Tropical Depression Nine approaches the Lesser Antilles

Tropical Depression Nine has formed in the waters a day's journey from the Lesser Antilles Islands, and is headed west towards the islands at 20 mph. This storm could be trouble for much of the Caribbean, and may affect the mainland U.S. next week. TD 9 is under light wind shear of 5 - 10 knots, and is over waters of 28°C. A large area of dry air lies just to the north, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and will keep development slow today. This morning's visible satellite loops show that TD 9's heavy thunderstorm activity is limited on the north side, due to the dry air. TD 9 has yet to develop a good upper level outflow channel, which will also keep development slow today. An analysis of upper level winds from the University of Wisconsin CIMSS shows that the winds at high levels are blowing fairly uniformly from the east over the storm, and these winds would have to take on a more counter-clockwise pattern over TD 9 to produce the most efficient upper-level outflow. The clump of heavy thunderstorms to the southeast of TD 9 is also a limiting factor; this clump of heavy thunderstorms is competing for moisture and interfering with the low level inflow and upper level outflow of the storm. However, if TD 9 manages to wrap in this extra clump of heavy thunderstorms and add their spin to its own spin, it could become a very large and dangerous storm. Heavy thunderstorms are now attempting to fire up around TD 9's circulation center, but this is being hampered by dry air. The center of TD 9 passed about 20 miles to the north of buoy 41040 at 10 am this Tuesday morning. The buoy recorded top winds of just 18 mph this morning, suggesting that this is not yet Tropical Storm Isaac. The first hurricane hunter mission into TD 9 is scheduled for Tuesday afternoon.


Figure 1. Morning satellite image of TD 9.

Intensity forecast for TD 9
The latest 8 am EDT run of the SHIPS model predicts that wind shear will be low to moderate, 5 - 15 knots, for the next five days. Ocean temperatures will warm from 28°C this morning to 29°C by Wednesday afternoon, and the total heat content of the ocean will increase sharply during that period. The low wind shear and warm waters will favor strengthening. The main impediment to development through Wednesday will be dry air to the north, though the storm will also have trouble separating from the clump of thunderstorms to its southeast. I expect that TD 9 will continue to struggle with dry air through Wednesday. The official NHC forecast of a 60 mph tropical storm moving through the Lesser Antilles on Wednesday afternoon is on the aggressive side, and I put the most likely range of strengths for TD 9 Wednesday afternoon at 45 - 60 mph. Once TD 9 enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to a hurricane. Intensity forecasts 4 - 5 days out are low in skill, though, and it would not be a surprise at all to see TD 9 struggle like so many other storms have over the past two years, and remain a tropical storm over the next five days. Conversely, rapid intensification into a Category 2 hurricane five days from now, as the official NHC and HWRF model forecasts are suggesting may happen, would also not be a surprise.

Impact of TD 9 on the Islands
The entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday, as TD 9 and the associated area of heavy thunderstorms to its southeast passes through. Sustained tropical storm-force winds of 40 - 60 mph will occur in the islands only on Wednesday, with Guadaloupe, Antigua and Barbuda, Martinique, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

TD 9 will make its closest approach to the Virgin Islands and Puerto Rico on Thursday, and heavy rains from the storm will affect these islands Thursday through Saturday. Tropical storm-force winds should remain just south of these islands.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic. If the center of TD 9 remains offshore, as appears likely, heavy rains from TD 9 will still be a danger to the country. Rainfall amounts of 4 - 8 inches will likely affect the Dominican Republic Thursday through Saturday, creating flash floods and mudslides. We can't rule out a direct hit on the Dominican Republic on Thursday evening or Friday morning, since the country is well within the NHC cone of uncertainty.

TD 9 is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from TD 9 will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 5 - 10 inches are possible, which will be capable to causing extreme flooding on the vegetation-denuded slopes of Haiti.

Longer-range outlook for TD 9
Heavy rains from TD 9 will begin in Jamaica and eastern Cuba on Friday night, but our vision of where the storm might be headed after Friday gets blurry. The official NHC track forecast at long ranges is heavily weighted towards the GFS and ECMWF models, which have done the best job predicting TD 9's path so far. In general, the models have been predicting a track too far to the north for TD 9, and I expect the storm will remain south of Hispaniola, avoiding the disruptive impact of that island's high mountains. While the current NHC forecast has TD 9 hitting Cuba just north of Jamaica five days from now, keep in mind that the average error in a 5-day forecast is 260 miles. Given the tendency of the models to predict a track too far to the north for this storm, and this season's general steering pattern that has already taken two storms into Mexico's Yucatan Peninsula, we should not be surprised if TD 9 takes a more southerly path than the official NHC forecast, and potentially become a threat to western Cuba or Mexico's Yucatan Peninsula. A trough of low pressure capable of pulling TD 9 to the north enters Western Canada Thursday, and the exact timing and amplitude of this trough as it crosses the U.S. this weekend will determine the ultimate landfall location of TD 9. The ECMWF model has a more northwards position for the trough, and thus keeps TD 9 moving more slowly and father south than the GFS. Both of these models predict an eventual landfall for TD 9 over Florida, and TD 9 is a threat to affect Tampa during the Republican National Convention, August 27 - 30. I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. The odds in the current situation are higher, somewhere in the 1% - 3% range. It would take a "perfect storm" sort of conditions to all fall in place to bring TD 9 to the doorstep of Tampa as a hurricane during the convention, but that is one of the possibilities the models have been suggesting could happen.

Disturbance 95L in the Gulf near the Texas/Mexico border
A region of disturbed weather (Invest 95L) is in the Gulf of Mexico off the coast, just northeast of Tampico, Mexico. The disturbance is due to a trough of low pressure and its associated cold front which moved off the coast over the weekend, but has been fortified via moisture from Tropical Storm Helene, which made landfall Saturday near Tampico. In their 8 am EDT Tropical Weather Outlook, NHC gave 95L a 30% chance of developing into a tropical cyclone by Thursday morning. An Air Force hurricane hunter aircraft is scheduled to investigate 95L this afternoon. Winds at Tampico this morning were light out of the west, suggesting that a surface circulation may be trying to form. Radar out of Altamira, Mexico does not show any organization to the precipitation echoes, though, and visible satellite loops show that 95L is small and disorganized, and some arc-shaped low clouds expanding away from the storm on its north side suggest 95L is ingesting dry air that is creating strong thunderstorm downdrafts, robbing the storm of moisture and energy. The computer models show that 95L should drift westwards over Mexico by Wednesday.


Figure 2. Morning satellite image of 96L.

Disturbance 96L off the coast of Africa
A tropical wave in the eastern Atlantic 450 miles southwest of the Cape Verde Islands (Invest 96L) is headed west at 15 mph. This disturbance has an impressive amount of spin, as seen on visible satellite loops, and a respectable amount of heavy thunderstorm activity. The storm is under a light 5 - 10 knots of wind shear. Given that TD 9 has moistened up the atmosphere ahead of 96L, this disturbance should have less of a problem with dry air. In their 8 am EDT Tropical Weather Outlook, NHC gave 96L a 60% chance of developing into a tropical cyclone by Thursday morning. This disturbance will track nearly due west the next three days, then is expected to turn more to the west-northwest late this week, bringing it close to the northern Lesser Antilles Islands by Sunday.

Typhoon Tembin a threat to Taiwan
In the Western Pacific, we have Typhoon Tembin, which put on a very impressive burst of rapid intensification on Monday, going from a 50 mph tropical storm to a Category 3 typhoon with 125 mph winds in just 24 hours. Tembin is currently a Category 3 storm with 115 mph winds, and is expected to hit Taiwan on Thursday as a Category 2 typhoon. Farther out to sea, Typhoon Bolaven has formed northwest of Guam. Bolaven is expected to become a Category 3 typhoon as it heads toward China, and may pass very close to Taiwan this weekend.


Figure 3. Typhoon Tembin near the Philippines on Monday August 20, 2012. Image credit: NASA.

I'll have a new post this afternoon.

Jeff Masters