The Insiders' Guide to St. Barths: Weather August 14th and so on

TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
800 PM EDT WED AUG 13 2008

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

AN AREA OF LOW PRESSURE...ASSOCIATED WITH A TROPICAL WAVE...IS
CENTERED A COUPLE HUNDRED MILES EAST OF THE LEEWARD ISLANDS.
SHOWER ACTIVITY IS PRESENTLY LIMITED AND DISPLACED WELL TO THE
NORTHEAST OF THE LOW CENTER. HOWEVER...UPPER-LEVEL WINDS ARE
FORECAST TO GRADUALLY BECOME CONDUCIVE FOR DEVELOPMENT OVER THE
NEXT COUPLE OF DAYS...AND THIS SYSTEM COULD BECOME A TROPICAL
DEPRESSION ON THURSDAY OR FRIDAY AS IT MOVES THROUGH OR JUST NORTH
OF THE NORTHERN LEEWARD ISLANDS.

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.

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.

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