Disturbance 92L continues towards Lesser Antilles Islands

A tropical wave about 650 miles east of the Lesser Antilles Islands (92L) continues be a threat to turn into a tropical depression as it moves west-northwest at 10-15 mph. This morning's QuikSCAT pass (Figure 1) showed that 92L did not have a closed circulation, but rather an elongated 600-mile long "convergence zone" along which the winds moved in opposite directions. Visible satellite loops have shown an increase in heavy thunderstorm activity and organization since yesterday. Surface spiral bands have formed to the west and north, and there is now upper-level outflow to the north visible as cirrus clouds streaming away from the center. 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 1. QuikSCAT image from the morning of Tuesday August 12, 2008. Two sweeps of the QuikSCAT satellite are shown. The data on the right swath was taken at 7:48 GMT (3:48 am EDT), and shows the center of circulation of disturbance 93L at the far right of the image. The colors of the wind barbs show winds of 10-20 knots circulating around the center. After the QuikSCAT satellite scanned 93L, it passed all the way around the planet and returned over the Atlantic again at 9:29 GMT (5:29 am EDT), when it scanned the region over disturbance 92L. Note the elongated region with winds moving in opposite directions where 92L is. This disturbance does not have a closed center of circulation. Also note the presence of some bad data along the right edge of the data swath; bad data is common at the edges of QuikSCAT swaths. Image credit: NOAA/NESDIS/ORA.


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 5-10 knots, and is forecast to remain below 10 knots for the next two days, then increase to 15-20 knots Thursday night through Saturday, thanks to strong upper-level winds from the northwest. The models are split on whether 92L will develop or not. The 2 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 1 hurricane that plows into the Bahama Islands on Saturday. In contrast, the 2 am EDT HWRF model has 92L at tropical depression strength on Saturday, when it enters the Bahama Islands. The National Hurricane Center is giving 92L a high (>50% chance) of becoming a tropical depression by Thursday morning. Given the continued trend in visible, infrared, and water vapor satellite imagery, 92L will probably become a tropical depression by Wednesday night. A big question is how the storm will adjust to formation of a closed center of circulation; it will lose much of the heavy thunderstorm activity currently along its long axis of converging winds, and this will ultimately affect the track of the storm. 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 their first visit to 92L on this afternoon.

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 shown a decrease in heavy thunderstorm activity since yesterday. Wind shear has fallen to a modest 10 knots over the disturbance. Water vapor satellite imagery shows that 93L is embedded in a large area of moist air, with some dry air and Saharan dust to its north.


Figure 1. Latest satellite image of 93L.

The forecast for 93L
Water temperatures are a warm 28° and forecast remain near 28°C for the next five days. Wind shear is forecast to remain 10-15 knots through Thursday morning, then drop below 5 knots. This should allow 93L to develop into a tropical depression by Friday. Dry air may begin to be a problem for 93L beginning on Thursday, as it works its way a bit further to the north where a dry Saharan Air Layer (SAL) exists. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Thursday morning. This storm could threaten the Lesser Antilles 6-7 days from now, but may also recurve out to sea.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 7 days from now.

I'll have an update between 3-5 pm EDT today.
Jeff Masters