Ernesto is on the way

Tropical Depression Five is looking much more organized this morning, and will likely be named Ernesto with the NHC 11am EDT advisory. Satellite intensity estimates from NOAA using a the standard "Dvorak technique" are the same for Debby and TD 5 this morning. Since Debby has a name, Ernesto should getting his name, as well. The visible satellite images from this morning show a large and expanding area of intense thunderstorms, some solid spiral bands forming, and the beginnings of some decent upper-level outflow. A QuikSCAT satellite pass at 7:40am EDT showed winds of up to 50 mph near the center, and I imagine this will be the maximum sustained wind speeds for the 11am advisory.


Figure 1. Current satellite image of TD 5.

Wind shear
As usual, the major question is, "what is wind shear doing, and what is the wind shear forecast?" My top hurricane bookmark is the real-time wind shear analysis done by the University of Wisconsin's CIMSS group. This image, updated every 3 hours from data taken by the GOES-East satellite, shows yellow contour lines of what the shear is, in knots. Pink arrows show what direction the shear is coming from. The shear is the difference between the winds at high altitude (200 mb) and low altitude (850 mb). Hurricanes like wind shear close to zero, but can tolerate shear up to 20 knots and still slowly intensify. As we can see from the winds shear plot from this morning at 9Z (5am EDT) (Figure 2), Tropical Storm Five was under about 10 knots of shear, from the west. This is low enough to allow some intensification, and that is what we are seeing this morning. However, Tropical Storm Debby was under about 20 knots of shear, and has not been able to intensify this morning.


Figure 2. Wind shear estimate at 9Z (5am EDT) Fri Aug 25 2006. Image credit: University of Wisconsin's CIMSS group.

To the west of TD 5 we see a large area of very high shear of up to 50 knots, south of Jamaica. This area of high shear is associated with a strong upper level trough of low pressure that has been a common feature over the Caribbean this season. The trough is moving westward, away from TD 5, but TD 5 is racing fast enough westward that it may catch up to this high shear area and get torn apart on Sunday. The 8pm UKMET and 2am GFS models both have TD 5 dissipating by Sunday, and this is reasonable forecast. However, the trough is forecast to split apart on Monday and leave a region of low shear very favorable for intensification of TD 5 in the western Caribbean. If the storm does manage to survive into Monday, we could end up with a dangerous hurricane in the Gulf of Mexico by the middle of next week. The latest runs of the GFDL and Canadian models prefer this solution, bringing the storm across Cuba and into the eastern Gulf of Mexico as a hurricane. This is also a reasonable forecast. If you have travel plans to Jamaica, Cuba, or Cancun in the coming days, I would wait as long as possible to change them until we see if TD 5 will survive.

Debby
Tropical Storm Debby remains a minimal tropical storm today, with 20 knots of vertical wind shear putting a damper on her intensification. She could slowly intensify in response to some warmer waters along her path the next two days. By the time Debby turns north this weekend, she could attain hurricane status, but this is looking unlikely now. Early next week, Debby is expected to get caught up in the jet stream and die in the North Atlantic.

New African wave
Several of the computer models are predicting that the tropical wave coming off the coast of Africa today will develop into a tropical storm early next week.

Next update
The next Hurricane Hunter mission into TD 5 is at 2pm EDT this afternoon, and I'll have an update when the reports from their mission come in. Also, I'm getting a lot of questions about which models are best, where to find model output, etc, and I will try to post a quick summary of this info this afternoon.

Jeff Masters