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Subject: A20) What does AL90, AL91, or 92L refer to in the tropical
discussions?
Contributed by Neal Dorst
Oftentimes, hurricane specialists become curious about disturbances in the
tropics long before they form into tropical depressions
and are given a tropical cyclone number. In order to
alert forecasting centers that they are investigating such a disturbance and
that they wish to have it tracked by the various forecast models, the
specialist will attach a 9-series number to it. The first such disturbance of
the year will be designated 90, the next 91, and so on until 99. After that,
they restart the sequence with 90 again. The purpose of these numbers is to
clarify which disturbance they are tracking as there are often more than one
happening at the same time.
To further clarify matters, each number is accompanied by a two-letter code
designating which tropical cyclone basin the disturbance is in. "AL" is
used for the Atlantic basin (including the Caribbean Sea and Gulf of Mexico),
"EP" for the Eastern Pacific, "CP" for Central Pacific, and "WP" for the
Western Pacific.
Many times in discussions these designations will be shortened to 90L, 91L,
and so forth. Also they may be referred to as 'Invest 90L'. However, once
a disturbance is designated a tropical depression this 9-series number will
be dropped and an ATCF code number will be assigned in
its place.
Computer model forecasts for AL90
You may also occasionally see an 8-series number, such as AL82. This means
that this is a test investigation. There is no particular disturbance that
the specialists are interested in, they're just running a test of the system
to make sure communications and software are running properly.
Revised August 14, 2009
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