The National
Weather Service issues hurricane and tropical storm probabilities in
public advisories. The probabilities are used 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 44 selected locations
from Brownsville, Texas to Eastport, Maine.
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 open and begin evacuation. The probability figures are available
through the news media.
Probabilities are issued four times a day. The times are: 6 A.M. Noon,
6 P.M., and 10:30 P.M. Eastern Daylight Time. The probability advisories
are appended to the public advisories in tabular form.
There are several key points to remember. First, if you live between
two listed locations you may estimate your chances of being affected by
averaging the number 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
greatest. Finally, be sensitive to increasing values from one advisory to
the next. Increasing probabilities indicate a greater risk than when
probabilities remain the same or decrease.
When the hurricane is 36 to 72 hours from predicted landfall,
probabilities are quite low. The number increases more rapidly as the
storm is closer than 36 hours. If a storm is forecast to be directly over
your location in 72 hours, the maximum probability is 13 to 18 percent. At
36 hours the maximum probability is 20 to 25 percent. When the storm is
less than 24 hours from forecast landfall, the value increases even more
rapidly to 60 to 70 percent probability.
If you live in an area that requires a long time to evacuate, you may
have to leave when probability values are low. Listen carefully to your
elected officials concerning evacuation for your community. Heed their
advice. If you wait too long, your escape route may be cut by rising storm
surge 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.
Probability figures do not tell you about intensity.
Intensity information is given in the advisory. Secondly, you must not
confuse hurricane and tropical storm probabilities with precipitation
probabilities. Precipitation probabilities are routinely included in
Weather Service forecasts. Not only are the two probabilities computed
differently, but the implications of being rained on and being hit by
hurricane are markedly different.