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Atlantic Tropical Weather Update 8/30/08

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Hurricane Gustav has passed over the deep, very warm waters between Jamaica and Cuba. In doing so, he rapidly strengthened overnight from a Category 1 hurricane to a Category 3 storm this morning, making him the second major hurricane of the 2008 Atlantic season. Bertha was the other major hurricane, but remained well out to sea her entire lifetime. Gustav’s vitals as of 8:30A MDT:

Center located at 20.8N, 81.6W; maximum sustained winds of 120mph; moving NW @ 11mph; minimum pressure is down all the way to 955mb, which is 33mb less than the reading yesterday. That is quite the rapid intensification. In fact, since the pressure has continued to drop steadily this morning, expect Gustav to strengthen even further today before eye-wall replacement cycles, a slightly cooler pool of water and the island of Cuba work to weaken him somewhat.

Gustav should still make landfall over Cuba later tonight as a Category 3 storm. A solid eye-wall did develop overnight. Gustav has an impressive outflowing cloud shield. Hurricane force winds were found on the northeast quadrant on an earlier hurricane hunter flight. As they get going again today, a clearer picture of where they are found will be available. Tropical Storm force winds extend all the way to Cuba, a decent distance from the storm’s center. Western Cuba is under a Hurricane Warning today.

Model solutions have come to closer agreement as of today. There is a much narrower band encompassing the solutions through the Gulf of Mexico, which Gustav will enter 12-24 hours from now. While Gustav crosses the central Gulf, he is still forecasted to overrun the Gulf Loop Current’s warmest stretch. The model intensity forecast has him strengthening to a Category 4 hurricane during this time, but I wouldn’t be surprised to see him go all the way to a Category 5. After that, there is very good agreement about the likely landfall location: the middle of the Louisiana coast. He should make landfall between Morgan City and Lake Charles. The timing is coming into better focus. The suite of models are also in pretty good agreement that the landfall could occur Monday night.

Post-landfall may not offer a better story than during landfall. Due to the strength and orientation of the ridge that will remain over the eastern U.S., Gustav could stall over Louisiana. We’ve already dealt with one slow moving storm: Fay dropped incredible amounts of rainfall over Florida just a week or so ago. Gustav shouldn’t dump quite that much rain, but the longer he lingers, the more flooding far inland will be a concern. New Orleans residents are smartly already evacuating. I’m sure other Gulf coast residents will follow similar measures throughout the next two days.

Tropical Storm Hanna’s future has changed slightly. Let’s start with her vitals as of this morning:

Center located at 21.9N, 66.4W; maximum sustained winds of 50mph; moving W @ 8mph; minimum pressure of 1000mb.

Hanna has stubbornly not strengthened in the past day. Vertical wind shear is the likeliest cause for this. Regardless, she has turned to the west today and should maintain that movement over the next few days before striking the Bahamas sometime Tuesday. The danger involved in that path is it looks like Hanna could slide along the northern coast of Cuba and enter the Gulf of Mexico through the Florida Strait. That won’t happen until later next week, but it is one possibility. Another possibility is she could move over Cuba or even southern Florida. It depends upon the strength and location of the ridge to her north during that time period.

A few models have that ridge shifting position, allowing Hanna to turn from a southwest course to northwest course, threatening Florida and the U.S. east coast. Some models have her intensifying to hurricane strength during the next five days, but the official NHC forecast keeps her at Tropical Storm strength.

A disturbance continues to move across the Atlantic, currently located about 1100 miles ENE of the Leeward Islands. The NHC has assigned a 20% chance of development in the next 24-48 hours.

Invest-97 has moved just to the southwest of the Cape Verde islands, off the west coast of Africa. It actually looks fairly well organized on satellite imagery, clearly displaying a somewhat centralized center of rotation and the nascent features of banded clouds. It is centered at 14.2N, 25.5W and has maximum wind gusts of 35mph. It is moving quickly across the Atlantic at 17mph. The NHC has designated this disturbance as having a >50% chance of development in the next 24-48 hours and I definitely agree with that forecast. If it continues to grow and organize, it would be identified as Tropical Depression Nine.

*****

[Update 7:30P]:

Hurricane Gustav underwent rapid intensification today prior to landing on the western portion of Cuba’s southern coast. At the time of landfall, Gustav was rated as a Category 4 storm with sustained winds of 150mph! The center of circulation at this moment in time is just about ready to enter the Gulf of Mexico. Gustav’s well defined eye-wall that was present before and during landfall has disappeared now that he has spent some time over land. Expect that eye-wall to be reestablished within a few hours of reemerging over the warm Gulf waters. Let’s move onto his 6P vitals:

Center located at 22.7N, 83.4W; maximum sustained winds of 150mph; moving NW @ 14mph; minimum central pressure has continued to steadily drop and is currently at 941mb. The continual lower pressure indicates that Gustav isn’t finished strengthening. In fact, the 150mph sustained winds is only 6mph shy of a Category 5 classification. This is very impressive since Gustav didn’t have to run over the deepest warm sector of the Gulf Loop Current. Gustav isn’t currently a very large hurricane. Hurricane and tropical storm force winds don’t extend a great distance away from the center, as far as hurricanes are concerned.

Okay, what are the models showing? The suite of solutions has really come into quite good agreement. Part of that is the amount of time to U.S. landfall has steadily fallen away. It looks more and more likely that Gustav will make landfall along the southern Louisiana coast. There is still time for Gustav to alter his trajectory prior to landfall. As such, the NHC has issued a Hurricane Watch from the FL/AL border all the way west into the Texas coast. As Gustav pushes further into the Gulf of Mexico, he should weaken just slightly as a result of moving over Cuba, then restrengthen, likely up to Category 5 status within the next six hours. Thereafter, eye-wall replacements will be the most likely factor affecting Gustav’s strength until he moves closer to the U.S. coast and the continental shelf affects ocean and thus heat depth.

Monday is looking like the big day for Gustav. According to the most recent model runs, Gustav should make landfall Monday afternoon. 12 hours prior to that, Gustav should start weakening as the shallower Gulf waters work against maintaining his intensity. The most current information indicates that Gustav could strike as a major hurricane, either Category 3 or 4. If he moves over the shallower waters at the same time the inner-most eye-wall is collapsing and being replaced, he could make landfall as a Category 2 hurricane. But those are details for a couple of days from now.

After landfall, Gustav should be slowed down considerably by the high pressure that is resident over the eastern U.S. The four and five day forecasts indicate that Gustav could shift westward after landfall, moving into eastern Texas and weakening to a Tropical Storm and then a Tropical Depression. Staying in the region would mean very heavy rainfall over a large area, making flooding the major concern inland.

Tropical Storm Hanna continues to move across the basin as well. Here are her vitals:

Center located at 22.8N, 67.3W; maximum sustained winds remain at 50mph; moving NNW @ 5mph; minimum pressure of 1000mb.

Hanna is forecasted to continue moving NNW for the next 36 hours or so, after which she should begin moving more due west. At the same time, her forward motion is being forecasted to diminish, meaning she will appear to crawl across the Atlantic toward the Bahamas. She could begin affecting the Bahamas with tropical storm force winds in two days or so, but the center isn’t expected to pass over any islands until Tuesday now.

It is after Tuesday that her track forecast gets important for U.S. residents. After Tuesday, Hanna is now forecasted to move NW again. A pick up in speed is also being forecasted during this time period. By Thursday, Hanna should really be affecting a lot of the Bahamas. As the ridge to her north shifts around, Hanna’s eventual track could do many things. She could stay out to sea or she could make landfall or she could skirt the U.S. coast, maintaining herself and impacting numerous communities. At this time, it is way too early to say which of these will happen. But it looks to me right now that she won’t move through the Florida Strait and into the Gulf. It is important to note that the closer to the U.S. coast she gets, the deeper the ocean has significant heat content, which could allow Hanna to strengthen significantly prior to impacting the continental U.S. The model intensity forecasts have Hanna slowly strengthening over the next five days, which makes sense.

Invest-97 and the other tropical disturbance continue to move across the Atlantic as well.  Development isn’t likely for these two systems any time soon.

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