Tropical wave approaching the Caribbean a threat; Debby gets her name

Forget about newly-named Tropical Storm Debby, now churning west-northwestward into oblivion in the open Atlantic. The area we need to focus on today is a tropical wave near 10N 53W, about 500 miles east of the southernmost Lesser Antilles Islands. This new wave is a threat to develop into a tropical depression later this week, once it crosses into the Caribbean. NHC has assigned this disturbance the name "Invest 97L", and has tentatively tasked the Hurricane Hunters to investigate it on Thursday afternoon.

While the wave does have the potential to eventually become a serious hurricane, it also has a number of hurdles to overcome, and it is more likely that it will never become a hurricane. Firstly, while satellite imagery does show some rotation at middle levels of the atmosphere, a pass by the QuikSCAT satellite at 5:11am EDT today showed no rotation at the surface, and just a slight wind shift. The system will have to develop a lot more spin, which will take time. There have been some intermittent bursts of heavy thunderstorm activity, which generated winds of up to 30 mph seen on the recent QuikSCAT pass, but the cloud pattern is very disorganized at present. Wind shear is favorable, a low 5-10 knots, but is a very high 30-40 knots just to the north, and any movement of this high shear zone to the south--or 97L to the north--might disrupt it. A large area of dry air and Saharan dust lies to the north, and this will interfere with development. The system should move through the Lesser Antilles Islands just north of the South American coast on Thursday and Friday, and this region of the Caribbean has been climatologically unfriendly to developing tropical storms. The presence of the South American land mass so close cuts off a key source of moisture for a developing storm, and many vigorous looking disturbances and tropical storms have died in the southeastern Caribbean.

The computer models forecast that wind shear over the south half of the Caribbean will remain low the rest of the week, so the further south 97L can stay, the more likely it is to develop. The 8pm EDT run of the GFDL last night did develop it into a tropical storm by tomorrow, but this model did a poor job with the track and took it too far north. The GFDL, UKMET, and NOGAPS models do not develop it. The Canadian model develops it early next week once it enters the central Caribbean south of Jamaica, and forecasts it will move into the Gulf of Mexico. If this system is going to develop, I suspect that the Canadian model has the right idea, and development will not occur until 97L reaches the central or western Caribbean.


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 got a name last night, and is a modest 45-mph tropical storm headed west-northwest towards the north central Atlantic. Debby is expected to turn more northwestward over the weekend, and get pulled northwards and recurved into the prevailing westerly winds at high latitudes over cold waters early next week. Debby is not a threat to any land areas. The storm is in a moderately favorable environment for intensification, and could eventually make it to Category 1 hurricane status.

I'll have an update this afternoon.

Jeff Masters