Hurricane Isaac is ramping up in intensity rather rapidly, with winds of 80 mph and a central pressure of 975 mb as of 5pm. Unfortunately for residents along the south central Gulf, the dry air we’ve been talking about for the past few days is currently being evacuated from the storm center and the peripheral outflow has significantly improved this hour, resulting in a much more concentric and more classical hurricane look on satellite:

The eye appears to actually be contracting and the storm is tightening up and slowing down on the latest radar from the NWS New Orleans, as it approaches the mouth of the Mississippi at the southeastern most tip of the state:

This contraction and decrease in forward speed is not an uncommon occurrence, and has been seen with previous Gulf of Mexico storms such as Hurricane Ike which hit the Galveston, TX area in 2008 (credit goes to Garret Bastardi for the radar link from Colorado State University):

as well as Hurricane Charley, which devasted Punta Gorda, FL in 2004 (courtesy of Weather Underground hurricane archives):

The good news is the center of the storm only has another 6-8 hours over open water, assuming it maintains it’s forward speed of 8 pm to the northwest. The bad news is that there still can be some significant strengthening during that time, and I would not be surprised if Isaac makes it to category 2 status, and tops out at near or over 100 mph at landfall. Compounding the problem is the slow movement of the storm, which some models have raking the Louisiana area for the next 24-36 hours, leading to torrential rains, as evidenced by the rainfall forecast from the NHC:

and the storm surge, also discussed in previous posts:

The predicted surge from the National Hurricane Center illustrates the danger well, showing a 50% chance of the following levels being exceeded:

with a general forecast from the NHC of a surge of 6 to 12 feet for southeast Louisiana.

The bottom line is that Isaac will come ashore later this evening or in the early morning hours overnight, and crawl to a position very close to the New Orleans area, with winds approaching 100 mph sustained and gusts to 110-115 mph. The real story with this storm, however, will be the flooding, with 10-20 inches of rain over the next 24-36 hours and the storm surges discussed above. With all that, the new levee and pump system will get quite the test. Here’s to hoping it passes.

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