A tropical disturbance (Invest 99L) near the Cayman Islands is drifting eastwards towards Jamaica, and has changed little in organization this morning, but is very close to tropical depression status. The storm is bringing heavy rain to the Cayman Islands; 3.85" inches has fallen over the past 48 hours at Savannah on Grand Cayman Island. Heavy rains will continue over the Cayman Islands today and spread to western Jamaica this afternoon. Recent satellite imagery shows that 99L has a well-defined surface circulation, but the center is exposed to view and 99L has a relatively meager amount of heavy thunderstorm activity. Wind shear is marginal for development, 15 - 20 knots, due to the clockwise flow of air around an upper-level high pressure system near the coast of Honduras. The high is bringing strong upper-level winds out of the southwest to 99L. Water vapor satellite loops show considerable dry air to the west and north of 99L, and the strong southwesterly winds over the storm are bringing some of this dry into into the core of the storm, keeping all the heavy thunderstorm development confined to the east side of the center. The waters beneath 99L are very warm, 29°C, but 99L will not be able to take advantage of these warm waters until the shear relaxes. The Hurricane Hunters will be in 99L around 11am EDT this morning to see if the storm is indeed a tropical depression.

Forecast for 99L
The latest SHIPS model forecast predicts that wind shear over the Western Caribbean will stay marginal for development, 15 - 25 knots, for the remainder of today, then decline to the moderate range, 10 - 20 knots, on Tuesday, as 99L positions itself more underneath the upper-level high near the coast of Honduras. Any motion by 99L to the southwest will tend to decrease the shear over 99L, and any motion to the north or east will increase the shear, so 99L's current eastwards drift is detrimental for development. Steering currents will be weak Wednesday through Friday in the Western Caribbean, making it difficult to predict where 99L may wander to, and how much shear might affect the storm. By Saturday, a ridge of high pressure is expected to build in to the north of 99L, forcing the storm on a generally westward track. This should allow 99L to find an environment with less shear. The GFDL and HWRF model predicts a more west-northwestward track, with 99L passing through the Yucatan Channel between Cuba and Mexico on Sunday or Monday as a hurricane. The GFS, ECMWF, and NOGAPS models predict a more west-southwesterly path, with 99L making landfall in Belize Sunday or Monday. NHC is giving 99L a 70% chance of developing into a tropical depression by Friday; I'd put these odds at 80%, and expect this will become Tropical Storm Richard by Thursday.


Figure 1. Morning satellite image of Invest 99L.

A tropical disturbance (Invest 99L) centered 160 miles southwest of the Cayman Islands is moving south to southeast at 5 - 10 mph. A Hurricane Hunter flight arrived in the storm at about 11am this morning, and found a closed circulation with top winds at flight level (700 feet) of 33 mph. A closed circulation and 30 mph surface winds are necessary conditions for a tropical depression to exist, but the storm must also have a great deal of heavy thunderstorm activity near the center that persists for many hours. In the judgment of NHC, 99L does not qualify as a tropical depression in that regard. The storm is bringing heavy rain to the Cayman Islands; 4.14" inches has fallen over the past 2 1/2 days at Savannah on Grand Cayman Island. Heavy rains have diminished over the Cayman Islands, but have spread to western Jamaica and west-central Cuba this afternoon. Recent satellite imagery shows that the surface circulation center is exposed to view, and 99L has a relatively meager amount of heavy thunderstorm activity. The center is more than 80 miles west of the heaviest thunderstorm activity, and it is likely that 99L's center will relocate itself to the east to be more underneath the heaviest thunderstorms. Wind shear is marginal for development, 15 - 20 knots, due to the clockwise flow of air around an upper-level high pressure system near the coast of Honduras. The high is bringing strong upper-level winds out of the southwest to 99L. Water vapor satellite loops show considerable dry air to the west and north of 99L, and the strong southwesterly winds over the storm are bringing some of this dry into into the core of the storm, keeping all the heavy thunderstorm development confined to the east side of the center. The waters beneath 99L are very warm, 29°C, but 99L will not be able to take advantage of these warm waters until the shear relaxes. A new hurricane hunter aircraft will be in the storm tonight near 8pm EDT.

Forecast for 99L
The current southward movement of 99L is carrying the storm into a region of lower wind shear, and we should see 99L accumulate more heavy thunderstorm activity near its center beginning tonight. The latest SHIPS model forecast predicts that wind shear over the Western Caribbean will decline below 15 knots Thursday afternoon through Saturday afternoon, which should allow the storm to become a tropical depression by Thursday. Steering currents will be weak today through Friday in the Western Caribbean, making it difficult to predict where 99L may go. The models are split into two camps, with the GFDL and HWRF models taking 99L to the west-northwest over the western tip of Cuba or the Yucatan Peninsula on Sunday as a hurricane. The rest of the models take 99L to the south over Honduras on Sunday, and keep the storm below hurricane strength. Given 99L's current southward motion and the possibility that the center will relocate farther to the east later today, this makes a track to the southwest towards Honduras more likely, I predict. NHC is giving 99L a 70% chance of developing into a tropical depression by Friday, which is a reasonable forecast. I expect this will become Tropical Storm Richard by Friday.


Figure 1. Afternoon satellite image of Invest 99L.

The Hurricane Hunters are in Tropical Depression Nineteen, and have found winds of tropical storm force that support upgrading the depression to Tropical Storm Richard. Between 8:15 - 8:30am EDT, the Air Force hurricane hunter aircraft measured surface winds of 40 - 70 mph with their SFMR instrument in the heavy thunderstorm region on the east side of TD 19's center. Winds at the aircraft's flight level of 1400 feet peaked at 46 mph. These measurements support upgrading TD 19 to at least a 40 mph tropical storm. Winds have been steadily rising this morning at NOAA buoy 42057, located about 50 miles west-southwest of the center of TD 19, on its weak side. Winds were 27 mph, gusting to 29 mph, at 7:43am EDT this morning. Recent satellite imagery shows that the surface circulation center of TD 19 is nearly exposed to view, thanks to moderate wind shear of 10 - 20 knots from upper-level southwesterly winds. TD 19 has a moderate and increasing amount of heavy thunderstorm activity which is getting more organized, with a curved spiral band forming on the storm's south side. The storm is bringing very heavy rain to Jamaica. Water vapor satellite loops show considerable dry air to the west and north of TD 19, and the southwesterly winds over the storm are bringing some of this dry into the core of the storm, keeping all the heavy thunderstorm development confined to the east side of the center. The waters beneath TD 19 are very warm, 29°C, but TD 19 will not be able to take full advantage of these warm waters until the shear relaxes and stops pushing dry air into the core.


Figure 1. Morning satellite image of TD 19.

Intensity forecast for TD 19
The latest SHIPS model forecast predicts that wind shear over the Western Caribbean will remain in the moderate range, 10 - 20 knots, through Friday morning. As the storm moves westwards on Friday, it will position itself beneath an upper-level high pressure system, which will allow shear to drop to the low range, less than 10 knots. With water temperatures a very warm 29°C and warm waters extending to great depth, TD 19 should be able to attain at least Category 1 hurricane strength by Saturday. NHC is currently giving TD 19 a 3% chance of becoming a major Category 3 hurricane. Given the latest data from the Hurricane Hunters and the latest set of computer models runs, I believe the odds are higher, near 30%. The main inhibiting factor for intensification will be the possibility of the dry air to the west of TD 19 getting wrapped into the core of the storm while it is trying to organize.

Track forecast for TD 19
Steering currents are weak in the Western Caribbean, and will remain weak through Friday morning, resulting in a slow, erratic movement for TD 19. A slow drift to the south is the most popular track expected by the major models. Thus, Jamaica can expect rains to increase and become torrential at times until Saturday, when the storm will finally move off. The Cayman Islands and possibly the north coast of Honduras can also expect very heavy rains of 3 - 6 inches Friday through Saturday. By Friday afternoon, a ridge of high pressure is expected to build in, forcing TD 19 to the west or west-northwest, bringing the storm to a landfall in Belize or Mexico's Yucatan Peninsula Sunday night. The models are divided on what might happen after Sunday, with the GFS, NOGAPS, and UKMET models indicating a continued west-northwest track taking TD 19 across the Yucatan Peninsula and into the Gulf of Mexico, where high wind shear would destroy the storm early next week before it could make landfall in the U.S. The other model solution, provided by the ECMWF, GFDL, and HWRF models, is for TD 19 to get caught up by a trough of low pressure moving across the Eastern U.S. early next week, which would take the storm to the northwest through the Yucatan Channel and into the west coast of Florida as early as Monday night. A band of very strong upper-level winds associated with the jet stream will be over the Gulf of Mexico early next week, so it is likely that if TD 19 follows this track, the storm will be weakening quickly as it approaches Florida. Either solution is possible, and we will have to wait to see what future model runs show will happen. The 5am EDT NHC wind probability forecast is giving the highest odds for tropical storm-force winds at Guanaja in Honduras and Cozumel in Mexico, at 34% and 37%, respectively. Key West is the only U.S. city being given odds, and these are just 3%. These odds will very likely rise with the 11am NHC advisory.

Invest 90L
A tropical wave that emerged off the coast of Africa yesterday (Invest 90L) has a modest amount of spin and some growing thunderstorm activity. NHC is giving the system a 10% chance of developing into a tropical depression by Saturday. This system is not a threat to cross the Atlantic and affect the Lesser Antilles or North America.