Tropical Cyclone Wind Speed Probabilities Products

Contents

1. Overview
2. Description of the tropical cyclone wind speed probabilities text product
3. Description of the tropical cyclone wind speed probabilities graphical products
4. Description of the tropical cyclone NDFD wind speed probabilities products
5. Potential advantages as compared to previous products
6. Method for computing the wind speed probabilities

1. Overview

The tropical cyclone surface wind speed probabilities text and graphical products issued by the Tropical Prediction Center/National Hurricane Center (TPC/NHC) became operational at the start of the 2006 hurricane seasons in the Atlantic and eastern North Pacific basins. The underlying techniques were developed by researchers at NOAA and Colorado State University, while the products themselves were developed by the Technical Support Branch (TSB) at TPC/NHC.

The tropical cyclone surface wind speed probabilities products provide probabilities of sustained (1-minute average) surface (10-meter elevation) wind speeds of at least 34 kt (39 mph, tropical storm force), 50 kt (58 mph), and 64 kt (74 mph, hurricane force) at individual locations in the Atlantic and eastern North Pacific basins. Also included as a graphic and in the text product (PWS) is the Maximum Wind Speed (Intensity) Probability Table that provides probabilities, in percent, for the maximum sustained (1-minute average) surface (10-meter elevation) wind speed of a tropical cyclone for various intensity categories (dissipated, tropical depression, tropical storm, and hurricane) and for the five categories of the Saffir-Simpson Hurricane Scale. The probabilities in this table apply to the maximum sustained surface wind associated with the cyclone, and not to winds that could occur at specific locations. Since these probabilities take into account land affects (e.g. weakening after landfall), the intensity probabilities should not be used to infer the intensity of a cyclone at the time of landfall. The probabilities represent the chances of a cyclone having certain maximum winds at a specific time. When a cyclone is forecast to be near land, the maximum wind speed probabilities tend to be more evenly distributed among the various stages (or categories) of tropical cyclones (or hurricanes). While these probabilities accurately represent the chances of a cyclone having a maximum intensity in the various stages (or categories) they do accurately reflect the chances of a storm at specific categories when the center approaches or crosses a coastline (see example in section 3).

The location-specific probabilities and the maximum wind speed (intensity) probabilities are based on errors during recent years in the official track and intensity forecasts issued by the TPC/NHC. Variability in tropical cyclone size (wind radii) is also incorporated into the location-specific probabilities.

One separate wind speed probabilities text product is issued with each advisory package for every active tropical cyclone in the Atlantic and eastern North Pacific basins. Each text product contains two sections, Section I is the Maximum Wind Speed (Intensity) Probability Table, and Section II is the location-specific probabilities for all three wind speed thresholds (34, 50, and 64 kt). The text products are available at about the same time as the other advisory package text products. Also, TPC/NHC issues a set of storm-centered graphical wind speed probability products with each advisory package for each active tropical cyclone in the Atlantic and eastern North Pacific basins. The graphical wind speed probabilities are immediately available as a preliminary graphic when the tropical cyclone advisory is issued. These preliminary graphics only contain data for one tropical cyclone. Approximately 30 to 45 minutes after advisory time, a set of basin-wide wind speed probability graphics become available which supercede the preliminary graphics. Although the probability data for each tropical cyclone remains the same, the .final graphics. illustrate the wind speed probabilities for all tropical cyclones in the area of interest. The delay that occurs between the preliminary and final probability graphics results from the need to wait a few minutes after the advisory release deadline (e.g., 11:00 PM EDT). This pause ensures that all tropical cyclone advisories have been issued.

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