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 , 8:06 PM GMT on September 12, 2007
Tropical Storm Humberto is steadily gaining strength as it closes in on the Texas coast near Freeport. Radar animations from Houston show a well-defined circulation, with plenty of low-level spiral bands wrapping around the center. Humberto is organizing quickly, and it is a good thing it has less than 12 hours remaining over water. Despite the rather intimidating appearance of Humberto on radar, the winds have not shown a major increase yet. A buoy 70 miles south of Freeport has measured sustained winds of 25 mph since 8:50am EDT. The region of winds above 35 knots (40 mph) as estimated by the Houston Doppler radar have shown a steady increase, doubling in area over the past two hours (keep in mind that due to the curvature of the earth, these winds are measured at an altitude of roughly 2,000 feet 50 miles from the radar). Top winds reported by the Hurricane Hunters in their 2:55pm EDT center fix were 38 knots at flight level, which corresponds to about 35 mph at the surface. They measured a pressure of 1001 mb, 4 mb lower than the estimated pressure from the 2pm EDT advisory from NHC. Winds from the SFMR instrument on the aircraft were 35 knots (40 mph) in several locations, with one spot of 51 mph winds.
Figure 1. Latest radar image from Houston, Texas.
Wind shear is a low 5-10 knots, and Humberto has time to develop into a 50-55 mph tropical storm before it makes landfall tonight. Since this system is 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 widespread destructive flooding. Rainfall amounts of 2-3 inches have already occurred along the coast, from Galveston to just beyond the Louisiana border, as estimated by radar.
Tropical Depression Eight
Tropical Depression Eight formed this morning, and has the potential to grow into a hurricane in the next 3-4 days. This morning's 8:18am EDT pass of the European ASCAT satellite captured the circulation of TD 8 nicely, and showed that it was still somewhat elliptical, which will slow down intensification until the circulation grows more circular. ASCAT estimated winds up to 25 knots (30 mph) on the northwest side of TD 8. Satellite loops of TD 8 have shown a slow but steady improvement in organization since early this morning. A large area of thunderstorms reach high in the atmosphere (as evidenced by very cold cloud tops on infrared satellite imagery), an upper-level outflow channel has opened to the south, and some low-level spiral banding is now apparent. Several satellite intensity estimates put TD 8 at tropical storm strength already, and I expect that this will be a tropical storm later today.
Wind shear is about 10 knots over TD 8, and is forecast to remain below 10 knots through Saturday morning. This should allow TD 8 the opportunity to grow to a Category 1 hurricane by Saturday, as predicted by the latest (12Z, 8am EDT) rund of the GFDL and HWRF intensity models. By Saturday afternoon, the models have different solutions on what will happen to TD 8. An upper-level trough will lie north of the Lesser Antilles at that time, and may bring hostile wind shear to TD 8. The GFDL model predicts that this shear will be strong enough to reduce TD 8 to a weak tropical storm. The HWRF model disagrees, and keeps the storm at Category 1 hurricane strength. The SHIPS intensity model says the shear will increase to about 15 knots, which would slow but perhaps not completely halt intensification. The farther north the storm goes, the better--wind shear will be stronger to the north.
The NOAA Hurricane Hunters are scheduled to fly a research mission into TD 8 Thursday evening.
I'll have an update Thursday morning.
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