August Hurricane Outlook Part III: Wind Shear and African Dust; update on 92L and 93L

By: Dr. Jeff Masters , 8:40 PM GMT on August 11, 2008

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Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.


Figure 1. Departure of wind shear from average for the 31 days ending August 9, 2008. Image credit: NOAA. Units are in meters per second; multiply by two to convert (approximately) to knots.

Wind shear during the past 31 days over the Main Development Region (MDR), between 10 and 20° N, extending from the coast of Africa to Central America, is shown in Figure 1. Recall that the MDR is where 85% of all major hurricanes form, and 60% of all tropical storms and weaker hurricanes. Shear has been above average over the Caribbean (blue colors), peaking at 4-6 m/s above average over the Western Caribbean. Wind shear has been below average over most of the rest of the MDR. Overall, wind shear has been a little below average so far this hurricane season over the tropical Atlantic.

However, the GFS model is predicting that very low levels of wind shear will affect much of the Main Development Region (MDR) during the coming two weeks (Figure 2). In particular, the Caribbean will see some of the lowest levels of wind shear it has seen all season. The long-range wind shear forecast from NOAA's CFS model (Figure 3) foresees wind shear values of 2-4 m/s (4-8 kt) below average across most of the Main Development Region during the peak months of hurricane season.


Figure 2. GFS model wind shear forecast for Tuesday, August 19, 2008. The forecast was made on Monday, August 11, at 2am EDT. Wind shear is the difference in wind between 200 mb (roughly 40,000 feet in altitude) and 850 mb (roughly 5,000 feet in altitude), in meters per second. Multiply this number by two to get the approximate wind shear in knots. In most circumstances, wind shear below 12 knots (6 m/s, the lighter red colors) is conducive for tropical storm formation. Note the very low wind shear, less than 10 knots, forecast for most of the Atlantic Main Development Region (white box) a week from now.


Figure 3. Forecast wind shear anomaly from NOAA's CFS model for the peak months of hurricane season, August, September, and October 2008. Wind shear is forecast to be 2-4 m/s (4-8 kt) below average across most of the Main Development Region (MDR) for Atlantic hurricanes. Image credit:NOAA's Climate Prediction Center.

If these wind shear forecasts come true, we are in for a very active main portion of hurricane season, since wind shear is typically the single most important factor inhiibiting tropical storm formation.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and dust activity begins to subside in August. The Sahel region of Africa has seen three straight years of average to above-average rains, which should result in soil stabilization and fewer dust outbreaks than average this year. So far this August, dust activity has (subjectively) appeared near average. Dust levels are very difficult to forecast, as they depend upon levels of soil moisture. Measurements of soil moisture in western Africa are almost non-existent, so the models must guess how dry the soil is. There are several groups that produce short-term forecasts (3-5 days) of Tropical Atlantic dust levels, including the Tel-Aviv University and the U.S. Navy (Figure 4). I don't find these forecasts particularly useful for forecasting tropical storm development, and I don't know how accurate they are. They may be valuable to residents of the region who want to know when they might be impacted by African dust.


Figure 4. Forecast dust concentrations (in micrograms per cubic meter) from the U.S. Navy's NAAPS model. The model was initialized at 8 am EDT Monday August 11, 2008. The model is predicting low levels of African dust over the eastern Atlantic at week's end.

Update on disturbance 92L approaching Lesser Antilles
A tropical wave about 800 miles east of the Lesser Antilles Islands (92L) has had its center reform today farther to the north. This location is difficult to pinpoint (I put it at 13.5N 48W at 4 pm EDT), but is farther away from the dry air to its west. The center was forced to re-form after dry air disrupted the original circulation early this morning. Thanks to the center re-location, dry air is troubling 92L much less, and visible satellite loops show a steady increase in heavy thunderstorm activity and organization, and surface spiral bands have formed to the west and north. Water vapor satellite loops show that a large area of dry air and Saharan dust lies to the west of 92L's center, and this dry air does not appear to be getting drawn into the storm's center any more.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a warm 28.2°C and forecast to increase to 28.8°C five days from now. Wind shear is forecast to remain below 10 knots for the next four days, then increase to 10-15 knots five days from now. This environment is favorable for intensification. 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 55 mph tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday. In contrast, the 2 am EDT HWRF model does not develop 92L at all. The National Hurricane Center is giving 92L a medium (20-50% chance) of becoming a tropical depression by Wednesday afternoon. This is a step down from their forecast last night of a high (>50%) chance of developing, and reflects the uncertainty that 92L will be able to get organized in the face of significant dry air to its west. However, given the recent trend in visible, infrared, and water vapor satellite imagery, I believe 92L has sufficiently insulated itself from the dry air to the west, and is destined to become a tropical depression by Tuesday night or Wednesday. Residents of and visitors to the northern Lesser Antilles should anticipate the possibility of a tropical depression or tropical storm arriving in the islands as early as Wednesday afternoon (though Thursday morning is more likely). The southern islands are not likely to be affected. Puerto Rico could be affected by Thursday, the Dominican Republic by Friday, and the Bahamas by Saturday. The U.S. East Coast could be affected by this system early next week, although it is too early to assess whether the system may end up recurving out to sea or not. The Hurricane Hunters are scheduled to pay their first visit to 92L on Tuesday afternoon.

Disturbance 93L off the coast of Africa
A tropical wave near 11N 29W (93L), about 400 miles southwest of the Cape Verde Islands, has shown a steady increase in heavy thunderstorm activity today, although there is almost no heavy thunderstorm activity near the center of circulation. Wind shear is a high 20 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 drop to 5-10 knots by Tuesday morning, and remain below 10 knots through Friday. Dry air may begin to be a problem for 93L beginning on Wednesday, as it works its way a bit further to the north where a dry Saharan Air Layer (SAL) exists. Odds are, 93L will be able to develop into a tropical depression by Wednesday. The GFDL, HWRF, and SHIPS intensity models all predict 93L will be a hurricane by Saturday. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Wednesday afternoon. This storm could threaten the Lesser Antilles 6-7 days from now, but preliminary indications are that it will pass to the north of the islands.

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.

Tune in tomorrow!

Jeff Masters

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.