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Implications

• How can you use this?
• Please let us know how you plan to use it.
• Thoughts as to how one might use it
• The probability of storm surge gt 5 feet product
  can show where it is likely to flood, and how
  susceptible an area is to storm surge.
• The height exceeded by 10 of storms product
  can show the extent of the potential flooding,
  approximating how bad it might get.
• Feedback
• psurge_at_noaa.gov (currently arthur.taylor_at_noaa.gov)
  
• http//www.weather.gov/survey/nws-survey.php?code
  phss