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Dangerous Caribbean disturbance 94L: deja vu?

By: Dr. Jeff Masters, 1:47 PM GMT on August 25, 2008

Heavy thunderstorms associated with a tropical wave (94L) near 15N, 70W, in the central Caribbean south of the Dominican Republic, have grown more concentrated over the past day. This morning's QuikSCAT pass (Figure 1) did now show a closed surface circulation, but did show sustained winds of 52 mph north of the center. Visible satellite loops show evidence of rotation in the clouds at middle levels of the atmosphere, but no surface circulation as yet. The area covered by the heaviest thunderstorms is relatively modest. The storm has moistened its environment considerably, and dry air should no longer be a problem for 94L. Wind shear has fallen to a very low 2 knots over 94L. NHC is giving this system a high (>50% chance) of developing into a tropical depression by Wednesday morning, and has scheduled a Hurricane Hunter flight to investigate the storm this afternoon.

Figure 1. QuikSCAT image from 6:33 am EDT Monday August 25, 2008. The red "L" denotes the center of disturbance 94L, which has no closed circulation. The wind barbs show there are south winds to the east of the center and winds from the east to the north of the center, but there are no winds from the west to the south of the center, which one would have to have in order for there to be a closed surface circulation. Note the purple wind barb just north of the center, with four long tines and one short tine, indicating that we had winds of 45 knots (52 mph) there. Image credit: Paul Chang, NOAA/NESDIS/ORA.

The deja vu forecast for 94L
The models have had a tough time properly handling 94L, due to the fact they have been getting this system entangled with the other disturbance (95L) a few hundred miles to the northeast of Puerto Rico. The latest (2 am EDT Monday) GFDL forecast appears believable--and presents a strong case of deja vu. It's an almost exact repeat of Fay's track. The GFDL predicts 94L will continue to move northwest and hit the Haiti/Dominican Republic region on Tuesday, then get turned to the west by a strengthening ridge of high pressure. The storm will cross over to eastern Cuba on Wednesday, then travel along the length of Cuba through Friday night. On Saturday, the GFDL has 94L popping off the coast of Cuba at the same spot Fay did, then intensifying into a Category 1 hurricane that moves over Key West towards a landfall in Southwest Florida. While it is unlikely that the exact details of this deja vu forecast will come true, it does give one a general idea of the land areas 94L is likely to affect. The Dominican Republic and Haiti can expect 4-8 inches of rain on Tuesday and Wednesday from 94L, with isolated amounts up to 12 inches. These rains will cause flash flooding and dangerous mudslides. Wednesday through Friday, 94L is likely to bring heavy rains to Cuba, Jamaica, the northern Cayman Islands, and the southeast Bahamas. There are some models calling for 94L to track through the Bahamas and then northeast out to sea, so it may end up that the Bahamas will end up taking the brunt of this storm. However, I don't think this is likely, and a more westerly track into the Gulf of Mexico will occur.

Sea surface temperatures and total oceanic heat content in the Central Caribbean are very high, and 94L is in a environment very favorable for intensification. Wind shear is predicted to remain very low to moderate, 0-15 knots, for the next five days. An upper level high pressure system is currently sitting over central Cuba, and if 94L can position itself under this high, it will provide very favorable upper-level outflow conditions for the storm later this week. The main restriction on 94L's development will, like for Fay, be interaction with land. The islands of Hispaniola and Cuba will provide formidable obstacles to intensification.

Links to follow
Buoy 42059, north of 94L, has measured sustained winds of 30 mph this morning.

Tropical Depression Fay continues to spin away over the Southeast U.S., and is now centered over southern Mississippi. With its center over land, Fay is cut off from its oceanic source of sustenance, and now has no hope of reaching tropical storm strength again. Satellite loops show that Fay still has a large circulation with plenty of rain bands soaking the Southeast, and the storm is now headed east-northeast towards Alabama and Georgia. It will take 2-3 days for the huge amount of angular momentum energy stored in such a large vortex to dissipate as the storm slowly spins down. No models are calling for Fay's center to emerge over water again, and the "The Joker" is finally finished. Some maximum rain amounts from Fay as of 1 am CDT Monday (by state): Florida: 25.28" (Melbourne Beach); Georgia: 17.43" (Thomasville); Alabama: 6.55" (Camden); South Carolina: 5.84" (Beaufort); Mississippi: 2.88" (Starkville); Louisiana: 2.80" (Baton Rouge).

On the plus side, Fay's rains are now moving into some of the most drought-stricken regions of the country--northern Alabama and northern Georgia. In fact, there is now concern about flooding in these regions later this week, if the remains of Fay continue to linger and bring eight or more inches of rain (Figure 2). Fay's rains have now increased the level of Lake Okeechobee by 2.2 feet. The lake level stands at 13.41 feet, which is still about 2 feet below normal.

Figure 2. Total forecast rainfall for the five days beginning at 8 am EDT Monday August 25. Image credit: National Weather Service.

Disturbance 95L northeast of Puerto Rico
Another tropical wave (95L) a few hundred miles northeast of Puerto Rico, remains disorganized. Wind shear has increased to a marginal 15 knots over 95L, and is forecast to increase to a high 15-25 knots in coming days. NHC is giving 95L a low (<20% chance) of developing into a tropical depression by Wednesday.

Elsewhere in the tropics
Several of the computer models forecast that a tropical wave that moved off the coast of Africa yesterday will develop late this week.

My next blog will be this afternoon, once the Hurricane Hunters find out more on 94L.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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