Gustav and Hanna--and many more to come

By: Dr. Jeff Masters , 5:21 PM GMT on August 28, 2008

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It's time to get familiar with the names Hanna, Josephine, Ike, and Kyle, because the tropical Atlantic is about to put on a rare burst of very high activity in the coming weeks.

The atmosphere pulled a major surprise last night and this morning, substantially altering the short and long-term fate of Tropical Storm Gustav. The ridge of high pressure that was forcing Gustav to the west shifted positions, and is now oriented southwest-to-northeast. This has 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 the its south side, away from the dry air and Haiti. As a result, the center of Gustav is now passing very close to Jamaica, and Gustav will pound that island today with winds near hurricane force. Rainfall will continue to be the main threat from Gustav today, as it was when it hit Hispaniola. Gustav's torrential rains accumulated to over 12 inches in southern Hispaniola, triggering floods and landslides that killed 22 people. The rain are mostly over in Hispaniola, but are just getting cranked up over Jamaica. It appears now that eastern Cuba will not get much rain from Gustav.


Figure 1. Current satellite image of Gustav.

Visible satellite loops and radar from Gran Piedra, Cuba show that Gustav has become better organized. A large Central Dense Overcast (CDO) of high cirrus clouds has covered the storm, and an eye has formed, right on the east coast of Jamaica. Several well-formed spiral bands have developed on the east and south sides of the storm, but the northwest side of the storm shows evidence that dry air is interfering with Gustav's organization. The latest 7:33 am EDT center report from the Hurricane Hunters found a significant drop in pressure, to 985 mb. This will likely mean that Gustav's winds will increase to hurricane force later this afternoon. The highest surface winds measured by this morning's mission were 70 knots (80 mph), with one spot of higher winds. However, winds at the aircraft's flight level of 10,000 feet were quite a bit lower.

The track forecast for Gustav
The latest 6Z (2 am EDT) model runs show a significant shift westward in Gustav's track, thanks to the southwestward motion and center re-formation of the storm. 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 across 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. The ridge will at least slow Gustav down, and the long term fate of Gustav remains highly uncertain.

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 is 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 not be able to maintain that strength all the way to landfall, if it slows down close to landfall, as some of the models are predicting. The GFDL is currently predicting a landfall in Alabama as a Category 2 hurricane; the HWRF is predicting a Category 3 storm making landfall in central Louisiana.

A NOAA Hurricane Hunter research aircraft is scheduled to drop a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico today 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).

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
Gradn Piedra, Cuba radar

Tropical Storm Hanna
Tropical Storm Hanna formed this morning near 19N 57W, a few hundred miles east-northeast of Puerto Rico. This storm may intensify into a hurricane and become a threat next week to the Bahamas, Cuba, and Florida. This morning's QuikSCAT pass found top winds of 45 mph. 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.

The forecast for Hanna
The upper-level low to the west of Hanna that is creating the shear is expected to most westward over the next few days, relaxing the shear over Hanna. Wind shear is expected to fall below 10 knots later today and remain below 10 knots for the next four days. This should allow Hanna to strengthen into a hurricane three to four days from now, as predicted by the HWRF, GFDL, and SHIPS intensity models. 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, and the models respond by weakening the system to a tropical storm. In the very long range, the ECMWF model predicts Hanna will hit Cuba, then move northwest again over Florida by September 6. We don't have any other models that extend out long enough in time to show what the eventual long-term fate of this storm. At this time, it appears that the Bahamas are the main region at risk from Hanna. 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 a few hundred miles north of the northern Lesser Antilles Islands by Monday Tuesday.

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.

Finally, there's a new blob (96L) in the southern Gulf of Mexico, in Mexico's Bay of Campeche. I haven't had time to look at this one yet, and this blog is way overdue, so I'll report on 96L in my 4 pm blog.

Jeff Masters

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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