Bill Read's Blog: Isaac's inland... now what?

Published On: Aug 29 2012 05:57:10 AM CDT
HOUSTON -

Early Wednesday, the center of Hurricane Isaac made a slow landfall over extreme southeast Louisiana, near Port Fourchon.

Typically, as a hurricane moves inland, the wind speeds drop off rather quickly to the low end of tropical storm force – 40 to 45 mph – then more slowly until it finally is no longer identified as a tropical cyclone.

This is primarily due to the increased friction over land and also due to the loss of the heat source that the warm ocean provided. In Isaac’s case this decay is expected to happen more gradually, mainly because it is moving inland over very flat land with lots of lakes and marshes. Thus the impacts over Louisiana and Mississippi will linger longer.

The high water levels generated by the storm surge along the coast will only slowly recede as the onshore winds decrease. Wind damage to trees and loss of power will continue to occur over much of southeast and south central Louisiana.

Wednesday may see an increase in the threat of tornadoes north and east of where the center of Isaac moves.

This threat extends well to the east due to the large size of Isaac. The reason the threat is higher today is that although the winds at the surface will be decreasing, very strong winds several thousand feet up in the atmosphere will decrease much slower.

When this occurs wind shear, a key ingredient in tornado generation, becomes significant. In general, the north and east sections of the circulation are most at risk for these tropical cyclone produced tornadoes.

Isaac will bring tremendous amounts of rainfall across the lower Mississippi valley region today and likely into Friday as the storm and its remnant move only slowly northward away from the coast.

Amounts of 5 to 10 inches will be common with some locations seeing upwards of 20 inches. We will no doubt see reports of locally devastating flooding as a result of the heavy rainfall. Later this week and into the weekend the remnants of Isaac may produce heavy but potentially beneficial rains to portions of the Midwest.

So, for the Houston –Galveston area, what is Isaac going to produce?

Well, today will be another hot and breezy day as the northerly winds on the western side of Isaac reach into Texas.

Only a remote chance of showers as typically this far west of a north moving tropical storm is subject to drier and sinking air – not conducive for rain. Winds may be strong enough to cause a lowering of water levels by as much as 1 to 2 feet by late tomorrow.

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