Stormy Weather Next Week?

Posted on: 6:25 pm, November 21, 2012, by

It’s hard to think about next week when we still have a beautiful Thanksgiving Day ahead of us, but November is notorious for wild swings in weather from week to week!

First things first, the initial change for us will be for some much, much colder air Friday night through Sunday and Monday. The leading cold front arrives Friday with a few spotty showers, but most communities will either see no rain at all or just enough to notice (less than a tenth of an inch). There is a lot more detail on the short and long-term forecast on WHNT.com/Weather if you need to make some weekend plans.

Check out those Saturday afternoon temps! A brief “pop” of Arctic Air will blow through the Tennessee Valley on Saturday and Sunday. Saturday’s high is forecast to be 47º, and the low Saturday night/Sunday morning will be around 26º in Huntsville (some of the usual cold spots may touch 21-23º by sunrise Sunday).

As often happens in the business of forecasting the weather, you have to see what happens with an earlier event to accurately predict a later event. In this case, we have to see how much of the cold, dry, Arctic air is left in this area on Tuesday of next week as a potent storm system arrives:

The above image is the surface map forecast for 6:00 AM Tuesday, November 27th. The bright yellowish, orange areas indicate where forecast models predict high instability known as CAPE (convective available potential energy). High CAPE fuels severe storms; low cape usually means a low severe weather threat.

The Forecast Challenges:

  • Arctic air is dense and hard to move; what often happens in situations like this is one half (the western part) of the Tennessee Valley will destabilize while the eastern areas in the rougher terrain stay stable because the Arctic air just can’t be scoured out fast enough.
  • This storm system may not be in good “phase.” Major severe weather events occur when strong dynamics (lift) show up in the same place as strong thermodynamics (CAPE). Based on the latest GFS model output, it could be a close call, but it does look like the best dynamics will race out ahead of the best thermodynamics. If that happens, we will likely get a round of heavy rain and some lightning & thunder; the severe weather threat would be limited.
  • There are always very small details that can make up for an “imperfect” storm. Those are details too fine to see at a distance. We will start seeing those better by the weekend, so be sure to keep in touch with us through Saturday, Sunday and especially Monday to see if our severe weather threat is increasing or decreasing.





Speaking of the storm system’s phase, these are three panels of the midday GFS. The first is dew point, the second is CAPE, and the third is shear between the surface and about 7,000 feet up into in the lower atmosphere. Dewpoints are rising quickly over Mississippi, western Alabama and West Tennessee at midday Tuesday, but the over-all instability parameter (CAPE) is low, and the best wind shear (the dynamic forcing that promotes storm maintenance and even rotation) is well east of the higher dewpoint and CAPE.

Surface Dewpoints forecast for midday Tuesday, 11/27. Dewpoints above 60º F are usually where we would expect a risk of stronger storms.

GFS model forecast for CAPE at midday 11/27. Higher CAPE values indicate regions of severe weather potential. The chance of the CAPE getting over 150 j/kg in Huntsville is only about 10-20% based on ensemble probability.

GFS wind shear forecast for midday 11/27. Note that the stronger wind shear is escaping to the east just as the instability in the previous images increases.

We will keep an eye on these features all through the holiday weekend, so go enjoy yourself if you have some time off! There is no need to worry about this situation right now, but we do want you to check in with us this weekend for the latest!

-Jason
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