Severe Weather Threat Marginal on Thursday

When it comes to severe weather, “marginal” is one of the best words you can hear. Marginal means that some of the ingredients are in place, but they are either not in balance or just not mixing well. A “marginal” threat means we probably will not have to deal with widespread strong or severe storms; however, it only takes ONE storm to make a big impact, so we will remain vigilant and watch each storm closely to make sure you are informed of any threat that may be coming in on you.

Last night’s model runs indicated a trend toward slower movement of the storms, and that led us to believe the threat could be a bit more elevated than previously thought. Now, we are seeing that those indicators were outliers – they do not fit the trend, so we are discounting that elevated threat.

This morning’s data suggests we could still have some relatively strong storms, and there is still a low chance (under 10%) of one or two of those storms turning severe by mid-morning. It is something we will monitor on a case by case basis.

The Storm Prediction Center has not included any of the Tennessee Valley in the Day 2 Severe Weather Outlook; there is little reason to believe this will change through tonight or tomorrow:

If you are not interested in the technical aspects of this event, scroll on past the pictures to The Bottom Line.

Here’s what we are looking at now:

Two of the primary factors to look at in a case like this are wind shear and dewpoint. Wind shear is the change in wind direction and speed as you go up from the surface to around 20,000 feet in the atmosphere. We’ve got a LOT of wind shear forecast for Thursday; this is a map of the NAM forecast 0-1 kilometer helicity (wind shear in the lowest 1,000 meters or 3,200 feet of the atmosphere). Since the storms will not have a lot of fuel in the form of warm, moist air, they will be low-topped – that means they won’t have the look of those big, towering thunderstorms of Spring and Summer. They will only be about 10,000 to 15,000 feet tall, so that low-level helicity is important in determining if those storms can rotate:

Helicity greater than 250 m2/s2 is significant; that means storms could rotate. When it is approaching 600-800, many storms will rotate. Not all rotating storms produce tornadoes! Many storms rotate and never produce any kind of wind damage; that is why the National Weather Service does not issue “rotating storm warnings;” they issue “Tornado Warnings.” If you simply base the forecast on this factor alone, you will miss the big picture.

I mentioned the fact that we are lacking fuel for the storms. There’s a guideline we go by around here that says if the dewpoint is less than 60 degrees, severe storms are not very likely. They can occur, but it is rare to have significant severe weather with dewpoints less than that.

The dewpoint is a good way to look at how warm and moist an air mass is. The higher the dewpoint, the higher the moisture content of the air, and the more unstable that air mass can become in a situation like this. I’ll switch forecast models on you just to show you the most aggressive one out there right now: the RPM. The RPM has been overdoing the moisture return lately, and that means the numbers you see here may be a little high:

Dewpoints are expected to get up close to 60º over most of North and Central Alabama on Thursday morning. That higher dewpoint air will probably stay south of Tennessee, though.

So the guideline says look for 60º or higher, but we can even take it a step further. Look how far north the 64º dewpoint line is. It never gets north of US 278 through North Alabama. That is a good sign!

We can have all the shear you can imagine, but if there is no fuel, it becomes just like a jet sitting on a runway with no fuel: those engines won’t go!

The Bottom Line:

• Timing: Right now, storms are expected to move into The Shoals between 5 AM and 8 AM. Some showers and storms will develop over South and Central Alabama in this time frame as well. Those developing storms will cut off a lot of the good low-level inflow of warm, humid air, and that should prevent significant severe weather in the Tennessee Valley. Showers and storms will rumble across North and Central Alabama through mid-morning and move out between 12 PM and 3 PM.
• What you need to do: Just be alert. We don’t recommend sitting up all night worrying about this. It is a “marginal” threat. Have a NOAA weather radio ON and programmed, or check out some of the smart phone apps that can alert you to a severe weather warning: WHNT App Recommendations.
• What kind of storms? Most likely, we will deal with a decaying (weakening) squall line moving across the Tennessee Valley after sunrise Thursday. The tornado threat is not totally zero, but is tiny. The severe weather threat is not zero, but is is quite low as well. Any severe weather would like come in the form of wind gusts exceeding 60 MPH; that would be a very isolated incident if it occurs.
• The wind ahead of the storms will be gusty. We could have some non-thunderstorm winds in excess of 40 MPH on the ridgetops tonight and Thursday morning. That could blow down trees and knock out power to some communities even where no “severe” weather occurs. Be aware as you are out driving that tree branches, garbage cans, and other objects can get blown out in the road even when no storms are present!

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-Jason
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