I am sure there is a lot of buzz and chatter about the potential for flooding in Atlanta and much of Georgia from a supposed 10 inches of rain from Issac. However, don’t get carried away. The future path and strength of Issac are far from certain. Number 1, Issac has continued to defy expectations of intensification. Number 2, the track of the storm keeps getting moved farther West with each passing day. Number 3, the models are in somewhat LESS agreement in their latest configurations rather than in greater agreement as would be expected or at least hoped for.
In my 25 years of forecasting in Atlanta, I can tell you that 8 out of 10 times the effects of a hurricane or tropical storm end up being much less than advertised. There are of course those other times when it’s as bad or worse than forecast. I also note that quantitative rainfall forecasts 5 days in advance are not very accurate. The forecast for rain to occur are good, just not the AMOUNT or the location of the heaviest.
As the remnants of Issac move north across Mississippi and Alabama into Tennessee next week Georgia’s weather will feel the side effects late Tuesday into Friday. But it’s too early to say the rain will be more harmful than helpful, given the severe drought in Metro Atlanta. At last calculation, 12 inches of rain were needed to zero out the drought.
Computer model track projections from the various forecast guidance agree that Isaac will track across the Florida Keys, likely between Key West and Marathon late today with hurricane conditions expected across the Florida Keys and southwest Florida into tonight.
Monday morning Isaac will be located about 100 miles northwest of Key West. Isaac is expected to track northwestward across the eastern Gulf of Mexico where it will should strengthen. The forecast model guidance from Monday through Tuesday diverge quite a bit and the various guidance cannot agree on a final landfall location on the northern Gulf Coast. The reason for the model spread in the guidance is due to how they are handling the movement of a trough of low pressure and a high pressure ridge that is located to the west and to the east of this trough. The GFS and Canadian model guidance forecast that Isaac will miss this trough and that the high pressure ridge will steer Isaac towards a landfall in southeastern Louisiana Wednesday afternoon. The European and NAM model guidance project enough of a gap in the ridges of high pressure that leads to a turn to the north and a landfall somewhere between Mobile Bay and Pensacola–Fort Walton Beach.
Yesterday’s European model runs forecast Isaac making landfall near Apalachicola. The most recent forecast run of the European model shows a landfall very close to Mobile Bay. So the European model is edging westward towards the idea of a landfall somewhere between New Orleans and Pensacola.
The ensembles of the GFS and European models have a forecast track that is in-between the operational models with the GFS ensemble forecasting a landfall near Gulf port, Mississippi early Wednesday morning while the European ensemble guidance is forecasting a landfall near the Mississippi-Alabama state line.
The farther west it goes and the stronger it gets the greater the threat of disruption to the energy industry as there are many oil and natural gas rigs, pipelines and refineries in the region from Mobile, Alabama west to Texas. This could end up being a multi-billion dollar storm and the strongest in the area in at least 7 years.