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 , 8:26 PM GMT on August 12, 2008
A tropical wave about 550 miles east of the Lesser Antilles Islands (92L) has changed little in organization today. Between 2-4:30 pm EDT, today's Air Force Hurricane Hunter mission could not find a surface circulation. The highest surface winds measured were 32 mph, and the lowest surface pressure was 1008 mb. As noted on this morning's blog, 92L has an elongated 600-mile long "convergence zone" along which the winds moved in opposite directions, rather than a closed surface circulation. However, visible satellite loops show that a circulation has developed at mid levels of the atmosphere (about 18,000 feet in altitude), and this circulation will probably be able to migrate down to the surface by Wednesday morning. Heavy thunderstorm activity has decreased some today, but modest upper-level outflow continues to the north, visible as cirrus clouds streaming away from the center. Low-level spiral bands are not present, but appear to be beginning to emerge again to the north. Water vapor satellite loops show that a large area of dry air and Saharan dust surrounds 92L on three sides, and this dry air may periodically interfere with 92L's organization over the next two days.
Figure 2. Latest satellite image of 92L.
The forecast for 92L
Water temperatures are a warm 28°C and forecast to remain above 28°C the next five days. Wind shear is a modest 10-15 knots, and is forecast to remain 5-15 knots for the next five days. The models are split on whether 92L will develop or not. The 8 am EDT GFDL model is the most aggressive, intensifying 92L to a tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday, then intensifies into a Category 2 hurricane that plows into the Bahama Islands on Saturday. In contrast, the 8 am EDT HWRF model develops 92L into a minimal 40 mph tropical storm on Saturday, when it enters the easternmost Bahama Islands. The SHIPS intensity model is in between, forecasting a 70-75 mph storm by Saturday. The National Hurricane Center is giving 92L a high (>50% chance) of becoming a tropical depression by Thursday afternoon. Given the moderately favorable wind shear and water temperatures, 92L will probably become a tropical depression by Wednesday night, and has about a 20% chance of reaching hurricane strength by Saturday. In its current disorganized state, 92L will be difficult for the computer models for make an accurate track forecast for, since we don't know where along the long axis of converging winds that the center will form at. We can expect the northern Lesser Antilles to get some heavy weather on Thursday, and these conditions may spread to Puerto Rico late Thursday into Friday. 92L may represent a threat to the U.S. East Coast early next week. The Hurricane Hunters are scheduled to pay another visit to 92L at 2 am EDT Wednesday. Beginning Wednesday afternoon, there will be both a NOAA P-3 and an Air Force C-130 Hurricane Hunter aircraft in the storm, as the P-3s collect detailed radar data to aid in research efforts to improve hurricane computer forecast models.
Disturbance 93L off the coast of Africa
A tropical wave near 13N 31W (93L), about 400 miles west-southwest of the Cape Verde Islands, has continued to show a decrease in heavy thunderstorm activity. Visible satellite loops show a large swirling circulation, but little heavy thunderstorm activity remaining. A large area of stratocumulus clouds is visible to the northwest, indicating the presence of cool, dry, stable air. This stable air is inhibiting development of 93L. Wind shear has been a low 5-10 knots today. Water vapor satellite imagery shows dry Saharan air to the northwest.
Figure 1. Latest satellite image of 93L.
The forecast for 93L
Water temperatures are a warm 28°, and forecast to cool below 27°C Thursday. Wind shear is forecast to remain below 10 knots through Thursday afternoon, then increase to 15-25 knots Friday through Saturday. The National Hurricane Center is giving 93L a medium (20-50% chance) of becoming a tropical depression by Thursday afternoon. I believe that 93L has a less than 20% chance of becoming a tropical depression this week. There is too much stable dry air to overcome between now and Thursday, followed by higher wind shear later in the week.
Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 5-6 days from now.
I'll have an update Wednesday morning.
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