Probabilistic Hurricane Storm Surge (psurge) NOAA’s National Weather Service

1. Probabilistic Hurricane Storm Surge (psurge) NOAA’s National Weather Service Arthur Taylor MDL / OST April 13, 2006

2. 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.

3. Example: Katrina Advisory 23

4. Probabilistic Storm Surge Methodology • Create an ensemble of SLOSH runs based on NHC’s 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 FEMA’s 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.

5. Varying Katrina’s 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

6. 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%)

7. 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.

8. Katrina Adv 23: Probability > 5 feet of storm surge

9. 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.

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

11. Arlene Adv 10 Probability > 5 feet of storm surge

12. Arlene Adv 10: 10% of storms can exceed this height

13. 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.

14. 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

15. 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 > 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@noaa.gov (currently arthur.taylor@noaa.gov) • http://www.weather.gov/survey/nws-survey.php?code=phss