Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 7:33 PM GMT on September 21, 2007
Tropical Depression Ten has gradually been acquiring tropical characteristics today, and officially made the transition from a subtropical storm to a tropical storm at 2 pm. One could still argue that the storm is subtropical, since the satellite presentation still shows a large band of strong thunderstorms well removed from the center. However, TD 10 has developed some heavy thunderstorms right at its center, which is a key characteristic of a tropical system. This transition means that TD 10 is likely to intensify, and surface winds measured by the Hurricane Hunter aircraft have increased about 10% since this morning. An Air Force aircraft measured a 37 knot surface wind at 1:05 pm EDT, and a NOAA aircraft measured 38 knots at 1:35 pm EDT. It's a judgment call on NHC's part to decide when this slow increase in winds merits an upgrade of the system to Tropical Storm Jerry. The storm still has subtropical characteristics, and this will keep its intensification rate far slower than Hurricane Humberto's. I think a 50 mph tropical storm is the strongest we'll see TD 10 get.
Top surface winds measured at buoys in the Gulf of Mexico were all below 25 knots (29 mph) early this afternoon. Long range radar loops from the Florida Panhandle show a slow improvement in the organization of the low level spiral bands. Satellite loops show that heavy thunderstorm activity is rather limited but slowly increasing. Radar estimated precipitation over the Florida Panhandle (Figure 1) has been up to three inches.
Figure 1. Estimated rainfall from the Tallahassee, Florida radar.
Elsewhere in the tropics
An area of disturbed weather in the western Caribbean between the Yucatan Peninsula and Jamaica is associated with a surface trough of low pressure. The counterclockwise flow of air around TD 10 is feeding moisture from this disturbance across Cuba and into South Florida today. This disturbance will bring heavy rains to Cozumel, Cancun, and western Cuba on Saturday as it crosses the Yucatan Peninsula. The disturbance is under 20 knots of wind shear at present, but the NOGAPS and GFS models predict this shear may fall enough on Sunday or Monday to allow some development to occur. Texas and Louisiana may get heavy rains from this system on Monday.
A non-tropical low pressure system about 1000 miles east of Bermuda is being watched by NHC for tropical development. SSTs beneath the low are 80 degrees F, which is right at the border where tropical storm formation can occur. This low is expected to move northeastwards out to sea 2-4 days from now.
I'll have an update Saturday morning.
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