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Carolina tropical depression strengthening, Caribbean disturbance disorganized

By: Dr. Jeff Masters, 3:43 PM GMT on July 19, 2008

Tropical Depression Three continues to organize off the coast of South Carolina. Wilmington, NC long range radar shows the steady development of solid spiral bands of thunderstorms on all sides of the center, and these bands have begun bringing heavy rains to the North Carolina coast. Rainfall amounts as high as 3 inches have fallen just north of Wilmington, NC, as estimated by weather radar (Figure 1). TD 3 is over waters of marginal warmth, about 27°C, a degree above the threshold of 26°C needed to sustain a tropical storm. The warm waters are very shallow, and the Tropical Cyclone Heat Potential is near zero, meaning we're unlikely to see rapid intensification of TD 3. Wind shear is about 15 knots over the storm, and is forecast to fall below 10 knots by Sunday. There is some dry air over the Southeast U.S. that may get entrained into TD 3's circulation. TD 3 should be able to intensify into a weak tropical storm, if the center manages to remain over water. None of the models forecast that TD 3 will intensify beyond a 50 mph tropical storm, due to the relatively cool water temperatures, moderate wind shear, and the presence of dry air nearby. I think TD 3 is organizing a bit more than the models are expecting, and has a 50% chance of becoming a 50 mph tropical storm by Monday. Coastal areas of North Carolina are likely to get heavy rains from TD 3, but it remains to be seen if these rains will penetrate far enough inland to significantly alleviate moderate drought conditions that exist over eastern portions of the state. Heavy rains may also develop over South Carolina, but the main rain will probably remain offshore as the storm passes that state. TD 3 is unlikely to affect any other states, with the possible exception of Cape Cod and Nantucket, Massachusetts Monday/Tuesday.

Links to follow:
Wilmington, NC long range radar
Southeast U.S. Marine observations and forecasts
Wilmington, NC weather

Figure 1. Latest radar estimated rainfall from TD 3.

Caribbean disturbance 94L
A well-organized disturbance in the Central Caribbean (94L) is bringing heavy rain and gusty winds to Haiti and Jamaica today. Kingston, Jamaica recorded sustained winds of 31 mph at 7 am local time, but the winds have since died down. Winds at the Central Caribbean buoy south of Jamaica have been sustained at 34 mph this morning. Satellite loops show no surface circulation, and the cloud pattern is disorganized. This morning's QuikSCAT pass missed 94L. An upper-level cold low centered over Cuba is bringing about 20 knots of wind shear to the northern portion of 94L, but the southern portion of the disturbance is under only about 10 knots of wind shear. A Hurricane Hunter flight will examine 94L this afternoon.

Links to follow:
Kingston, Jamaica weather
Central Caribbean buoy

The upper-level low bringing high wind shear to 94L has weakened considerably in the past six hours, and is forecast to slide westward and continue to weaken over the next two days. Very low wind shear of 5 knots or less should be over the disturbance Sunday. The low shear combined with the warm (28.5°C) water of the Western Caribbean should allow 94L to finally organize into a tropical depression by Sunday. NHC is giving 94L a high (>50% chance) of developing into a tropical depression by Monday morning. The disturbance has time to potentially strengthen into a tropical storm with 50 mph winds before coming ashore on the Yucatan Peninsula sometime between Sunday night and Monday afternoon. Passage over the Yucatan will no doubt significantly disrupt what should be a relatively weak system, and it is unclear what threat, if any, the storm will pose to the Gulf of Mexico coast. Once it does cross into the Gulf, wind shear should be low enough to permit development. The models favor a second Mexican landfall just south of Brownsville, Texas. There is a excellent chance that 94L will bring welcome drought relief to the parched regions of Southern Texas and northern Mexico along its path, regardless of whether the system develops into a tropical storm.

Bertha sets a new record
Hurricane Bertha is now the longest-lived July hurricane on record (and the longest-lived hurricane so early in the season). Bertha has been a hurricane 7.5 days, which eclipses the previous record of 7 days held by Hurricane Emily of 2005. Bertha is also the longest-lived tropical storm on record for July (and for so early in the season), as well as the farthest east forming tropical storm and hurricane for so early in the season. I find it amazing we've had a July storm that has lasted nearly 17 days! Bertha's days are numbered, though. Bertha will reach cold water less than 70°F by Sunday, which should finally kill it.

Elsewhere in the tropics
Three of our four reliable computer models for are predicting a new tropical depression will develop off the coast of Africa 5-7 days from now. The level of tropical activity the past week has been quite remarkable, and is more typical of September than July. It's a very good thing that sea surface temperatures are more than 1°C cooler than during the record-breaking Hurricane Season of 2005. As it is, the nature of this unusually high early season activity likely means we are in for a busier than average main portion of hurricane season, August-October.

I'll have a full update Sunday morning, and perhaps a short update later today if we get a new named storm.

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.