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

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

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

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1. NOAA’sNational Weather Service:Probabilistic Storm Surge (P-Surge) Arthur Taylor, Bob Glahn, Wilson Shaffer MDL / OST November 30, 2005

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 • An ensemble of SLOSH runs is created based on NHC’s official advisory and historic errors. • Create a SLOSH input track based on the advisory. • Create a set of SLOSH input tracks by varying the various input parameters based on historic errors. • Assign a probability to each SLOSH input track based on the likelihood of that track. • Run the SLOSH model on all the input tracks, and join the results together to compute the probability of surge exceeding various thresholds.

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.

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%) • 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. • We are examining the need to calibrate the probabilities.

11. Potential Products • Product Types: • Probability of storm surge > X feet at any time during the run. • Probability of storm surge > X feet from time T0 to T1. • Formats: • GRIB2 (WMO’s GRIdded Binary) with multiple choices of X, and multiple time slices. • Images in the form of .png files • GIS data in the form of .shp files • Dissemination Methods: • Use the National Digital Guidance Database (NDGD). • Put images / data on the NHC web / ftp site. • Display Methods: • Improve the SLOSH display program to display / animate GRIB2 • Web browser for the .png images, GIS for the .shp files • Plans: FY06 Experimental Products, FY07 Operational Products

12. Calibration • To produce better probability forecasts, we can calibrate the method. • If we forecast 50% chance of exceeding X feet, does it actually exceed X feet 50% of the time? • For all calculated probabilities (in 10% bands), find the actual relative frequency of occurrence. • For observations, we can use SLOSH’s best track analysis. • Use all historic storms making landfall over the last 10 years. • Since a single basin doesn’t have a large number of historic cases, we work on a single uniform grid • The uniform grid also has the advantage that each cell is the same size, so it can be weighted equally.

13. Preliminary Calibration Results • Combining: • Bonnie98 • Floyd99 • Isabel03 • Charley04 (FL)

14. Preliminary Calibration Results • Combining: • Bonnie98 • Floyd99 • Isabel03 • Charley04 (FL)