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  • Fred Gossage

Stormy weather pattern by Monday. Details unclear, but can't rule out a severe weather risk.

So far, this "cool season" (fall and winter) has been mostly quiet in terms of severe weather threats across the Tennessee Valley. There have been a few events to our south that have had damaging winds and a few tornadoes, but our immediate area has been spared. In fact, our viewing area hasn't had a tornado warning since the remnants of Hurricane Laura back in August! We are watching a storm system that will move through at the beginning of next week that has at least the potential to change that, but the details are unclear at this time.


See that big U-shaped dip in the jet stream flow over the Four Corners area? That is an upper-level storm system that will eject out of the Southwest into the Plains between Sunday night and Monday morning. In the days leading up to that, the little storm systems that ride along our frontal boundary and provide us with rain chances this week will keep a southerly flow in place across the Gulf of Mexico that originates from down in the Caribbean. This primes the Gulf of Mexico with a warm, humid, unstable low-level air mass with dewpoints way up through the mid and upper 60s, and in some cases, the lower 70s out over the open water of the Gulf. With the frontal boundary near us never moving back south through the Gulf, but rather stalling on the coastline, that humid air will be sitting out over the Gulf and waiting for the storm system to eject out into the center of the nation. As that upper-level system approaches Texas on Sunday, that will start southerly flow across much of the Southeast which will allow that humid and unstable air mass from the Gulf of Mexico to begin to race northward ahead of the ejecting storm system.


As that happens, in the mid-levels of the atmosphere, the southwesterly flow aloft will transport a plume of unstable air off the Mexican plateau out over the Southeast ahead of the storm system. This plume of air is what we call the elevated mixed layer (or EML). It is characterized by being dry and from the bottom of its layer to the top of its layer, the temperature cools rapidly with height... making that whole layer of air very unstable. When this mid-level layer of air moves out overtop rich Gulf moisture in the low-levels, it sets up a large-scale environment that has unstable air needed for thunderstorm development.


Here's where the forecast gets tricky. Above is a look at that same upper-level system as it ejects out into the Southern Plains and Mississippi Valley by midday on Monday. Notice how the GFS model loses a lot of that U-shape to the trough and it's mostly flat, but the other main models we use have a much more defined U-shape to the trough. The GFS has the system weakening as it approaches, but the Euro model, Canadian model, and UKMET model have the system holding its strength. This is actually a known and documented bias in the GFS and its ensemble system for troughs like this to be too flat and sheared apart compared to what really happens in reality. However, the other models have had a little trouble with inconsistency themselves. This is why the forecast at this point is really murky. How strong the system is when it approaches plays a VITAL role in the outcome in our area on Monday.


If the trough remains stronger as it approaches on Monday, the surface low pressure associated with it will be stronger and will be located farther north. This will allow the warm sector to expand farther north and the unstable air we talked about earlier will pull well north through Alabama, Mississippi, and Tennessee. This also means that the "muscle" of the storm system (lift, dynamics, and wind shear [spin in the atmosphere]) will be stronger and able to act in combination with that unstable air. This scenario would present the potential for strong to severe storms across all of the area, with all severe weather threat "types" being possible.


However, if the trough flattens on out closer to what the GFS shows, the system will be weaker. The jet stream will stay back on the cool side of the front instead of crossing out across the warm sector. The low pressure will be farther south. There will still be unstable air in the warm sector, but the warm sector won't move as far north AND there won't be as strong of "muscle" from the storm to combine with the unstable air. If this scenario plays out, the main threat would be heavy rain and thunderstorms that may even have the potential for flash flooding. We still couldn't 100% rule out a severe weather threat either, but the risk would be much lower and may be shifted south of the area, compared to what may be a more substantial risk... including a risk of tornadoes... if the system plays out closer to Scenario 1 outlined above.


The overall strength of that upper-level trough is KEY to this. A stronger trough means an overall strong system and one that is more favorable for a severe weather threat. A weaker system means a lower risk of severe weather and a risk that may stay south of our area. Regardless, rain and thunderstorms are likely... and if this evolves closer to Scenario 2 outlined above, flooding could even become a risk.


We are five days out; so, there is no possible way to know how the details of this will play out. But we ARE within the range where there is enough credibility to know that there IS a system to watch. For the next day or two, it's all about watching for trends and consistency and getting a handle on what the larger scale setup should look like. This will give us a general idea of whether this leans more toward heavy rain or more toward a severe weather risk. As we get closer to the weekend, we should start having a handle on the smaller scale details that will decide (IF there is a severe weather threat) what the magnitude of the risk is, the timing of the risk, and what areas will be involved. Make sure you keep checking back over the coming days as we update the forecast as details become more clear. There WILL be changes to the forecast as we become more certain about how this evolves. Remember, weather information is just like milk... it has an expiration date. Old information is BAD information!

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