Major steering current shift coming; update on Lesser Antilles disturbance 92L

By: Dr. Jeff Masters , 7:28 PM GMT on August 13, 2008

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There is little new to report on 92L, the tropical disturbance now just 200 miles east of the Antigua in the northern Lesser Antilles Islands. Heavy thunderstorm activity remains limited, and dry air surrounding the storm continues to interfere with it. The latest crop of 12Z (8 am EDT) computer model runs are similar to the last set of runs, except the new runs are becoming more definitive about a threat to the Bahamas on Saturday and Sunday, and the east coast of Florida or Florida Keys on Monday. The National Hurricane Center continues to give a medium (20-50% chance) that 92L will be a tropical depression by Friday afternoon. I give a 20% chance that 92L will be a hurricane by Monday. An Air Force and a NOAA Hurricane Hunter aircraft are scheduled to investigate 92L simultaneously Thursday afternoon.


Figure 1. Latest satellite image of 92L.

Major shift in steering currents coming
As I discussed in last week's blog on steering currents, the hurricane steering pattern for all of July and the first two weeks of August over the North Atlantic has predominantly acted to recurve hurricanes out to sea. The jet stream has been "stuck" in a standing wave pattern, where it dips southward over the East Coast of the U.S., creating a trough of low pressure capable of recurving tropical storms once they get north of the Caribbean Sea (20° latitude). This pattern is in contrast to the steering pattern that set up in 2004 and 2005, when a ridge of high pressure set got stuck over the Eastern U.S. A ridge in this location does not allow hurricanes to recurve, and the U.S. took a terrific battering those years.

This year's steering pattern is about to make a major shift towards the steering pattern observed in 2004 and 2005. According to recent 500 millibar (mb) upper-air forecasts from the GFS model. and ECMWF model, the trough of low pressure over the U.S. East Coast will be replaced by a ridge of high pressure 7-10 days from now. As a result, the surface Bermuda High will extend far to the west over the Eastern U.S. This pattern will mean that fewer hurricanes will be recurving beginning a week from now, and the threat to the U.S. Gulf Coast will increase. Conversely, the threat to Bermuda and the Northeast U.S. will diminish.

There is no way of telling how long this new steering pattern might stay in place. It could last only a few days, or remain in place for several months.


Figure 2. Upper air charts showing the height where a pressure of 500 mb is forecast to be found this Saturday (top) and next Saturday (bottom). The forecast was made at 8 pm EDT Tuesday August 12 by the ECMWF model. Note that the axes where dominant troughs and ridges are found are predicted to reverse over this 1-week timespan, with a ridge of high pressure setting up over the eastern half of the U.S. This upper-air pattern is conducive to more hurricane activity along the Gulf of Mexico coast.

Links to follow
Martinique radar
Wundermap for the northern Lesser Antilles Islands
St. Thomas, Virgin Islands.

I'll have an update Thursday morning.

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.