Tropical Cyclone Wind Speed Probabilities Products

Updated 7 January 2009

Overview
Description of the tropical cyclone wind speed probabilities text product
Description of the tropical cyclone wind speed probabilities graphical products
Description of the tropical cyclone NDFD wind speed probabilities products
Potential advantages as compared to previous products
Method for computing the wind speed probabilities

6. Method for computing the wind speed probabilities

The calculation of the wind speed probabilities is accomplished by creating a large set of alternative but plausible tracks and intensities roughly centered on the current official forecast. These alternate forecasts are determined by random sampling of historical track and intensity errors in official NHC forecasts (since 2001). The alternate intensity forecasts consider whether each alternate track is over land or water, and the alternate intensity forecasts are adjusted accordingly. The size of the tropical cyclone (set of wind radii) for each alternate track is determined by a climatology and persistence (CLIPER) model and its error components. This CLIPER model takes into account the size of the cyclone at that start of the forecast period as well as typical changes in size that occur as a cyclone experiences changes in strength, forward motion, and other factors. An adjustment is made, for purposes of calculating the probabilities, so that the wind radii represent the average, rather than the maximum, extent of winds in each quadrant. This process results in probabilities of actually experiencing certain wind speeds, not probabilities of falling within the traditional forecast wind radii that indicate the maximum extent of winds from the center. Swaths of particular wind speeds are then computed for each alternate forecast. Probabilities are computed on a 0.5x0.5 degree latitude-longitude grid by counting the fraction of alternate forecasts in which each point falls within a given wind swath (34, 50, or 64 kt). This output is then processed and expressed in text or graphical format via the products being produced by the TPC/NHC.

The Maximum Wind Speed Probabilities are created using the same set of alternate but plausible track and intensities. The maximum winds of each of the alternate scenarios at the standard forecast hours is computed and the probabilities are determined by the numbers of scenarios that fall within the various intensity stages versus the total number of scenarios.

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Tropical Cyclone Wind Speed Probabilities Products

Contents

6. Method for computing the wind speed probabilities