Tropical Cyclone Storm Surge Probabilities

Contents

1. Overview
2. Product Description
3. Methodology
4. Availability
5. Examples
6. Feedback and Comments

3. Methodology

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. 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. Currently, the graphic displays probabilities of storm surge of 5 feet or greater above normal tide levels.

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:

http://www.nws.noaa.gov/directives/sym/pd01006001curr.pdf.

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