Tropical Cyclone Wind Speed Probabilities Products
Updated 7 January 2009
Contents
 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
2. Description of the wind speed probabilities text product
The wind speed probabilities are provided in a stormspecific text product that contains two sections. The first section is the Maximum Wind Speed (Intensity) Probability Table. This table provides categorical maximum wind speed (intensity) probabilities at standard forecast hours (12, 24, 36, 48, 72, 96, and 120) for various intensity stages (dissipated, tropical depression, tropical storm and hurricane) and for the five categories on the SaffirSimpson Hurricane Scale. These probabilities apply to the maximum sustained surface wind associated with the cyclone, and not to winds that could occur at specific locations. These intensity probability values can not be used to infer the intensity of a cyclone at the time of landfall due to issues with the probability computations and land interaction.
The second section of the wind speed probabilities text product contains wind speed probabilities for selected coastal and inland cities along with each forecast issued by the NHC. Each wind speed probability text product provides probabilities (in percent) for wind speeds of at least 34 kt (39 mph, tropical storm force), 50 kt (58 mph), or 64 kt (74 mph, hurricane force) at each listed location. Two types of probability values are produced in this section of the text product: cumulative probabilities of occurrence, and individual period probabilities of onset.
Cumulative probabilities are provided in the text product for the following time periods: 012 hours, 024 hours, 036 hours, 048 hours, 072 hours, 096 hours, and 0120 hours (05 days). These cumulative probabilities indicate the overall chances that the stated wind speed will occur at each location during the period between hour 0 (the beginning of the forecast) and each listed forecast hour.
Individual period probabilities are provided for each of the following time intervals: 012 hours, 1224 hours, 2436 hours, 3648 hours, 4872 hours, 7296 hours, and 96120 hours. These individual period probabilities indicate the chances that the stated wind speed will start during each individual period at each location. Cumulative probabilities through each forecast time period are also just the sum of the individual period probabilities up to that time.
In other words, cumulative probabilities tell decisionmakers the chances that the event will happen at all. The individual period probabilities tell decisionmakers when the event is most likely to start.
View a complete example of the tropical cyclone wind speed probability text product.
Probabilities for a particular location and speed are provided only when the 120hour (5day) cumulative probability of sustained tropical storm and 50kt winds is at least 2.5% (rounded to 3%). Hurricane force probabilities are provided when the 120hour (5day) cumulative probability is at least 1%. Locations are listed in geographic order, and data for all wind speeds (with high enough probabilities) at one location are grouped together.
It is important for users to realize that probabilities that may seem relatively small may still be quite significant. The probabilities may indicate there is a chance that a damaging or even an extreme event may occur at your location. This 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.
Next: Description of the tropical cyclone wind speed probabilities graphical products
