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Two tropical depressions may form today

By: Dr. Jeff Masters, 1:46 PM GMT on September 12, 2007

The area of disturbed weather 100 miles south of Freeport, Texas (90L), has become well-organized this morning, and a tropical depression appears to be forming. NHC put out this special advisory at 8:45am EDT today:

Satellite and NWS radar observations indicate that the area of low pressure in the northwestern Gulf of Mexico is becoming better organized this morning. This system could become a tropical depression later today...and an Air Force Reserve unit reconnaissance aircraft will investigate the area early this afternoon. The low is moving slowly north-northwestward...and regardless of whether or not it becomes a tropical cyclone...heavy rains are expected to spread across southeastern Texas and Louisiana over the next couple of days. For information specific to your area...please consult statements issued by your local NWS forecast office.

Figure 1. Latest long-range radar image from Houston, Texas.

Long range radar out of Houston (Figure 1) shows a well-defined surface circulation exists about 100 miles south of Freeport, with some low-level spiral bands starting to get organized. A ship just east of the center measured 40 knot (46 mph) winds at 8am EDT, but this is likely a strong downburst wind from a thunderstorm, and is not representative of the larger-scale winds. A buoy 70 miles south of Freeport measured sustained winds of 25 mph at 8:50am EDT. Wind shear is a low 5-10 knots, and 90L does have time to develop into a tropical storm before it makes landfall tonight or Thursday along the mid or upper Texas coast. Since this system is very slow moving, it has the potential to drop rain amounts in excess of ten inches along the Texas and Louisiana coasts--including the Houston metropolitan area--over the next two days. These rains may cause serious widespread flooding. Rainfall amounts of 2-3 inches have already occurred along the coast, from Freeport to the Louisiana border, as estimated by radar. The Hurricane Hunters are scheduled to investigate this storm at 2pm EDT this afternoon.

91L could become a tropical depression today
A strong tropical wave near 12.5N 44W, 1100 miles east of the Lesser Antilles Islands, is headed west-northwest at 10 mph. This system (91L) may develop into a tropical depression today, and has the potential to become a large and dangerous major hurricane next week. This morning's QuikSCAT pass captured only the western side of the circulation, but did have some believable winds of 30-35 knots (35-40 mph). Last night's QuikSCAT pass showed an elliptical but better-defined circulation. The circulation looked more circular and better-defined in a pass from the European ASCAT satellite a few hours after that. A circular motion of winds around the center is much more conducive for strengthening than an elliptical circulation. Satellite loops show considerable improvement in organization has occurred since yesterday--a large area of thunderstorms that reach high in the atmosphere (as evidenced by very cold cloud tops on infrared satellite imagery) has formed near the circulation center. An upper-level outflow channel has opened to the south, and some low-level spiral banding is now apparent, particularly to the north. These spiral bands are also apparent on the latest microwave satellite imagery (Figure 1). Wind shear has dropped below 10 knots, and with warm 28 C waters underneath, 91L should become a tropical depression later today.

Figure 2. Microwave satellite image of 91L. Note the long spiral band forming to the north of the clump of heavy thunderstorms that lie near the center of 91L. Image credit: Navy Research Lab, Monterey.

Shear is forecast to remain below 10 knots for the next 3-4 days, which should allow 91L the opportunity to grow to a Category 1 hurricane or stronger before it reaches the Lesser Antilles Islands. However, the SHIPS, GFDL, and HWRF intensity models all keep 91L below hurricane strength for the next five days. I think this is unrealistic, given the favorable conditions for strengthening present. The big question concerns the track--there is a good chance that 91L will miss the Lesser Antilles, as the current steering currents favor a more northwesterly track for the storm over the coming days. The system has already moved north of Barbados' latitude, and the southern Lesser Antilles Islands are unlikely to receive a direct hit from 91L. Most of the models indicate a forward speed near 10-15 mph, which would bring 91L to the northern islands Monday. The U.S. East Coast may be at risk from this storm ten or so days from now, but it is far too early to speculate on the chance of this occurring, or what region might be most at risk.

The NOAA Hurricane Hunters may fly a research mission into 91L Thursday evening.

I'll have an update later today if either system develops into a tropical depression.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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