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A Halloween hurricane?

By: Dr. Jeff Masters, 10:11 PM GMT on October 27, 2007

A surface low pressure system (90L), near 16N 71W, about 150 miles south of the Haiti/Dominican Republic border, is moving west to west-northwest at about 10 mph. Long range radar of of Puerto Rico shows bands of heavy rain continuing to affect the region. Satellite loops show most of the heavy thunderstorm activity is to the east of the low's center of circulation, but these thunderstorms have gotten more organized in the past few hours, and a more circular center has developed. Wind shear has fallen to 20-25 knots this afternoon, and is expected to fall below 15 knots on Sunday. This should allow 90L to develop into a tropical depression on Sunday.

Figure 1. Latest satellite rainfall estimate of 90L.

The system will continue to bring heavy rains and the threat of flash flooding and mudslides to Puerto Rico through Sunday night. Heavy rains of up to 6 inches have already fallen in southeast Puerto Rico and the Virgin Islands (Figure 1). Heavy rains will also affect the Dominican Republic and Haiti Sunday and Monday, and are likely to trigger life-threatening flash floods in Haiti.

This morning's 12Z (8am EDT) computer model forecasts have made a major change: the three major intensity forecast models--the GFDL, HWRF, and SHIPS models--all agree that 90L will intensify into a hurricane. The GFDL and HWRF predict 90L will move northwesterly across Haiti and western Cuba, and into the Bahamas on Tuesday, and intensify into a hurricane over the central Bahamas on Tuesday. A trough of low pressure would then swing 90L northeastwards out to sea. This forecast track seems unreasonable, as 90L has headed more to the west today than these models predicted.

The ECMWF and GFS models predict 90L will track along the length of Cuba early next week, then pass within 50 miles of Miami on Thursday before recurving northeastwards out to sea. These models do not intensify 90L into a hurricane, due to the amount of time the storm spends over the mountainous terrain of Cuba. This is a reasonable forecast, should 90L track over Cuba for a long distance.

I believe the forecasts of the UKMET and NOGAPS models, which predict a more southerly track into the Western Caribbean, just south of Cuba, are the most reasonable ones. This track would favor 90L developing into a hurricane, possibly a major hurricane, since the heat content of the waters in the Western Caribbean is high.

One possible wild card is the disturbed area of weather that has formed in the extreme Western Caribbean, just east of the Yucatan Peninsula. This disturbance is currently very disorganized, but is under only 10 knots of wind shear. If it starts to develop early next week, it could alter the path and development of 90L.

I am expecting 90L to develop into a tropical storm by Monday, and a into hurricane later in week, if the system does not track directly along the length of Cuba. The eastern 2/3 of Cuba, Jamaica, and the Cayman Islands should anticipate the possibility of a tropical storm affecting them on Monday and Tuesday. Later in the week, these regions, plus western Cuba, South Florida, the Bahamas, and Mexico's Yucatan, are at risk of a hurricane.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.