Strong tropical wave blows into the Lesser Antilles

A powerful tropical wave that has the potential to become a serious hurricane is sweeping through the southernmost Lesser Antilles Islands today, bringing tropical storm force wind gusts and heavy rains. Barbados reported sustained winds of 32 mph at 5am this morning. Togabo had wind gusts to 36 mph, and wind gusts to 43 mph were observed on St. Lucia. A QuikSCAT satellite pass from 6:30am EDT shows a large area of 35 mph winds to the north of the center, which appears to be near 11N 59W, near the islands of Trinidad and Tobago and just off the South American coast. The storm's organization has steadily increased since yesterday, and the Hurricane Hunters are tasked to investigate this afternoon to see if a tropical depression has formed. The storm has two main areas of intense thunderstorms visible on satellite imagery this morning that are almost disconnected. I expect the northern mass will dissipate today, since it is much farther from the center of circulation.

Wind shear is favorable in a small area over the storm--5-10 knots--and is forecast to remain low through the next five days. However, there is a zone of very high shear to the system's north, so the forecast of low shear could easily change. A large area of dry air and Saharan dust covers much of the eastern Caribbean, and may be a modest impediment to intensification. The primary difficulty for the storm lies in its close proximity to South America. The storm center may hug the coast through Saturday, limiting its development. The storm should bring heavy rains and winds near tropical storm force over the ABC Islands of Aruba, Bonaire, and Curacao on Friday. After that, there is a lot of uncertainty.

The computer models
The latest 8pm and 2am EDT computer model runs have a variety of solutions. The Canadian model continues to be very consistent and very gung-ho, developing 97L into a strong tropical storm on Saturday, south of Jamaica, then taking the storm into the Gulf of Mexico as a hurricane. The NOGAPS model is also consistent, assuming a more southerly track will occur with no development due to close proximity to the South American coast. The GFS takes a weak tropical storm across the Dominican Republic on Saturday, then into the Bahamas. The GFDL has the same idea, but has a much stronger system that becomes a Category 1 hurricane in the Bahamas on Monday.

The run-to-run consistency of the GFDL has been poor, and both the GFDL and GFS have not done a good job forecasting the initial track of the storm so far. A more southerly track betwen Jamaica and Honduras like the Canadian and NOGAPS models are suggesting is probably more reasonable. If 97L can survive the next two days and separate from the South American coast--which it has at least a 50/50 chance of doing--I believe it will probably develop into a serious hurricane, as the GFDL and Canadian models have been suggesting. The track such a hurricane might take is highly uncertain, but it appears that Jamaica, Cuba, and the Yucatan would be at highest risk in the Caribbean. No part of the U.S. coast can be ruled out as a target in the longer term.

Got travel plans to the Caribbean this week? Don't change them yet. This appears to be an all-or-nothing kind of situation, and we could get nothing. It may not be until Saturday that we have a reasonable idea if this storm will be a major threat.


Figure 1. Current satellite image of Invest 97L.


Figure 2. Preliminary model tracks for disturbance 97L approaching the Lesser Antilles Islands.

Debby
Tropical Storm Debby remains a minimal tropical storm today, but is expected to 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. Early next week, Debby is expected to get caught up in the jet stream and die in the North Atlantic.

I'll have an update this afternoon after the Hurricane Hunters check out 97L.

Why the 2am run of the GFDL failed
The 2am EDT run of the GFDL model failed on 97L this morning, so I was quoting the results from the 8pm EDT run last night. I got an email from Morris Bender of the GFDL project this morning on why the 2am GFDL run failed:

The vortex initialized from our initialization was very weak as the initializaiton process spins up the storm to match the observed initial winds. Since the easterlies in the lower level were very strong in the GFS analysis, it initialized a disturbance with very weak vorticity.

As you can see at hour 0 there was almost no circulation intially. As a result, our grid movement shut down at 3 hours and the inner nests could no longer follow the storm for the rest of the forecast.

The previous runs also had a very weak initial disturbance but there was enough of a pressure gradient that the inner nests continued to follow the vortex and eventually with the high resolution it developed into a signficant tropical cyclone.

In the 6z run, the weak distrubance moved out of the high resolution inner grid, into the coarse resolution, and so all we had left was a very weak disturbance that could not be resolved in the corase outer mesh.


As you can see, getting the computer models to work on weak disturbances is a difficult business! We should not put too much faith in the computer models for any weak system; it is too difficult for the models to get the starting conditions of the storm correct.

Jeff Masters