RECONNAISSANCE AIRCRAFT FIND ISAAC NEARLY A HURRICANE… SIGNIFICANT STORM SURGE AND FRESHWATER FLOOD THREAT TO THE NORTHERN GULF COAST

7:00 AM CDT Tue Aug 28
Location: 27.8°N 88.2°W
Moving: NW at 7 mph
Min pressure: 976 mb
Max sustained: 70 mph

by Rob Lightbown of Crown Weather

Tropical Storm Isaac:
Isaac is not yet officially a hurricane this morning, although central pressures of 977 millibars and flight level winds of 85 to 90 mph have been recorded by reconnaissance aircraft. At this point, Isaac has less than 24 hours until it makes landfall along the Louisiana coast; so, I do expect it to become a hurricane within the next few hours and make landfall as an upper end Category 1 hurricane with 90 mph winds. With that said, this is not expected to be a “normal” minimal hurricane as it will bring a significant storm surge to the northern Gulf coast, especially across southeastern Louisiana and coastal Mississippi where up to 12 to 14 feet of storm surge is expected.

Isaac is generally tracking west-northwestward at 11 mph, however, center fixes from reconnaissance aircraft have shown a due west track since about midnight. This is likely a temporary track, however, it does put into question whether Isaac may make landfall further west along the central or southwestern Louisiana coast. Upper level wind analysis clearly shows a gap in the ridge from southeastern Louisiana to the western Florida Panhandle. Isaac is still expected to turn into this gap and make landfall in southeastern Louisiana during the predawn hours of Wednesday morning.

The latest forecast guidance continues to have some differences on where Isaac will come ashore. The NAM model forecasts a landfall in southeastern Louisiana late tonight. The GFS model guidance also forecasts a landfall in southeastern Louisiana just after midnight tonight. The Canadian model guidance forecasts a landfall in southeastern Louisiana and southwestern Mississippi early Wednesday morning. The European model guidance is furthest west of all of the guidance and forecasts a track that keeps Isaac just offshore of much of the southeastern Louisiana coastline and forecasts a landfall right in Vermilion Bay on Wednesday evening. Given the due west track seen by reconnaissance aircraft this morning, this trend will need to be monitored very closely.

For now, I’m going with a landfall in southeastern Louisiana as a 90 mph hurricane during the predawn hours of Wednesday morning. Now, I will caveat this by saying that if you extrapolate out the recon center fixes over the past several hours, it would give you a landfall area much further west between Morgan City and New Iberia, which is exactly what the European model is forecasting. I will be monitoring these center fixes throughout the day today and will update immediately if I see a shift in the forecast track.

The impact from Isaac will be widespread across the northern Gulf coast as already its large wind field is causing ocean water to rise across southeastern Louisiana and along the Mississippi coastline. A storm surge of upwards of 12 to 14 feet is possible across southeastern Louisiana and along the Mississippi coastline from Isaac. For those of you that are under mandatory evacuation orders, please heed that advice and evacuate.

Huge amounts of rainfall is expected across much the Gulf coast over the next few days. Rainfall totals of 15 to 25 inches are expected across the eastern half of Louisiana and the western half of Mississippi from today through Wednesday. Freshwater flooding is likely and please do not go out and drive around in the rising water.

So, weather conditions across much of the northern Gulf coast will continue to deteriorate throughout the day today and into tonight with wind, surge and rainfall induced flooding threats likely. It should be noted that the heavy rainfall and flash flood threat is expected to push northward into the lower Mississippi Valley by Wednesday and continue right through Thursday and Friday.

I am now constantly monitoring the progress of Isaac and will continue to have frequent updates on the storm.

For more on this story and updates go to:

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