an explanation of strike probabilities

[Houstonia]

This seems like a good explanation of strike probabilities. It's aimed at Hou/Galv residents, but the explanation probably applies to everyone...

Statement as of 8:00 PM CDT on September 20, 2005

... Hurricane/tropical storm probabilities...

National Hurricane Center issues hurricane/tropical storm
probabilities /miaspfat3/ to realistically assess the threat of a
particular storm affecting your community. Probabilities are defined
as the chance in percent that the center of the storm will pass within
65 miles of selected locations.

For the Texas coast... probabilities will be issued for Brownsville...
Corpus Christi... Port O Connor... Galveston... and Port Arthur. Nearby
Louisiana locations are New Iberia and New Orleans. Additionally... if
the storm track is expected to parallel the coast... probabilities will
be issued for points over the Gulf. Over the Texas coastal areas Gulf
points will be: 28n 93w /southeast of Port Arthur... 28n 95w
/southeast of Freeport/... 27n 96w /southeast of Corpus Christi/... and
25n 97w/southeast of Brownsville. Probabilities are primarily for
local government decision makers and those in private Industry who
must take early preventative action. Local emergency management
officials use them to make decisions on when to open shelters and/or
begin evacuation.

Probabilities are issued four times daily... when storms are in
existence... around 5 am... 11 am... 5 PM... and 930 PM and are in a
tabular form.

Remember the following points when using the probabilities...

A. If you live between two listed points... you may estimate your
probability by averaging the two.

B. To assess your threat... compare your probability with those of
neighboring locations... if you have the highest value... your threat
is the greatest.

C. You should be sensitive to increasing values from one advisory
to the next. Increasing values indicate a greater risk those that
remain the same or decrease.

When a hurricane/tropical storm is 36-72 hours from landfall
probabilities are quite low. Values increase 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.
Probabilities are low out to 72 hours due to forecast errors which may
occur through a long period of time. At 48 hours from predicted
landfall... the maximum is 13 to 18 percent. At 36 hours... the maximum
probability is 20 to 25 percent. At 24 hours... the maximum probability
is 35 to 45 percent. At 12 hours... the maximum probability is 60 to 70
percent.