Sunday Evening Tropical Thoughts

Be sure to check in with Meteorologist Brandon Chambers on WHNT News 19 at 9:00 on WHNT-2 and WHNT News 19 at 10:00 on WHNT for the latest on the forecast and how Isaac may or may not be a big player in the weather here through the end of the week. Here are some thoughts I have after a day full of analysis and some late-day model guidance.

First, some updates on the civil situation:

• Governor Bentley has declared a State of Emergency for Alabama as have Governors Bryant and Jindal of Mississippi and Louisiana.
• A mandatory evacuation order has been given for coastal Mobile and Baldwin Counties for Zones 1 and 2 beginning at 8 AM Monday.
• A Hurricane Warning is in effect for areas east of Morgan City, Louisiana to Destin, Florida including Metropolitan New Orleans, Lake Ponchartrain and Lake Maurepas. This covers the entirety of the Alabama Gulf Coast.

The 18Z (1 PM CDT) GFS model run still trends what should be Hurricane Isaac toward the Louisiana Gulf Coast by Wednesday morning. That would put Alabama and Florida’s coast at less of a risk for significant problems; however, there is still a large amount of uncertainty involved here. Plus, a lack significant problems doesn’t mean no problems at all.

The most impressive part of the 18Z GFS package is how consistent the ensemble members are with the set-up at 500 mb (that’s about 15,000 to 20,000 feet above the surface)! Ensembles runs are a way for us to simulate small changes in the atmosphere that could change the outcome of a weather event. In short, models have to do a lot of estimating; ensembles allow us to see a number of other “estimations” that are slightly different.

All of the little red circles around the thicker white circle are the ensemble positions of the upper-air “part” of Isaac’s circulation. This reflection matches up well with where the center of the surface circulation is most of the time, and in this case, we know it’s almost identical.

Notice how few little red circles there are over Alabama or Florida? That means this model is very, very confident that the storm will not get picked up in the disturbance passing by Monday and Tuesday that could pull it north. (That idea was discussed earlier in Isaac: This Way and That Way earlier today.) So, if you have read that post already, you know that this model is trying to close off one of the paths and tell us to look at the other one (the westerly one) more closely.

Once the European model (ECMWF) and the 0Z (7 PM) GFS come in late tonight, I believe the picture of Isaac’s future will start becoming very clear…that is, as long as they are in some agreement!

If Isaac does take the westerly route toward Louisiana, there will still be some storm surge all along the Gulf Coast. This is the latest SLOSH model that identifies areas of storm surge based on the forecast path from the National Hurricane Center. Most of Alabama’s coast could see 2′ to 6′ of surge; Louisiana and Mississippi could get up to 11′ of surge:

Here’s the latest track from NHC:

So, what does this mean for the Tennessee Valley?

If my hunch is right, and the storm passes to the west into Louisiana, there will be less of an impact here. As long as there is no quick right-hand turn that we don’t see, the risk of heavy rain and severe weather will be limited. Don’t stop paying attention here just because it sounds like the affects will be minor; a lot can change, and we never want to base a forecast on one single model run. It’s just that in this case, the model is the news because of its consistency…and that could be good news for most of Alabama.

-Jason
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