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Facts about Hurricane/Tropical Storm Probabilities

The National Weather Service issues hurricane/tropical storm probabilities in public advisories to realistically assess the threat of a hurricane or tropical storm hitting your community. The probabilities are defined as the chance in percent that the center of the storm will pass within approximately 65 miles of 45 selected locations from Brownsville, Texas to Eastport, Maine. In our area, these include Gulfport, Mississippi, Mobile, Pensacola and Panama City.

Probabilities are intended primarily for decision makers in local government and private industry who must begin protective actions early. State and local officials use probabilities to help decide when to begin evacuation and to open shelters. The probability figures are made available through the news media. Probabilities are issued four times a day, at 4 AM, 10 AM, 4 PM, and 10 PM Central Daylight Time and when special public advisories are issued.

There are several key points to remember. First, if you live between two listed locations you may estimate your chance of being affected by averaging the numbers on either side. Second, to assess your threat, compare the probability of your community with those of neighboring locations. If you have the highest value, your hurricane threat is the greatest. Finally, you should be sensitive to values that increase from one advisory to the next. Increasing probabilities indicate a greater risk than those that remain the same or decrease.

When the hurricane is 36 to 72 hours from predicted landfall, probabilities are quite low. The numbers increase more rapidly as the storm gets closer than 36 hours. If a storm is forecast to be directly over your location in 72 hours the maximum probability is 10 percent. At 48 hours from predicted landfall the maximum is 13 to 18 percent. At 36 hours the maximum is 20 to 25 percent, and at 24 hours 35 to 45 percent. When the storm is less than 24 hours from forecast landfall, the values increase even more rapidly to 60 to 70 percent.

Use caution when interpreting the probabilities. Potential loss of life and property will vary depending on the intensity of the storm. The figures do not tell you about intensity, that information is given in the advisory. Secondly, hurricane/tropical storm probabilities must not be confused with precipitation probabilities which are routinely included in Weather Service forecasts. Not only are the two computed differently, but the implications of being rained on and being hit by a hurricane are markedly different.


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