Rain Ends Tonight; Next Round of Rain Brings Severe Storm Potential

Showers will not be as heavy for the rest of the evening and overnight; most spots will get less than a quarter-inch of rain through Friday morning, and there is actually a decent warm-up coming as the sky clears Friday afternoon!

A nice warm-up is also coming for the weekend: highs in the 70s with some sunshine. We’ve got all of the forecast info posted (in detail) at WHNT.com/Weather.

Drier, warmer weather comes in this weekend thanks to a strong area of high pressure northeast of the Tennessee Valley. By the middle of next week, that same high will be in a position to push warm, humid air from the Gulf of Mexico north across the entire region. That warm, humid low-level air sets the stage for what looks like a multi-day severe weather threat from the Plains to the East Coast.

Please read and understand the following statement before you go on to the rest of this blog post:

At more than five days in the future, details are sketchy. There is no way for us to be certain how significant a particular thunderstorm could be, but we do see an environment that looks like it will be ripe for severe thunderstorm development. Seeing an environment is much different than tracking individual severe thunderstorms; in other words, I cannot possibly tell you if a particular town, county, region of a state or other geographical/geopolitical area will get hit with something severe.

The following pictures are general outlines of where severe storms could occur if conditions come together next week; this is NOT an outlook from the Storm Prediction Center. There are no statistics involved here; it’s based primarily on the morning run of the GFS model. The GFS has been very consistent with this system’s intensity and timing over the past two days, so it is currently the model of choice.

Here’s a quick model comparison between the morning GFS and morning ECMWF:

Screen Shot 2013-04-04 at 3.57.39 PM

The ECMWF is slower and less intense with the entire system, but it still sets up at least a low-end severe weather threat for the Tennessee Valley in the Thursday/Friday time frame. The GFS is faster and much more dynamic; if the GFS stays the course and ends up being the more-correct model, it could get ugly over a broad area.

One big reason the GFS is the model of choice right now is that the ensemble guidance puts the greatest instability (greater than 700 J/kg of CAPE) in the Tennessee Valley late Wednesday night through Thursday. That coincides well with the operational (pictured above) timing and intensity of the system. In fact, the operational puts CAPE up in the 1000-1500 J/kg range by Thursday morning. That’s more than sufficient for strong storms.

Screen Shot 2013-04-04 at 3.59.12 PM


This is something that is a long way off in the distance, and our forecast will almost certainly need to be adjusted. It could be adjusted to a simple rain event, or it could ramp up to something ugly. We just aren’t able to see the details this far in advance.

When it comes to thunderstorms, take these forecasts for what they are: a heads-up. The real statements you need to listen for are WATCHES and WARNINGS. Watches are issued for several hours over large parts of states; they are only posted when severe storms are possible within a 6 to 12 hour time frame.

Warnings are posted for small parts of counties when severe storms are impacting them in the immediate future (less than one hour).

None of that is on the table yet; the only thing you need to do now is make sure you’re aware of how quickly the weather could get out of hand next week if everything comes together. Don’t spend this beautiful weekend that is coming up worrying yourself sick over this; just check in with us from time to time and see how things are shaping up.

Like I said earlier, it could turn out to be what some call a “bust” with no severe weather at all, but it’s something we are going to keep watching closely over the next few days.

Connect with me!
Google+: Jason Simpson
Facebook: Jason Simpson’s Fan Page
Twitter: @simpsonwhnt