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Bridging climate science, citizens, and policy

Atlantic Tropical Weather Update 9/1/08

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[Update 11:45A MDT]:

Tropical Storm Hanna has strengthened to a Category 1 hurricane.  An Air Force reconnaisance plane investigating the storm found winds strong enough at flight level to support the decision to characterize the storm as a hurricane.  Her updated vitals:

Center located at 22.4N, 72.6W; maximum sustained winds of 75mph (just over category 1 threshold); moving WSW @ 4mph; minimum pressure of 985mb (9mb less than the previous update).  The lowering pressure should allow Hanna to continue to slowly strengthen into a moderate Category 1 hurricane.  The intensity forecast has been updated to reflect this status in the short term.  At this time, the models are not indicating a strengthening to Category 2 strength.

***

Hurricane Gustav made landfall within the past hour or two (currently 9:30A MDT) on Louisiana’s southern coast, near Cocodrie, Louisiana, which is south of Houma, Louisiana.  He made landfall as a strong Category 2 storm.  Reports are coming in that levees in New Orleans, to Cocodrie’s north, are being overtopped by the storm surge and waves.  I’m sure additional reports will come in throughout the day.  His vitals:

Center located near 29.2N, 90.8W; maximum sustained winds of 110mph; moving NW @ 14mph; minimum pressure of 955mb.  Those sustained winds are the threshold between a Category 2 and Category 3 storm.

Gustav’s future looks radically different than it did last night in the long-term.  He is expected to slow down over the next day or so as he moves across Louisiana and eastern Texas, weakening the entire way.  Between Tuesday and Wednesday, Gustav should turn toward the north (instead of the south, as was indicated last night) and his forward speed should continue to decrease.  Friday could see Gustav still moving slowly to the north into Oklahoma.  By the end of the week, another synoptic trough should be moving across the United States.  That trough right now looks less robust than the one currently making its way from west to east, but Gustav’s remnants should get caught up in the larger scale flow and advected out of the Texas/Oklahoma region.

So beyond additional levee failures in the New Orleans area, the main threat from Gustav will gradually shift toward flooding as he stalls and rain falls over the same area for many hours.

The next storm threatening the Bahamas and the U.S. is of course Tropical Storm Hanna.  I think Hanna has benefited from Hurricane Gustav’s proximity to the Gulf coast somewhat because she is a little stronger this morning than she was nine hours ago.  Her vitals:

Center located at 23.0N, 72.9W; maximum sustained winds of 60mph (10mph more than Hanna has displayed for over two days); moving WSW @ 4mph; minimum pressure of 994mb (not much change in the past 24 hours).

Tropical Storm Hanna is currently giving the Bahamas a hard time.  Her cloud shield extends now to over Cuba, Hispanola and Puerto Rico.  Those areas aren’t feeling too many effects from the storm, however.  Hanna looks good on satellite this morning, with consistent outflow in most directions, but still somewhat weighted toward the south as Gutav’s outflow is still ingested from the north.

For the next 24-36 hours, Hanna will continue her WSW motion, drenching the southern Bahamas.  Sometime tomorrow, Hanna is expected to begin moving back toward the northwest, slowly at first, then more quickly with time.  By Wednesday, Hanna could strengthen to a Category 1 hurricane and is likely to be affecting the central Bahamas.  Throughout Wednesday, Hanna is forecasted to continue picking up speed, allowing her to impact the northern Bahama Islands (east of southern Florida) by Thursday morning.

The model solutions are in better agreement this morning about her path betwen Wednesday and Friday.  Instead of being spread out from Cuba to North Carolina (yikes!), Hanna is more confidently forecasted to make landfall either over Georia or South Carolina.  Now, that track could shift over the next few days, but it is reassuring to see a smaller track envelope than yesterday.  The intensity forecast has Hanna as a moderate Category 1 hurricane from the Wednesay to Friday time period.  Thus, she should make landfall as a hurricane, but one that is nowhere near as strong as Gustav was.

The Atlantic basin remains very busy today.  After Gustav and Hanna, there are a number of disturbances strewn across the ocean.  As of this morning, Tropical Depression Nine formed from what was identified as Invest-97.  There is some good news involved with this T.D. – it is located fairly far north, meaning the Caribbean should be spared another storm.  The east coast of the United States isn’t completely out of the woods, and neither are the northern portions of Caribbean islands.  Let’s look at T.D. 9’s vitals:

Center located near 17.6N, 39.5W; maximum sustained winds of 35mph (4 below what’s needed for a Tropical Storm); moving W @ 16mph; minimum pressure of 1005mb.  The T.D. has good looking spiraling bands again today and has well established upper-level outflow.

Most of the model track solutions show T.D. 9 moving from east to west for the next five days.  Some northward motion is also very possible, bringing T.D. 9 above 20N latitude in 2-3 days.  Thereafter, model solutions indicate T.D. 9 could curve slightly back due west or just south of west.  It is that second option that could bring the system into eventual contact with the north coast of Hispanola or through the Bahamas.  That’s pretty far out in time, though, and a lot could happen between now and then.

The intensity forecasts have T.D. 9 strengthening during the entire time period, and since it’s located near 40W (where warmer temperatures are found), that’s not out of the question.  However, most disturbances are generously given increasing intensity by the models that don’t pan out in reality.  It’s a long ways from any land, so at this point it doesn’t matter too much.  If T.D. 9 does strengthen, as the official forecast calls for, it would be identified as Tropical Storm Ike.  We could be within 12 or fewer hours from such a designation.

The disturbance that was identifies as Invest-98 has split into two separate disturbances: one located 700 miles ESE of Bermuda, the other located 250 miles east of the Leeward Islands and moving toward the Caribbean Sea.  The NHC has given both disturbances <25% chance of development within the next 24-48 hours.

Lastly, another area of disturbance has entered the Atlantic basin off the west coast of Africa, near 12N, 20W.  This tropical wave has been identified as Invest-99.  It is located between Africa and the Cape Verde Islands.  It is characterized by strong convection, but doesn’t have a well-defined circulation or outflow yet.  Model solutions for this system have it moving west to northwest over the next five days, reaching 20N, 40W by the end of that time period.

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