Gustav clears Jamaica, grows significantly in size

By: Dr. Jeff Masters , 3:26 PM GMT on August 29, 2008

Tropical Storm Gustav has completed its traverse of Jamaica and the center has now exited the western end of the island. At 7:06 am EDT, the Hurricane Hunters found the center of Gustav on the western tip of Jamaica. The storm weathered the passage over the island with relative ease, with the pressure only rising 5 mb, from 983 to 988 mb. Gustav's top winds remain below hurricane force, near 65 mph. Top winds measured in Jamaica during Gustav's passage were in the capital of Kingston, where sustained winds of 50 mph were measured last night. Preliminary news reports indicate that Jamaica suffered no major damage or injuries. However, the death toll has risen to 59 on Hispaniola, where flooding from Gustav killed 51 people in Haiti and 8 in the Dominican Republic.

Figure 1. Current satellite image of Gustav.

Visible satellite loops show that Gustav has significantly expanded in size in the past few hours, and now presents a rather intimidating appearance. Gustav is going to be a large and dangerous hurricane by Saturday. Upper-level outflow is established in all quadrants, low-level spiral bands are multiplying and intensifying, and the amount and intensity of Gustav's heavy thunderstorms are steadily increasing. There is no eye yet, but that should appear by this evening. Radar from Pilon, Cuba shows the developing spiral bands of Gustav quite well. Dry air is not evident anywhere close to Gustav.

The track forecast for Gustav
The latest 0Z/6Z (8pm/2 am EDT) model runs are more divergent than yesterday's runs, and the uncertainty of the long-term landfall location on the U.S. Gulf Coast remains high. In the short term, we're confident that Gustav will track over the Cayman Islands today, then cross the western tip of Cuba Saturday. Passage over the relatively flat terrain of western Cuba will probably decrease Gustav's winds by 20 mph, which it will regain within 12 or so hours.

The trough of low pressure moving across the Midwest U.S. expected to pull Gustav northwest towards the Gulf Coast is expected to weaken and be replaced by a ridge of high pressire on Monday. This means that Gustav may stall just before or just after landfall near Louisiana, and be forced westwards. The models are split: the GFS, HWRF, and GFDL models all carry Gustav into Louisaina or Mississippi on Monday, with a possible turn to the west after landfall. The ECMWF, NOGAPS, and UKMET all foresee a turn to the west before Gustav reaches the coast. The ECMWF and NOGAPS models predict an eventual landfall near Corpus Christi, Texas, while the UKMET bends Gustav southwestwards towards Mexico. NHC usually makes their forecast a blend of the GFS, GFDL, NOGAPS, and UKMET model forecasts. A forecast of that blend would show a track to the Texas/Louisiana border region. However, the current (11 am EDT) forecast cycle throws this cautious approach out the window, and gambles that the GFS/HWRF/GFDL model solution is correct. These models are currently forecasting a stronger hurricane, which would be less likely to be forced westwards. Residents of Texas should be prepared in case the alternative solution materializes, and Gustav takes a sharp turn to the west on Monday.

The intensity forecast for Gustav
Gustav has low wind shear (<10 knots), the highest heat content waters of the Atlantic, and no land masses or dry air to interfere with it. I expect it will intensify into a Category 3 or 4 hurricane before hitting Cuba.

Yesterday, a NOAA Hurricane Hunter research aircraft dropped a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data are now feeding directly into the GFDL and HWRF computer models, but not into the other global models (GFS, NOGAPS, UKMET, and ECMWF). In theory, the intensity forecasts from the GFDL and HWRF models should be better, for the portion of Gustav's track over the Gulf. Both of these models are currently calling for Gustav to be a Category 3 hurricane at landfall. The HWRF is saying this landfall will be near New Orleans Monday night, and the GFDL predicts a Mississippi landfall Monday morning.

Gustav's impact on Cuba and Mexico's Cancun/Cozumel region
Cancun and Cozumel are no longer in the cone of uncertainty, so a direct hit from Gustav is unlikely. Remember, though, that the cone is only right about 2/3 of the time--historically, over the past five years, about 1/3 of storm positions have fallen outside the cone. For example, the NHC forecasts issued at 5 pm, 8pm and 11pm Wednesday all put the cone of uncertainty along the northern portion of Jamaica, and the eventual track of Gustav directly over the island fell outside the cone of uncertainty. With that caveat in mind, those of you planning to visit Cancun/Cozumel will probably only have one day of heavy rain (Saturday), with some wind gusts of 40-50 mph. The Yucatan will be on the weak (left) side of Gustav, where tropical storm force winds do not extend out as far. The odds of Cozumel getting sustained winds of tropical storm force (39 mph or greater) have decreased to 23%, as indicated in NHC's wind probability product. The odds of hurricane force winds are 3%.

The portion of the world most likely to suffer a major hurricane strike from Gustav will be western Cuba. If Gustav makes landfall as a major hurricane somewhere along this stretch of coast, it may bring a storm surge of 10-15 feet to the right of where the eye come ashore.

I'll have much more to say about Gustav this afternoon, including an analysis of its likely size at landfall, and discussion about when it would be good to evacuate New Orleans.

Links to follow
Wundermap for the Cayman Islands
Pilon, Cuba radar
Cayman Brac, Cayman Islands
Grand Cayman

Tropical Storm Hanna
Tropical Storm Hanna is beginning to get its act together. The upper level low to Hanna's west has weakened some and moved away, allowing Hanna to position its center of circulation underneath its heaviest thunderstorm activity. Visible satellite loops show that the amount and intensity of heavy thunderstorm activity has increased considerably, and there are now two prominent upper-level outflow channels visible, on the north and east sides. Strong upper-level winds from the upper-level low to Hanna's west are still interfering with the thunderstorm development and upper-level outflow on Hanna's west side. Radar out of Martinique shows a large area of heavy thunderstorms that are poorly organized (no spiral bands). This morning's QuikSCAT pass saw winds of 40-55 mph at the surface. The first Hurricane Hunter mission for Hanna is scheduled for Sunday.

The forecast for Hanna
Steering currents imparted by the counterclockwise flow around the upper-level low to its west will keep Hanna moving northwest, to a point midway between Bermuda and the Bahama Islands. About 3-4 days from now, a strong blocking ridge of high pressure is forecast by most of the models to build over Hanna, forcing it to the southwest towards the Bahamas. This is an unusual motion for a hurricane, and it would be surprise to see Hanna move as far south as some of the models are predicting--all the way into Cuba. However, the Bahamas are at high risk from this storm 4-5 days from now. Hanna may be weakening at that time, as wind shear from an upper-level trough to the north of the storm is expected to bring 15-25 knots of shear to the storm. In the longer term, both the ECMWF and GFS models are predicting Hanna will pass through South Florida or the Florida Keys into the Gulf of Mexico by Thursday or Friday next week.

A major complicating factor in forecasting both Hanna's track and intensity may be the possible development of a tropical disturbance behind it, near 18N 41W (see discussion below, under "Elsewhere in the tropics". This disturbance is forecast to develop into a tropical storm 3-5 days from now by some of the models. If so, the new storm could substantially alter the path and strength of Hanna. Don't believe that Hanna will be going through South Florida quite yet; the models do very poorly with hurricane-hurricane interactions, and the long term fate of Hanna is still highly uncertain.

Elsewhere in the tropics
A large circulation is located near 18N, 41W, in the mid-Atlantic Ocean. This morning's QuikSCAT pass show an elongated, poorly formed circulation, with top winds of 25 mph. Heavy thunderstorm activity has increased since yesterday, but the disturbance is battling marginal wind shear of 15-20 knots and dry air on its south side. Wind shear is expected to stay marginal for development over the next few days, limiting this system to only slow development. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Sunday. Some slow development is possible over the next few days, but the system has a better chance 3-5 days from now, when its environment will be moister. Several of the models develop it, and predict the system will be 500 miles or so north of the northern Lesser Antilles Islands by Monday or Tuesday.

Cape Verdes Islands disturbance, 97L
A low pressure system (97L) with a large circulation and plenty of spin emerged from the coast of Africa last night, and is now located near 13N 19W, a few hundred miles southeast of the Cape Verde Islands. This morning's QuikSCAT pass revealed a closed surface circulation and top winds of 25 mph. This system is already developing some concentrated heavy thunderstorms over the waters south of the Cape Verde Islands. Wind shear is a moderate 10-15 knots over the storm, and is expected to remain in the low to moderate range the next few days. NHC has given this system a medium (20%-50%) chance of developing into a tropical depression by Sunday. The models have been very aggressive developing this system over the past few days. It is too early to speculate whether this storm might end up recurving out to sea or not.

There are two other impressive African tropical waves lined up behind 97L that are also likely to be a threat to develop once they move offshore Africa next week. The long-range GFS model develops all three of these waves.

Disturbance 96L in the Bay of Campeche
Some good news: the blob in the southern Gulf of Mexico, in Mexico's Bay of Campeche, dissipated last night.

My next blog will be by 5pm this afternoon.

Jeff Masters

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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