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Gustav pounds Jamaica

By: Dr. Jeff Masters, 8:31 PM GMT on August 28, 2008

Jamaica is feeling the full force of Tropical Storm Gustav, as torrential rains and near-hurricane force winds pound the island. At 1:55 pm EDT, the Hurricane Hunters found the eye of Gustav on the eastern tip of Jamaica. Gustav's central pressure had dropped another 2 mb, to 983 mb, and the surface winds were at 70 mph, just below hurricane strength. Kingston, Jamaica recently reported sustained winds of 35 mph, and the pressure falling, at 995 mb.

It's a surprise the Gustav is here at all, because the atmosphere pulled a major trick out of The Joker's book last night. The ridge of high pressure that was forcing Gustav to the west shifted positions to be oriented southwest-to-northeast. This pushed Gustav to the southwest, and pumped in some dry air into the northwest side of Gustav. As a result of this dry air, and the weakening of the circulation due to interaction with Haiti's mountains, Gustav was forced to form a new center under heavy thunderstorms on its south side, away from Haiti and the dry air. As a result, Gustav is now pounding Jamaica. The ridge that forced Gustav southwest has now re-oriented itself, and is pushing Gustav due west again. Rainfall will be the main threat from Gustav in Jamaica, as it was when the storm hit Hispaniola. Gustav's torrential rains accumulated to over 12 inches in southern Hispaniola, triggering floods and landslides that killed 22 people. It appears now that eastern Cuba will not get much rain from Gustav.

Figure 1. Tropical Storm Gustav at landfall in Jamaica.

Visible satellite loops show that Gustav has already suffered from its impact on Jamaica. A large Central Dense Overcast (CDO) of high cirrus clouds has become asymmetric, and the eye that appeared briefly before landfall has disappeared. Radar from Gran Piedra, Cuba still shows several well-formed spiral bands on the east and south sides of the storm. However, the northwest side of the storm shows evidence that dry air is interfering with Gustav's organization.

The track forecast for Gustav
The latest 12Z (8 am EDT) model runs continue to show a significant shift westward in Gustav's track, thanks to the southwestward motion and center re-formation of the storm this morning. Gustav may now pass through the narrow Yucatan Channel, and not be significantly weakened by Cuba.

By Saturday, a trough of low pressure moving across the Midwest U.S. should weaken the ridge, and allow Gustav to turn north near the tip of western Cuba into the Gulf of Mexico. The final landfall location of Gustav depends on the strength and speed of the trough. This trough may not be strong enough to pull Gustav to a landfall in the central Gulf Coast. Instead, a blocking ridge of high pressure may build in over the southern U.S. by Monday, forcing Gustav to move slowly westward towards Texas. This continues to be the solution of the 12Z run of the GFS model, but it is now the only model predicting this. The best guess now is that the ridge will slow Gustav down, but allow it to make landfall Tuesday. The GFS is the only model calling for a Texas landfall; the other main models (UKMET, NOGAPS, ECMWF, GFDL, and HWRF) all foresee a landfall between Alabama and western Louisiana.

The intensity forecast for Gustav
Gustav has two factors inhibiting its development in the short term: land interaction with Jamaica, and dry air. Jamaica has some 7,000 foot high mountains on it, and may be able to keep Gustav from strengthening today, as the center passes near or over Jamaica. As we saw yesterday, Gustav is a very small storm, and is prone to disruption by mountains. A large region of dry air to the north is also hurting Gustav, and this dry air may continue to be a problem for the storm over the next 2-3 days. Wind shear is not a problem for Gustav--it is currently under low wind shear (5-10 knots). This shear is expected to remain in the low to moderate range (0-15 knots) for the remainder of the week. By Friday, as Gustav approaches the Cayman Islands, the storm will be underneath an upper level anticyclone, and over the highest heat content waters of the Atlantic. The intensity of Gustav as it passes through the Caymans depend critically on how much of a favor Jamaica does today by keeping Gustav from intensifying. My best guess continues to be that the Caymans will get hit by an intensifying Category 1 hurricane on Friday.

If Gustav crosses the western tip of Cuba, it will lose intensity-- by perhaps 25 mph or so--but should easily regain that lost strength within 12-24 hours. Gustav will likely be a major Category 3 or 4 hurricane in the Gulf of Mexico. However, Gustav may not be able to maintain that strength all the way to landfall, if it slows down close to landfall, as expected. The GFDL is currently predicting a landfall Monday morning near the Alabama/Mississippi border as a Category 2 hurricane; the HWRF is predicting a Category 3 storm making landfall in central Louisiana, on Tuesday morning. The GFDL continues to be a day or so faster than the other models in bringing Gustav to landfall, resulting in less time for intensification. Since it is the outlier, a landfall Tuesday seems more likely.

A NOAA Hurricane Hunter research aircraft is currently dropping 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 will feed directly into the GFDL and HWRF computer models, but not into the other global models (GFS, NOGAPS, UKMET, and ECMWF).

I just got an email from one of my old Hurricane Hunter colleagues, Terry Lynch, who is currently on the flight deploying the AXBTs. He says:

We're currently just a tad over 1/2 way through the pattern...AXBT #33 just went out. About 1/2 an hour into the flight the chief scientist started second guessing and wished that he had us load up 80 BT's. And I thought 60 was a lot!

How big will Gustav get?
Gustav is currently a very small storm, similar in size to Hurricane Andrew of 1992. Tropical storm force winds extend out only 50 miles from the center. NHC is forecasting that Gustav will remain small over the next three days, with tropical storm-force winds expanding out only to a 75 miles radius. For comparison, Hurricane Katrina's tropical storm-force winds extended out about 200 miles from the center. Can Gustav become as large as Katrina? Well, we don't understand the processes that govern hurricane size very well. Katrina started out much larger than Gustav, so I doubt Gustav will grow that large. It is a fact that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. However, I'm guessing that the wind field will be half the size of Katrina's.

Gustav's impact on Cuba and Mexico's Cancun/Cozumel region
If you're in the cone of uncertainty, you're at risk. Hurricane forecasts are uncertain, and this uncertainty is graphically represented by the cone of uncertainty around the central "best guess" forecast positions. Cancun and Cozumel are both in the cone of uncertainty, so could get a direct hit from Gustav. At present, though, it appears that Mexico's Yucatan will only have one day of heavy rain (Saturday) with some winds gusts of 40-50 mph. The odds of Cozumel getting sustained winds of tropical storm force (39 mph or greater) are not much higher, though, and have increased to 29%, as indicated in NHC's wind probability product. The odds of hurricane force winds are 6%.

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.

Gustav's potential impact on the oil and gas industry
The price of U.S. crude oil has jumped about 2%, and the price of U.S. natural gas has increased 11% in the past two days, in anticipation that Gustav might rip through the oil and gas production areas of the Gulf of Mexico. About 25% of U.S. crude oil and 15% of its natural gas are produced in the Gulf of Mexico. As seen in Figure 2, the oil production areas are concentrated along the Louisiana and Texas coast. If Gustav makes a landfall on the right side of its cone of uncertainty, in Alabama or the Florida Panhandle, the oil and gas infrastructure might not be significantly affected. However, most of the cone of uncertainty lies in the major oil and gas producing areas, and I give an increased 70% chance that Gustav will significantly hurt oil and gas production in the Gulf.

Figure 2. Location of major oil platforms in the Gulf of Mexico. The data was taken from the Department of Interior Mineral Management Services

Links to follow
Wundermap for Eastern Cuba/Jamaica
Gran Piedra, Cuba radar

Tropical Storm Hanna
Tropical Storm Hanna continues to suffer from wind shear, and remains a minimal tropical storm. Visible satellite loops show a well-defined surface circulation that is exposed to view, thank to strong upper-level winds from the west that are blowing all of Hanna's heavy thunderstorm activity to the east side of the storm. These upper-level winds are creating a moderate 10-20 knots of wind shear over Hanna. This morning's QuikSCAT pass found top winds of 45 mph.

The forecast for Hanna
Since the center of circulation is fairly far removed from Hanna's heaviest thunderstorms, Hanna's center could reform to the southeast later today. This re-formation would alter the storm's eventual track, and make it less likely to recurve out to sea. 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 the four 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. The exception is the GFS model, which takes Hanna northeast out to sea. I'm discounting this solution at present, since it is the outlier. About five day from now, wind shear is expected to increase due to an upper level trough to the north. Upper level outflow from Gustav may also create some shear. The models respond by weakening Hanna to a tropical storm. In the very long range, the ECMWF model predicts Hanna will hit Cuba on Tuesday, then pass through the Florida Keys next Thursday. The NOGAPS model brings Hanna just off the South Carolina coast, and the other models stall it out northeast of the Bahamas. At this time, Hanna's intentions are a mystery. No Hurricane Hunter missions are scheduled into Hanna yet.

Figure 3. Visible satellite image from 7:30 am EDT Thursday August 28, 2008. A long line of impressive tropical waves is lined up over the Atlantic and Africa. Image credit: U.S. Navy.

Elsewhere in the tropics
Most of the computer models forecast the development of at least one additional tropical wave between Africa and the Lesser Antilles Islands during the coming week. The first candidate is a large circulation located near 18N, 39W, 800 hundred miles west of the Cape Verde Islands. While the circulation of this system is impressive, the system is devoid of any heavy thunderstorm activity, and is surrounded by a large are of dry air to the west and north. Wind shear is a moderate to marginal 10-20 knots in the region, and forecast to remain in the moderate to marginal range for the next few days. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Saturday. 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 Tuesday. This system would probably only be a threat to Bermuda if it develops.

Like the ocean, the atmosphere also has waves. Large-scale atmospheric waves form over Africa during the African summer monsoon season, track east to west, and emerge over the Atlantic Ocean near the Cape Verde Islands, where they often serve as the nucleus for a powerful Cape Verdes-type hurricane. Well, the African Easterly Wave Factory is exceptionally busy right now, and there are three very impressive looking waves with plenty of spin lined up across the continent (Figure 3).
The western-most wave, just coming off the coast of Africa today, is particularly impressive. This system has a very large circulation with plenty of spin, and is already developing some concentrated heavy thunderstorms over the waters south of the Cape Verde Islands. This morning's QuikSCAT pass saw winds of 50 mph near the heaviest thunderstorms. Wind shear is a moderate 10-20 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 Saturday. The models have been very aggressive developing this system over the past few days, and chances are good that this system will become a large and powerful Cape Verdes-type hurricane next week. It is too early to speculate whether this storm might end up recurving out to sea or not.

The other two waves lined up behind the wave moving off the coast 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
Finally, there's a new blob (96L) in the southern Gulf of Mexico, in Mexico's Bay of Campeche. An ASCAT pass at 12:15 pm EDT captured the right side of 96L, and revealed that the storm probably has a surface circulation. A small area of heavy thunderstorm activity has developed on the north side of 96L, as seen on visible satellite loops. NHC has given 96L a medium (20%-50% chance) of developing into a tropical depression by Saturday. By Saturday, 96L will be very close to the coast of Mexico, several hundred miles south of the Texas border. There is a 20% chance that this storm will grow large enough to influence the steering currents and wind shear that might impact Gustav when it enters the Gulf of Mexico.

On Cayman Brac, in Gustav's path:
I got this report last night from wunderground user mangroveman, who lives on Cayman Brac: The general feeling here is to be prepared and wait and see. Pu blic Works has gotten most of the shutters in place on schools and other public buildings. They will finish up in the morning. Hurricane shelters are ready incl uding the new Humane Society shelter for pets. The shelters are almost all on th e Bluff, the limestone spine of the Brac which rises to 140 feet on the east end of the island and gives the Brac its name, and so are well out of the way of an y storm surge.

Since Hurricane Ivan virtually destroyed Grand Cayman in 2004, many folks from t here (as well as Brackers) have built themselves hurricane cottages on the Bluff , also well out of the way of any storm surge. I would say that about half the h omes here have their shutters up, and the rest should all be up -- or ready to g o depending on the storm's strength -- by noon tomorrow.

Cayman Airways has added several jet flights between the islands--we are usual ly served by Twin Otters from Grand Cayman--for people who wish to get out of here--or there!

Business at the grocery and hardware stores has been steady, with the usual stocking up of supples. A lot of people went through this exercise not so long ago with Fay, and so most folks already have plenty of supplies.

The biggest concern here about the nature of the storm is that it has slowed down to a crawl, and, experience tells us that the slower it moves, the stronger it can become. However, general feeling is that Gustav is disorganized and will ne ed to really pick up strength quickly if it is going to be a major hurricane that would threaten the Brac. One of the historic memory points on the island is 1932 when the Brac was whacked by a mega-storm which not only destroyed the island and killed 110 people, but also caused many families to leave. The '32 Storm has become a living legend, and most any hurricane that hits the Brac is compared to that storm. Most people feel Gustav will not measure up against the '32 monster. Which is a good thing, of course!

Next blog
OK, now I feel like the guy who paints the bridge and gets done, only to realize that coat of paint he put on at the beginning already needs to be re-done! Anyway, it's time to post this, and start painting the bridge again Friday morning--

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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