NOAA’s National Weather Service: Probabilistic Storm Surge (P-Surge)

1. NOAA’sNational Weather Service:Probabilistic Storm Surge (P-Surge) Arthur Taylor, Nicole P. Kurkowski MDL / OST July 26, 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. 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.

4. SLOSH’s Input Track • Location • Can get from NHC’s advisory • Forward Speed • Can compute from NHC’s advisory. • Radius of Maximum Winds (Rmax) • Not given in NHC’s advisory due to lack of skill in forecasting changes in Rmax. • Pressure • Can only get the current value (no forecast values) from NHC’s advisory.

5. SLOSH’s Rmax and Pressure • Since NHC’s advisory does not provide Rmax, or forecast Pressure, we need to compute them. • The SLOSH parametric wind model relates Rmax, Pressure, and Maximum Wind Speed (Vmax). Given any two, the third can be computed. • Vmax is provided in NHC’s advisory. • Since the current Pressure is provided, one can estimate the current Rmax. • We assume that Rmax remains constant, then calculate the resulting Pressures.

6. Example: Katrina Advisory 23

7. Varying Katrina’s Tracks • 1.645 standard deviations (sd) to left and right, is equivalent to 90% of storms • 0.67 sd to left and right would be average error • Spacing based on size of the storm • Calculations are done when 34 knot winds or greater are in the SLOSH basin

8. 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%)

9. Determine Which Basins to Run • We try all SLOSH input tracks in all operational basins: • For each basin, eliminate tracks which never forecast tropical storm force winds. • Remove basins where all the tracks were eliminated. • Treat eliminated tracks as if they generated no surge in a basin.

10. Calculate probability of exceeding X feet • To calculate probability of exceeding X feet, we look at each cell in each SLOSH run’s envelope. • If that value exceeds X, we add the weight associated with that SLOSH run to the total. • Otherwise we don’t increase the total. • The total weight is considered the probability of exceeding X feet.

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

12. 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.

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

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

16. When are our products available? • Beginning with the first NHC advisory forecasting landfall of a hurricane in 24 hours. • Available approximately 1 hour after the advisory release time.

17. 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: • nws.psurge@noaa.gov • http://www.weather.gov/survey/nws-survey.php?code=phss

18. Future Development • We are continuing to examine whether we need to calibrate the probabilities, by looking at reliability diagrams. • 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 the Gulf of Mexico and 0 feet in the Atlantic Ocean. After making the change, we will need to reinvestigate the calibration question. • We are working on adding the data to the NDGD (National Digital Guidance Database).