Tropical Cyclone Storm Surge Probabilities
The Tropical Cyclone Storm Surge Probabilities (P-Surge 2.0) is a graphical product developed by the Meteorological Development Laboratory (MDL) of the National Weather Service (NWS), in cooperation with the NHC, with the intent to provide the emergency management community; federal, state, and local government agencies; media; maritime interests; and the general public with information that enhances their ability to make preparedness decisions specific to their own situations with regards to storm surge. The Tropical Cyclone Storm Surge Probabilities product was created in response to user requests for additional tropical cyclone probabilistic information, and the National Research Council's Fair Weather Report encouraging probabilistic products.
The Tropical Cyclone Storm Surge Probabilities graphics show the overall chances that the specified storm surge height will occur at each individual location on the map during the forecast period indicated. The probabilities are based on errors during recent years in the official track and intensity forecasts issued by the NHC. Variability in storm size is also incorporated. Storm surge values used in the calculations are produced by the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model.
The calculation of the Tropical Cyclone Storm Surge Probabilities is accomplished by statistically evaluating a large set of SLOSH model runs based on the current NHC official forecast, and taking into account historical errors in official NHC track and intensity forecasts. Track forecast errors are considered in terms of both along-track (i.e., forward speed) and cross-track (i.e., landfall location) errors. The astronomical tide is explicitly modeled in P-Surge 2.0, which means that the timing of cyclone landfall relative to the tidal signal (i.e., along-track variability) is fully captured in each SLOSH model run. Variability in the radius of maximum wind, which is also known to have a significant impact on storm surge, is also considered. The result is a map of cumulative storm surge probabilities that indicate the overall chances that the indicated storm surge will occur at each location on the map during the period between hour 0 (the beginning of the forecast) and 3 days (72 hours) after the beginning of the forecast.
It is important for users to realize that probabilities that may seem relatively small may still be quite significant. The probabilities might indicate there is a chance that a damaging or even an extreme event could occur at your location. As a storm gets closer to land, relatively small probabilities may warrant making preparations to protect lives and property. Users are urged to consider the potentially immense cost (in terms of lives, property, etc.) of not preparing for an extreme event, even if the chances at an individual point are only perhaps 1 in 20 (5%) or 1 in 10 (10%) that the event will occur.
A full description of other NWS Tropical Cyclone Weather Services Program Products is provided in NWSI 10-601, which is available on the Internet at the following URL:
The NHC will produce a set of updated storm surge probability graphics whenever a hurricane watch or hurricane warning is in effect for any portion of the Gulf or Atlantic coasts of the continental United States. These graphics will take approximately 30 minutes (perhaps more) to update on the NHC website following the issuance of the NHC tropical cyclone advisories at 5:00 AM EDT, 11:00 AM EDT, 5:00 PM EDT, and 11:00 PM EDT (or 4:00 AM EST, 10:00 AM EST, 4:00 PM EST, and 10:00 PM EST).
These graphics can be found here when advisories are created for an active storm.