TD 18: the one to watch

The forecasters at the National Hurricane Center are pretty busy this weekend, writing advisories for two systems in the Atlantic and three in the eastern Pacific. Thankfully, Ophelia is finally gone, but we are watching another wave of the coast of Africa that might develop into a tropical depression. But for now, we can ignore all of these systems except one--Tropical Depression 18, which is expected to build into Tropical Storm Rita later today. The NHC agreed with this assessment last night, taking the unusual step of diverting a hurricane hunter airplane--the mission bound for Philippe was recalled and sent to TD 18 instead. The Florida Keys and Texas are at greatest risk from this developing storm.

Current status of TD 18
TD 18 is over the Turks and Caicos Islands, which one can think of at a western extension of the Bahama Islands. After an initial west-northwest motion last night, TD 18 has settled into a track 10 degrees north of due west which will take her through the Bahama Islands today and tomorrow, and threaten Cuba and the Florida Keys after that. TD 18 is devloping a little faster than Philippe did, and showing some solid deep convection over the center, good spiral banding on the northwest side, and some decent upper-level outflow on the north side. The shear over TD 18 has decreased steady the past 48 hours, and is now down to 5 - 10 knots, which should support some moderate intensification. This shear is expected to remain constant or decrease slightly over the next 24 hours. The upper-level winds look favorable--an upper-level anticyclone has developed on top and will provide favorable outflow. Water temperatures are very warm--30 to 31C. However, there are two factors which will probably prevent TD 18 from attaining hurricane status in the next three days:

1) An upper level low pressure system over Cuba and Jamaica. This upper low is positioned to the southwest of TD 18, and the counterclockwise flow of air around the upper low will bring 5 - 10 knots of shear to TD 18's southwest flank over the next few days. The upper low is forecast to move west and maintain it's position on the southwest side of TD 18. However, after three days, the low is forecast to weaken, which would generate the lower shear environment TD 18 would need to grow into a hurricane.

2) Interaction with Cuba and the Yucatan Peninsula. TD 18's current track may put it on top of Cuba Tuesday. One model, the UKMET, indicates TD 18 will cross Cuba and get tangled up with the Yucatan Peninsula. Interaction with either of both of these land areas will likely prevent TD 18 from attaining hurricane strength until it can clear them and get its entire ciculation into the Gulf of Mexico.

Track of TD 18
The strong ridge of high pressure steering TD 18 to the west is forecast to persist for the next five days. The computer models differ some on how strong this ridge will be after TD 18 clears the tip of the Yucatan Peninsula, with the UKMET and Canadian models taking the storm west-southwest into Mexico, the GFS and NOGAPS taking the storm towards south Texas, and the GFDL taking it into Louisiana. The strength of a trough of low pressure expected to swing across the Midwest U.S. late in the week will be crucial toward determining the where the storm ultimately makes landfall along the Gulf Coast. The strength of this trough is impossible to determine at this point. I believe that Texas is at greatest risk, judging by the current forecast guidance.

Intensity forecast for TD 18
Both of the major computer intensity forecast models, the SHIPS and GFDL models, make Rita a Category 1 hurricane by Wednesday, as it passes through the Florida Keys. It is unlikely TD 18 could intensify to a stronger than Category 1 hurricane before it hits the Keys, since it's current state of organization is relatively poor, and there will still be some shear of the southwest side affecting the storm the next two days.

Assuming the storm misses Cuba, there is nothing I can see to prevent Rita from becoming a major Category 3 or higher hurricane in the Gulf of Mexico by Friday. Shear is forecast to be light, and water temperatures are at record high levels. But if Rita hits Cuba or the Yucatan Peninsula, it will be difficult for her to regroup quickly enough to attain more than Category 2 hurricane strength.

Philippe
Tropical Storm Philippe continues to organize, and has all the appearance of a system on its way towards becoming a large a powerful major hurricane. Philippe's track looks good for everyone except the residents of Bermuda. I looked through the tracks of all hurricanes that passed close to the point where Philippe is forecast to be five days from now, a few hundred miles south of Bermuda. Only three hurricanes in the past one hundred years have gone on to hit the U.S. that passed through a location that far to the north and east. The longer range computer models do suggest Philippe may take a more westerly motion towards the U.S. late in the week, but the odds of a trough of low pressure picking up Philippe and recurving him out to sea are high. I seriously doubt Philippe will affect any land areas except perhaps Bermuda.

ITCZ disturbance
The Intertropical Convergence Zone (ITCZ), remains active. A tropical wave in the ITCZ located about 900 miles west-southwest of the Cape Verde Islands has acquired a low level circulation and some deep convection, and may grow into Tropical Depression 19 later in the week as it moves west-northwest over the open Atlantic Ocean.

Jeff Masters