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Jack’s

Stormy Weather Early Next Week

Posted on: 3:28 pm, November 9, 2012, by

There are still a lot of questions surrounding the storm system coming into the Tennessee Valley on Monday of next week; the biggest question is “will we have the fuel for intense thunderstorms?”

Right now, the risk of widespread severe weather seems rather low. Earlier in the week, I established it at around 20%, and I don’t see any reason to change that line of thinking at this point. Dewpoint is a fairly simple measure of the fuel; an airmass with dewpoint temperatures above 60º F has a lot more energy than one with dewpoints in the 50s.

Warmer, more humid air (higher dew points) lead to higher levels of instability. In trying to forecast where the potential for severe storms exist, we try to find the place where the leading edge of the warmest, most humid air is moving north into the highest wind shear. On Monday, we do not see extremely high levels of either instability or shear, but there is some of both. That leads us to believe there is a marginal threat of some storms that could have some strong gusty winds and heavy rain; the tornado threat is very low.

Timing: A line of storms 72 hours in the future is almost impossible to “nail” down to a specific place and time, but our best estimate right now is that rain and storms will arrive in The Shoals and travel to near I-65 between 6 AM and 10 AM. The line of heavier storms will be lined up from near Pulaski south and west through Cullman, Morgan, and Winston Counties around mid-morning, and it will continue on through Huntsville, Albertville, Fort Payne, Scottsboro, Fayetteville, and Winchester between 10 AM and 2 PM. Remember, there is uncertainty involved here, and the timing could be off by as much as 2 to 4 hours; check in with us through the weekend and on Monday morning as we narrow it down to more precise times.

How You Should Prepare: First of all, don’t worry about this event all weekend. There’s a big difference in worrying about it and staying aware that there is a risk. The majority of us will just see heavy rain for a brief period on Monday morning; it will total up to around 1/2 to 1 inch of rain in most communities.

Since this will be a line of storms, it will not lend itself to being a “tornado outbreak;” however, any severe weather deserves our attention and respect for the situation. WHNT News 19 offers several mobile apps that can keep you informed on the go: WHNT Weather Apps (at the bottom of the page).

Go about your business on Monday expecting rain; just have a way to stay informed in case one of the few stronger storms moves near you!

Here’s some of the model evidence supporting the “marginal” threat:

Higher dewpoints (above 60º) will begin to surge north by mid-morning Monday:

12Z NAM Wind & Dewpoint forecast for Monday at 9 AM.

The highest region of shear will be moving northeast away from the Tennessee Valley by late morning; however, there will still be sufficient shear to allow some rotation of individual storms moving east through North Alabama and Southern Tennessee. It’s important to remember that “rotation” does not equate to tornadoes; it is just a measure of potential. That means higher helicity leads to a greater threat (in most situations).

12Z NAM with 0-1km Helicity at 9 AM Monday

There will also be some dry air in the mid-levels of the atmosphere on Monday morning; that can often help increase the strength of straight-line winds within lines of storms. Here’s the forecast sounding for 9 AM in Rainsville; there is sufficient wind shear for those strong winds, but it is not a guarantee of severe weather in DeKalb County or in any other spot.

We’ll keep you updated!

-Jason
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