Nature is jumping the gun a bit this year, with the season's first tropical depression forming four days before the official start to hurricane season. The area of disturbed weather (91L) that we've been watching, about 250 miles east-northeast of Cape Hatteras, North Carolina, has developed enough heavy thunderstorm activity and spin to be classified as a tropical depression. QuikSCAT imagery from last night revealed a closed surface circulation, but top winds of only 20 - 25 mph. Satellite estimates (using a cloud pattern recognition method called the "Dvorak" technique) were saying this was a tropical depression this morning, though, so the NHC elected to upgrade the system.
The disturbance is over the relatively warm waters of the Gulf Stream (25°C) and has wind shear of 5 - 10 knots over it, and these conditions are marginally favorable for some slow development to occur until Friday, when the system will likely move over waters too cold to support intensification. TD One is not a threat to any land areas. I give the storm a 60% chance of becoming Tropical Storm Ana.
Figure 1. Latest satellite image of TD One.
Is the formation of TD One a harbinger of an active hurricane season?
Probably not. Early season storms occurring near the U.S. coast have not been shown to be correlated with an active main portion of hurricane season during August - October. However, the situation is different if we start getting June and July storms in the deep tropics between Africa and the Lesser Antilles Islands. This was the case last year, when the formation of Hurricane Bertha in the deep tropics in July presaged an active 2008 hurricane season. According to the Hurricane FAQ, "as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during June and July have been at least average and often times above average. So it could be said that a June/July storm in this region is pretty much a "sufficient" condition for a year to produce at least average activity."
