Weatherdem's Weblog

Bridging climate science, citizens, and policy

Atlantic Tropical Weather Update 8/31/08

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Hurricane Gustav came off the western portion of Cuba last night as a Category 4 hurricane and maintained that strength while continuing northwest. It appears that interaction with the landmass caused Gustav to weaken slightly since then. As of this morning, Gustav has weakened to a still very dangerous Category 3 storm. Starting with his vitals:

Center located at 25.3N, 86.0W; maximum sustained winds of 120mph; moving NW @ 17mph; minimum pressure of 960mb, about 20mb higher than the last time I looked near 12A MDT.

Gustav’s impressive satellite signature yesterday has degraded so far today. There is no longer a clear eye portion of the storm. Also, the convection and outflow isn’t as symmetric as it was late last night. The strongest convection looks to be confined to the southwestern portion of the storm.

What’s next for Gustav? He is still heading straight for the northern Gulf coast. As the hours tick away, his most likely landfall site is being forecasted with more confidence. Somewhat surprisingly, that location remains southern Louisiana, perhaps a bit to the east of where I thought he would strike the past two days. It looks more likely that the site will be close to New Iberia, LA. Models are forecasting a slight re-increase of strength back to Category 4 status while Gustav is still over the Gulf. Prior to landfall, his strength should begin slowly decreasing and continue to do so after landfall. Landfall is expected to occur during the day on Monday, currently sometime after 8A CDT. Due to the influence of the ridge of high pressure over the eastern U.S., his forward speed should decrease and the possibility of him stalling over Louisiana or Texas remains high.

Storm surge from Gustav is expected all along the northern Gulf coast. A maximum of 12-15 feet of storm surge is possible near the mouth of the Mississippi River and east of New Orleans. The height of surge east and west of this region decreases from there, with up to 12 feet forecasted a good distance away from the River, out toward Abbeville to the west and Biloxi, MS to the east. Combined with rain from convective bands, flooding then becomes a major risk from this storm across a very wide area. The initial storm surge and winds will weaken infrastructure and a stalled storm will only add to the challenges facing those who live and those who will help clean up in the region. Based on Gustav’s current projected path and expected effects, I don’t think the situation looks good for the New Orleans area. Damage from wind and flooding should occur once again. It is obviously too early to tell just how extensive that damage will be, which means that officials along the Gulf coast should be congratulated on acting promptly to move residents away from the area.

Tropical Storm Hanna is battling some adverse environmental conditions. A synoptic-scale upper air low and Gustav’s outflow are among the factors inhibiting Hanna from strengthening right now. Her vitals:

Center located at 23.3N, 70.0W; maximum sustained winds remain at 50mph; moving WNW @ 10mph.

Hanna is forecasted to keep on her WNW/W course over the next several days, until she is just east of the Bahamas. By Wednesday morning, she could move in a more steady NW direction as a trough to her north passes by and the ridge reestablishes itself. By Tuesday night, the Bahamas should start to feel a more direct effect by Hanna. Luckily, they are to her southwest where winds and surge will be somewhat minimized. After Wednesday, things could get more interesting. With the ridge back in place, Hanna should accelerate toward the northwest. Given her location prior to this, this means Hanna could begin to threaten the U.S. toward the end of the week. The official track forecast issued by the NHC places Hanna off northern Florida’s east coast by 8A Friday.

The suite of model solutions available this morning do indicate a more northerly trajectory. By Friday, that means Hanna could be taking aim at Georgia, South Carolina or North Carolina. With easing environmental obstacles, she could undergo some strengthening between now and then as well. The official intensity forecast calls for Hanna to be a hurricane by Tuesday morning. As was the case with Gustav three days ago, it is just too early to tell who is in Hanna’s sights (if anyone) or how powerful she might be if she does end up making landfall. Hanna has some serious work to do if she ends up making landfall along the U.S. coast as a hurricane.

Invest-97 continues to make its way across the basin. The “center” of the disturbance is located near 15.8N, 31.2W. It has winds nearing 35mph in strength and is moving W @ 13mph. The very weak banding features are still apparent around the approximate location of the “center”. I-97 should continue to move in a westerly direction, staying between 15N and 20N for the next three to four days. It should be west of 40W tomorrow, at which time it could have a better chance at organizing into a tropical depression. The NHC has assigned this disturbance a >50% chance of development within the next 24-48 hours.

Invest-98 has been designated for the disturbance between Hanna and I-97. It is located near 20.7N, 48.3W with winds near 30mph, and is moving W @ 6mph. There is no center of circulation evident at this time. The NHC has given it a <20% chance of development. Given its current location in the basin, the storm should get caught up in the westerlies within the week as it starts to move more northerly.


Hurricane Gustav is about 12 hours away from landfall (11P MDT).  Here are his vitals:

Center located near 27.3N, 88.1W; maximum sustained winds of 115mph; moving NW @ 16mph; minimum pressure of 954mb.

The models have been extremely consistent in identifying a narrow range along Louisiana’s southern coast for his landfall.  His center should come ashore in such a way that New Orleans and points east will be on the eastern side of the storm.  This is the more dangerous half of the storm in terms of the size of the storm surge and surface winds.  When Hurricane Katrina made landfall (3 years ago to the day!!!) in 2005, her center came ashore to the east of New Orleans.  That meant that the area was spared the worst of the storm’s fury.  And look what happened.  I fear that the levee system will not be strong enough to withstand Gustav’s onslaught.  Only time will tell.

After landfall, Gustav should turn more towards the west.  Then some interesting interaction with a westerly trough and eventual ridge rebuilding could occur.  The official track forecast has Gustav stalling over western Louisiana or eastern Texas.  The suite of model solutions predicts Gustav will move back toward the Gulf coast in the 4-5 day time period.  That isn’t reflected in the official track, due to the inherent complexities involved in accurately determining his position that far out in time.  If the model solutions are robust and the same pattern is shown over the next couple of days, Gustav could hang around the TX/LA area for a good while.

Tropical Storm Hanna looks a little better on satellite tonight, but still isn’t terribly organized.  The outflow from Gustav is still preventing her from organizing and strengthening any more.  Once Gustav moves ashore, his outflow will be considerably reduced, allowing Hanna more opportunity to pull herself together.  Here are her vitals:

Center located near 23.7N, 72.2W; maximum sustained winds of 50mph; moving W @ 8mph; minimum pressure of 997mb.

Hanna’s extended forecast is becoming more interesting and more important for the U.S.  The latest model suite has her continuing to meander near the Bahamas over the next day or so.  After that, movement to the WNW to NNW is predicted.  This rather large envelope of future tracks has implications in her potential landfall.  One model has her briefly touching Cuba’s northern coast before heading over Florida.  Another has her making landfall over central Florida, another has her coming ashore over Georgia.  Two predict she will make landfall over South Carolina and one has her coming ashore over North Carolina.  Obviously, these forecasts will become more accurate after another day or so.  But for now, the entire east coast of the U.S. from Florida to North Carolina should monitor this storm as closely as the northern Gulf coast did with Hurricane Gustav.


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