Hanna is struggling with some extremely high wind shear of 30 knots, thanks to strong upper-level winds from the north. It's amazing that the storm has held together in the face of this shear, but Hanna is definitely suffering. Satellite loops show the shear has destroyed all of Hanna's heavy thunderstorm activity except on the south side. These heavy thunderstorms are currently pummeling Haiti.
The track forecast for Hanna.
The current steering flow driving Hanna to the west-southwest is very weak, and we can expect erratic motion over the next day. By Wednesday, a rather strong high pressure ridge will build over Hanna, forcing it northwest to a landfall in the Southeast U.S. Due to the storm's expected rather random motion over the next day, plus the expected track of Hanna parallel to the Southeast U.S. coast, the location of final landfall has a much higher uncertainty than usual.
The intensity forecast for Hanna.
The wind shear of 30 knots is forecast to gradually relax to 10-15 knots by tomorrow morning, and remain in the 10-20 knot range until landfall Friday. This reduced shear should allow Hanna to intensify, as the storm will be over warm 29°C waters with a Tropical Cyclone Heat Potential (TCHP) of 50-70, just below the value of 80 typically associated with rapid intensification. The GFDL and HWRF models respond by intensifying Hanna to a Category 2 hurricane by landfall; the SHIPS model foresees a Category 1 hurricane. However, these models did not anticipate Hanna's current disorganization. Given the current disorganized state of Hanna, Category 1 strength is probably the maximum the storm has time to achieve before landfall.
By Jeff Masters at the weather lab.
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