Thursday, August 6, 2009

Hurricane Season Outlook Revised Lower

NOAA this morning revised its Atlantic tropical season outlook downward from the forecast released in May. The new prediction from the Climate Prediction Center and National Hurricane Center calls for the following probabilities (May numbers in parentheses):
Above normal: 10% (25%) 
Near normal: 50% (50%)
Below normal: 40% (25%)
There is a 70% chance of the following numbers of storms (normal in parentheses):
Named storms:     7-11 (11)
Hurricanes: 3-6 (6)
Major hurricanes: 1-2 (2)
The intensity of the hurricane season is measured using the Accumulated Cyclone Energy (ACE), which is a combination of the duration and intensity of storms, rather than simply the number of storms.

The downward revision in the forecast is based mainly on the continued development of El Niño conditions in the tropical Pacific. The higher wind shear (increase of wind with height) from El Niño is expected to outweigh the effects of warmer than average Atlantic and Caribbean sea surface temperatures.

Despite the indications of lower than average tropical activity, NOAA cautions residents of the Atlantic and Gulf Coasts that they should be prepared every year, regardless of the overall level of activity. To paraphrase on old mariners' saying, "One nasty hurricane can spoil your whole day." Some examples of hurricane landfalls in El Niño years include Betsy (1965), Bob (1991), Danny (1997), and Lili (2002). The most notorious example of a disastrous storm during a below-average season was Andrew (1992). Others include Hurricane Donna (1960) and Hurricane Betsy (1965).

The latest dates for the first storm of the season are:
For records back to 1851:
Tropical storm: September 15 (1914)
Hurricane: October 8 (1905)

Satellite era (since 1966):
Tropical storm: August 30 (Arlene, 1967)
Hurricane: September 11 (Gustav, 2002)

Image (click to enlarge): Recent sea surface temperatures, from NOAA/Climate Prediction Center

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Seasonal Outlook

Latest seasonal forecast: Click here.

Latest 3-month temperature outlook from Climate Prediction Center/NWS/NOAA.