A severe lengthy lashing of storm surge, pounding waves, gusty winds and heavy rain for Coastal Louisiana and adjacent states. Long-term flooding in the days ahead not the wind will be the big story. Why the hurricane center has not declared this a hurricane is a mystery to the rest of us who went to meteorology school, they are needlessly picking nits.
Isaac is not yet officially a hurricane this morning, although central pressures of 976 millibars, flight level winds recorded by reconnaissance aircraft, satellite and hunter confirmation of an open eye all indicate that for all intents and purposes Isaac IS a minimal hurricane right now. It will bring a significant storm surge to the northern Gulf coast, especially across southeastern Louisiana and coastal Mississippi where up to 6 to 12 feet of storm surge is expected.
Eye 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 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 by 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 in Vermilion Bay Southeast of Abbeville, LA Wednesday evening. Given the due west track seen by reconnaissance aircraft this morning, this trend will need to be monitored very closely.
The hurricane center still predicts landfall up the mouth of the Mississippi River just southwest of New Orleans but 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 what the European model is showing.
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.
Huge amounts of rainfall is expected across much the Gulf coast over the next few days. Rainfall totals of 5 to 15 inches are expected across the eastern half of Louisiana and the western half of Mississippi from today through Wednesday with isolated 5-day totals around 20 inches.
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.
In Georgia the circulation around Isaac is pulling moisture into the state off both the Gulf and the Atlantic with outer spiral bands to impact metro Atlanta at times into next week. Often times the computer models overestimate the rainfall amounts mistaking moisture that will be more cloud cover than rain thanks to how the model physics handle the thermodynamics of latent heat release and convective feedback problems. So the model may show widespread steady flooding rainfall but the real result is often periods of showers on and off, sometimes drizzle or mist with dry spells in between and intermittent heavy downpours. Thunder and lightning is at a minimum, and only isolated strong or severe storms occur. The east side of Isaac is where the risk of brief weak small tornadoes will exist so we wil monitor this potential but I don’t expect it for for Atlanta as of now.
Metro Atlanta needs a foot of rain to eliminate the drought. So IF we could get a week of scattered on and off showers and thunderstorms and total up 2 or 3 inches of rain on average and isolated 6 inch totals accumulated over a 5 day period instead of just a couple days we could avoid major flooding but put a real dent in the drought. However, at this point it does not look like the rain will be all that heavy in the metro with 3 day total on one inch on average, with isolated 2 or 3 inch totals between now and Sunday. As always every tropical cyclone behaves differently so we’ll have to take it one day at a time. I’ll continue to do updates on Isaac daily on the radio throughout the week even when I don’t blog.