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

Probabilistic Storm Surge 2005 Introduction NHC begins operational SLOSH runs 24 hours before landfall. Provides a storm surge estimate for non-evacuation applications.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.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.Track and intensity prediction errors are the largest cause of errors in SLOSH surge forecasts and can overwhelm the SLOSH results.

Probabilistic Storm Surge 2005 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 SLOSH input track based on the advisory. Create a set of SLOSH input tracks by varying the various input parameters based on historic errors.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.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.Run the SLOSH model on all the input tracks, and join the results together to compute the probability of surge exceeding various thresholds.

Probabilistic Storm Surge 2005 SLOSH’s Input Track Location Can get from NHC’s advisoryCan get from NHC’s advisory Forward Speed Can compute from NHC’s advisory.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.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.Can only get the current value (no forecast values) from NHC’s advisory.

Probabilistic Storm Surge 2005 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.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.Vmax is provided in NHC’s advisory. Since the current Pressure is provided, one can estimate the current Rmax.Since the current Pressure is provided, one can estimate the current Rmax. We assume that Rmax remains constant, then calculate the resulting Pressures.We assume that Rmax remains constant, then calculate the resulting Pressures.

Probabilistic Storm Surge 2005 Example: Katrina Advisory 23

Probabilistic Storm Surge 2005 Varying Katrina’s Tracks standard deviations (sd) to left and right, is equivalent to 90% of storms standard deviations (sd) to left and right, is equivalent to 90% of storms 0.67 sd to left and right would be average error 0.67 sd to left and right would be average error Spacing based on size of the storm Spacing based on size of the storm Calculations are done when 34 knot winds or greater are in the SLOSH basin Calculations are done when 34 knot winds or greater are in the SLOSH basin

Probabilistic Storm Surge 2005 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%)

Probabilistic Storm Surge 2005 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.For each basin, eliminate tracks which never forecast tropical storm force winds. Remove basins where all the tracks were eliminated.Remove basins where all the tracks were eliminated. Treat eliminated tracks as if they generated no surge in a basin.Treat eliminated tracks as if they generated no surge in a basin.

Probabilistic Storm Surge 2005 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.If that value exceeds X, we add the weight associated with that SLOSH run to the total. Otherwise we don’t increase the total.Otherwise we don’t increase the total. The total weight is considered the probability of exceeding X feet.The total weight is considered the probability of exceeding X feet. We are examining the need to calibrate the probabilities.We are examining the need to calibrate the probabilities.

Probabilistic Storm Surge 2005 Katrina Adv 23 Probability > 5 ft (approx. 24hr before landfall)

Probabilistic Storm Surge 2005 Arlene Adv 10 Probability > 5 ft (approx. 24hr before landfall)

Probabilistic Storm Surge 2005 Potential Products Product Types: Probability of storm surge > X feet at any time during the run.Probability of storm surge > X feet at any time during the run. Probability of storm surge > X feet from time T0 to T1.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.GRIB2 (WMO’s GRIdded Binary) with multiple choices of X, and multiple time slices. Images in the form of.png filesImages in the form of.png files GIS data in the form of.shp filesGIS data in the form of.shp files Dissemination Methods: Use the National Digital Guidance Database (NDGD).Use the National Digital Guidance Database (NDGD). Put images / data on the NHC web / ftp site.Put images / data on the NHC web / ftp site. Display Methods: Improve the SLOSH display program to display / animate GRIB2Improve the SLOSH display program to display / animate GRIB2 Web browser for the.png images, GIS for the.shp filesWeb browser for the.png images, GIS for the.shp files Plans: FY06 Experimental Products, FY07 Operational Products

Probabilistic Storm Surge 2005 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?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.For observations, we can use SLOSH’s best track analysis. Use all historic storms making landfall over the last 10 years.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 gridSince 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.The uniform grid also has the advantage that each cell is the same size, so it can be weighted equally.

Probabilistic Storm Surge 2005 Preliminary Calibration Results Combining: Bonnie98 Bonnie98 Floyd99 Floyd99 Isabel03 Isabel03 Charley04 (FL) Charley04 (FL)

Probabilistic Storm Surge 2005 Preliminary Calibration Results Combining: Bonnie98 Bonnie98 Floyd99 Floyd99 Isabel03 Isabel03 Charley04 (FL) Charley04 (FL)