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 , 1:06 PM GMT on September 19, 2009
A tropical disturbance (98L), is located midway between Africa and the Lesser Antilles Islands. This disturbance has shown a modest increase in heavy thunderstorm activity over the past day. This morning's QuikSCAT pass showed an elongated circulation, with top winds around 30 mph. Wind shear is low, 5 - 10 knots, and Sea Surface Temperatures are 28°C, which is about 2°C above the 26°C threshold needed to support a tropical cyclone. There is a large amount of dry air to the north and west of 98L, and this dry air is interfering with development.
Wind shear over 98L is expected to remain in the low range, 5 - 10 knots, through Sunday evening, according to the SHIPS model. This may allow the storm to organize into a tropical depression, assuming it can fight off the dry air that surrounds it. By Monday, the SHIPS model predicts shear will increase to the high range, 15 - 30 knots, so in is unlikely 98L will become anything stronger thatn a weak tropical storm over the coming 5-day period. The models predict that a strong trough of low pressure will turn 98L to the northwest and then north beginning on Monday, with the result that 98L misses the Lesser Antilles Islands by at least 500 miles. NHC is giving 98L a high (greater than 50%) chance of developing into a tropical depression by Monday. At this time, it does not appear that 98L will ever threaten any land areas.
The remains of Hurricane Fred are still spinning away about 700 miles east of Florida. There has been a modest increase in heavy thunderstorm activity on the south side of Fred's circulation over the past day, but high wind shear and dry air have kept the thunderstorms from building over Fred's center. Wind shear is moderate, 15 - 20 knots, and there is substantial dry air surrounding ex-Fred on all sides. This morning's QuikSCAT pass showed top winds of 30 mph.
None of the computer models develop ex-Fred, and conditions for development are expected to remain marginal over the next three days, with wind shear of 15 - 20 knots and plenty of dry air around. Most of the models predict ex-Fred should move over Florida on Tuesday, but steering currents may weaken early next week, and ex-Fred could end up slowing down and turning northwest towards South Carolina. A hurricane hunter aircraft this afternoon was cancelled.
Figure 1. Morning visible satellite image of 98L and the remains of Hurricane Fred.
Twenty years ago today
On September 19, 1989, Hurricane Hugo moved away from Puerto Rico, and headed northwest at 15 mph. An upper-level low over Georgia, in combination with the steering currents imparted by the Azores-Bermuda High, were responsible for the northwesterly motion of the storm. Wind shear from strong upper-level winds continued to weaken the hurricane, and Hugo diminished to a Category 2 hurricane with 105 mph winds.
Figure 2. GOES visible satellite image of Hurricane Hugo taken on September 19, 1989. Wind shear had weakened Hugo to a Category 2 storm with 105 mph winds. Image credit: Google Earth rendition of the NOAA HURSAT data base.
I'll have an update this afternoon if there's any major developments to report.
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