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By: Dr. Jeff Masters , 4:15 PM GMT on July 06, 2008
Tropical Storm Bertha continues west-northwestward over the open Atlantic, and has now moved over some warmer Sea Surface Temperatures (SSTs) of 26°C. Wind shear has remained about 10-15 knots, and the storm appears to be responding to these slightly more favorable conditions by generating a larger area of heavy thunderstorm activity near the center. The tops of these more vigorous thunderstorms reach higher into the atmosphere, and thus show up as colder in infrared satellite loops. This morning's QuikSCAT pass showed winds of about 50 mph north of the center. Visible satellite loops show some increased organization of Bertha's low level spiral bands, and it appears the storm is beginning to intensify. Additional data on Bertha's current state will be available Monday afternoon, when the Hurricane Hunters are scheduled to pay their first visit.
Figure 1. Oceanic heat content along Bertha's forecast track. At 8am EDT today, Bertha had just finished crossing a region of near zero heat content (black colors), but is poised to cross a region of increasing heat content over the next 120 hours. The oceanic heat content is a measure of the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. A high heat content in excess of 80 kilojoules per square centimeter is very beneficial to rapid intensification. Bertha is not passing over anything that warm in the next five days, so only slow intensification is anticipated. Image credit: NOAA/CIRA/RAMMB.
The intensity forecast
SSTs will warm quickly to 27°C by Monday morning and 28°C Tuesday morning underneath Bertha, and shear is expected to remain moderately low, in the 10-15 knot range. This should allow Bertha to intensify into a Category 1 hurricane by Tuesday. On Wednesday, Bertha is expected to hit a branch of the subtropical jet stream that will raise shear levels to 15-20 knots. The models have been steadily weakening the trough of low pressure associated with this shear in recent runs, and it now appears that the shear will not be strong enough to weaken Bertha, although it may be able to keep Bertha from intensifying further.
The track forecast
The models are now in better agreement that Bertha will track well north of the northernmost Lesser Antilles Islands, and tropical storm conditions are not likely in the islands. Bertha should continue to the west-northwest through Thursday, when a moderately strong trough of low pressure is forecast to exit the East Coast of the U.S. This trough is expected to turn Bertha to the northwest, and there is the potential for Bertha to affect Bermuda 6-8 days from now. Climatology, and the few models that we have that run out that far--the GFS and the ECMWF--suggest that this trough will be strong enough to fully recurve Bertha so that it misses the U.S. However, long range forecasts of this nature are highly erratic in their reliability, and if this trough is not strong enough to recurve Bertha, the storm may pose a risk to the U.S. East Coast 7-10 days from now.
Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic, and none of the models are forecasting tropical storm formation in the next seven days.
I'll post an update Monday morning.
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