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UNDERSTANDING HURRICANE/TROPICAL STORM PROBABILITIES

From the National Weather Service

The National Hurricane Center issues Hurricane/Tropical Storm probabilities in Public Advisories to realistically assess the threat of a Hurricane or Tropical Storm affecting your community. The probabilities are defined as the chance in percent that the center of the storm will pass within approximately 65 miles of selected locations.

Probabilities are intended primarily for decision-makers in local government and private industry who must begin protective actions early. Your Local Emergency Management Officials use probabilities to help decide when to begin evacuation and open shelters. The figures will be available in local weather service statements and through the news media.

Probabilities are issued four times a day around 6 a.m.; noon; 6 p.m.; and 1030 p.m. EDT or at three hourly intervals depending on the degree of threat and are appended to the public advisories in tabular form.

There are several key points to remember concerning the use of probabilities. First, if you live between two listed locations you may estimate your probability by averaging the numbers on either side second. To assess your threat, compare the probability of your area with those of neighboring locations. If you have the highest value, your hurricane threat is greatest. Finally, you should be sensitive to increasing values 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 only 10 percent. The probabilities are low out to 72 hours due to the forecast errors which may occur through such a long period. 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 the maximum probability is 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.

If you live in an area where it takes a long time to evacuate be cause of traffic congestion or if you live in an area which floods very easily you may have to leave when probabilities are low. You should listen carefully to your elected officials concerning evacuation for your community and heed their advice. If you wait too long your escape route may be cut off by rising storm tides as the hurricane gets closer.

Use caution when interpreting the probabilities. Potential loss of life and property will vary depending on the intensity of the storm. The probabilities do not tell you about intensity. That information is given in the text of 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.