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Posted at 11:14 AM ET, 08/28/2012

Isaac nearly a hurricane as it bears down on Louisiana, northern Gulf coast

Visible satellite image of Isaac as of 10:15 a.m. EDT (NOAA)
Throughout the morning, aircraft reconnaissance data have indicated Isaac is gradually becoming better organized. Nevertheless, as of the 11 a.m. EDT advisory, the National Hurricane Center keeps Isaac just below hurricane strength. Tropical storm Isaac’s maximum sustained winds are still 70 mph, even though its central pressure of 976 mb is much more characteristic of a hurricane.

Extent of tropical storm force winds at 11 a.m. EDT shaded in orange. Blue shaded areas indicate tropical storm warnings, red shaded areas indicate hurricane warnings. (National Hurricane Center)
Landfall is expected to occur later this afternoon or this evening in southeastern Louisiana as a Category 1 hurricane. The area covered by tropical storm force winds, as well as the latest watches and warnings, are shown to the right.

It is extremely important not to focus on the exact location of the center of the storm. Heavy rain, damaging wind, tornadoes, and storm surge will extend far away from the center.

The center has been a bit disorganized, but is now within range of the New Orleans radar for convenient and constant tracking.

Disorganized or not, any tropical cyclone is capable of producing large amounts of rainfall. As we saw in Haiti and Florida, a tropical storm quickly unloaded 6”, 12”, and even 20” of rain. This morning’s updated rainfall forecast for the south-central U.S. is shown below, with a peak of almost 18” over southern Mississippi.

Rainfall forecast over the next five days. (NOAA/HPC)

Probability of Isaac’s storm surge exceeding 6’: red is 70-80%, orange is 60-70%, etc.
The northern Gulf coast has another vulnerability: storm surge. This large dome of water bulldozed out ahead of a storm by its winds has nowhere to go but inland when it reaches a coastline. This map shows the probability of the storm surge caused by Isaac exceeding 6’.

The surge is in addition to whatever the normal lunar tide is at the time; so if the peak surge occurs during high tide, that’s much worse than if it occurs during low tide.

Coastal areas in MS and AL (Bay St. Louis, Gulfport, Biloxi, Mobile) will generally be near low tide this afternoon/evening, while Lake Ponchartrain at New Orleans will have its highest tide at 5:22pm CDT... rather poor timing.

Here is a video update on Isaac from CBS News:

Tropical storm Isaac is set to make landfall as a hurricane on the seventh anniversary of Hurricane Katrina

Stay tuned to the CWG blog throughout the day for further updates on Isaac and its impacts.

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

By Brian McNoldy  |  11:14 AM ET, 08/28/2012

Categories:  Latest, Tropical Weather

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