Monday, August 27, 2012

8/27 - 9:30pm - Update on Isaac's Expected Gulf Coast Landfall

Tropical Storm Isaac is teetering on the edge of becoming a hurricane this evening with winds of 70 mph as of this writing. The threshold for a Category 1 hurricane is 74 mph. The storm motion has slowed down over the past few hours, now moving northwest at only 10 mph. Issac continues to run into issues that prevent it from rapid intensification, which is certainly good news. The latest issue is a batch of dry air that is currently being pulled into the center of the storm. Without an uninhibited source of moist air the storm cannot form a defined eye or intensify very well. The dry air is why Isaac is lopsided right now with most of the intense winds on the south side.

How long will it be until this dry air is finished working through the storm? Probably a few more hours, possibly lasting into the early morning hours tomorrow if current model forecasts are correct. Speaking of models, the current track from the National Hurricane Center takes Isaac right into the Eastern Louisiana/Mississippi Coast areas late tomorrow night into early Wednesday morning as a weak Category 2 storm with 100 mph winds. With the lack of organization we're seeing right now it may only be a Category 1 at landfall. The latest computer model tracks, the spaghetti plots as you've probably heard about and seen over the past few days, have tightened up on a landfall location somewhere around New Orleans. It's worth noting that some of these models are now trending eastward after a massive amount of successive westward jogs in recent days. It doesn't appear that this landfall will take place further east than Biloxi, Mississippi given the current motion of the storm and model data, but the exact landfall location won't be the big story with this storm.

Storm surge exceedance probabilities
The big story here is how big and how slow this storm is. Sure, there will be some wind damage along the coast as the storm makes landfall, but the large area that Isaac covers means that quite an expansive area along the Gulf Coast will experience other effects from it. Storm surge at or above ten feet is possible in locations from New Orleans to Pascagoula, MS and rainfall amounts over a foot are forecast over much of Central and Southern Mississippi and Eastern Louisiana. Isaac will slow down even more than its current speed once it gets over land so heavy rain will be a good bet across much of the Gulf Coast states until we get into the weekend. Inland flooding will be a problem with the deluge that Isaac with unleash on the region as it crawls northward.

Rainfall through 7pm CDT Saturday
As far as local effects in North Mississippi and West Tennessee, Isaac will bring very heavy rain and winds with gusts in some cases near 40 mph. Starkville, MS may be looking at a six or more inch rainfall scenario through Saturday evening and areas in West Tennessee like Jackson will likely see more than three inches of rain. This highly depends on Isaac's exact track so some areas will see much more rain than others.

With just about any landfalling hurricane you can expect the threat of isolated tornadoes, and Isaac will be no different. The turning of winds with height in a hurricane or tropical storm create favorable wind shear for tornadoes to form in stronger parts of it. Due to this potential threat, the Storm Prediction Center has issued a Slight Risk for most of Mississippi and parts of Louisiana, Alabama, Georgia, and Florida.

So overall we're not looking at Isaac to be a major hurricane at landfall, but the effects from it will be widespread due to it's large size. It's a good thing that folks in a wide area are preparing for this storm since it could be a long haul given the slow motion that's expected from it.

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