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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