The Storm Prediction Center maintains a Level 2 out of 5 risk of severe weather from near Clinton, TN to Pyburn, TN and westward. The entire rest of the area is under a Level 1 of 5 risk of severe weather. Because of how the outlooks run from 6am to 6am (and 7am to 7am in the spring and summer months) and how the risk timing straddles that timeframe for what they consider a "day" in their outlooks, the Storm Prediction Center has had to draw outlooks on both their Sunday and Monday forecasts. It's all still one event, with a general timing for our area that runs from after midnight until about 8:00-9:00am or so. We will detail timing in this blog post. The overall main threats are one or two storms being capable of a 50-60 mph wind gust or a spin-up tornado, but most storms will be producing wind gusts in the 30-40 mph range. We can't rule out a storm or two producing hail up to quarter size because of the very cold air aloft, or isolated flooding because of the heavy rain, but those are lower threats.
Here is an overall timing of today, tonight, and into Monday with our high-resolution Baron Futurecast model. We think it has a good handle on things. You can scroll through the images for a breakdown of the timeline of everything, to go along with our description here.
We start out warm (by December standards), cloudy, and breezy by daybreak on our Sunday morning, with foggy conditions and maybe a sprinkle or shower. Temperatures will be in the upper 50s for most of us, but a few folks may already be near 60 by that time. As the winds continue to kick up and we work later into the morning, the fog breaks up and we get a few sun breaks. Temperatures by the afternoon climb into the upper 60s and lower 70s for everyone. It will be a windy day with southerly wind gusts as high as 20 to 25 mph at times. We can't rule out a stray shower or storm during the afternoon and evening, but the more widespread activity holds off until the overnight hours.
Scattered thunderstorms start increasing in coverage over west Tennessee late in the evening, and really expand after midnight. It's at this time, as the storms intensify over west Tennessee after midnight, that we start opening up the door in our western counties for a few stronger storms. The heart of the action with the main line, however, doesn't move into our region until after 3:00-4:00 am, and maybe closer to 5:00am. From then until about 8:00-9:00am, the main line sweeps east and southeast along and ahead of the cold front across middle Tennessee and northwest Alabama. This is the main round of storms that has the potential to pop a few severe thunderstorm warnings, or maybe even a stray tornado warning or two.
This timeline of wind gusts across the area from the Baron Futurecast shows that, even during the day, wind gusts will be in the 15 to 25 mph range out of the south, way ahead of any shower or storm activity. However, as that main line moves through early Monday morning, those darker red areas on the map are wind gusts in the 30-40 mph range, which look to be widespread with the line, and then the purple to blue areas are isolated spots with wind gusts of 40, 50, maybe even 60 mph.
Temperatures in the mid/upper-levels of the troposphere will be very cold, and this will help to offset temperatures only in the low to mid 60s during the predawn hours, to still provide some instability. Especially with dewpoints looking to get into the 61-62 degree range areawide now, it does appear likely that we will get 300-500+ j/kg of CAPE across the area (instability/fuel for storms), which is enough for maybe a few storms to be strong to severe. The updraft helicity product from the Futurecast there shows tracks for where the model thinks storms may have rotating updrafts. The exact specific locations shown are not important, but the overall idea is there that a few storms could be in rotation, and that means we can't rule out a tornado or two being possible.
If there is any good news to point out, it is that the threat of flooding, overall, is low. While we can't rule out isolated instances of flooding or flash flooding, we expect rain totals to generally be between three-quarters of an inch to an inch and a half. Totals up near 2 inches may be possible in isolated areas where the heaviest rain occurs. With how dry we have been over the past month, that will help to mitigate any potential for more widespread flooding or flash flooding.
The overall risk of severe weather across our area is low. This is a lower-end, minor type of threat. However, the risk is NOT zero, and there may be a few warnings and a few storms that produce damage across the area, either from damaging winds or a couple of tornadoes. That means that you need to review your safety plans and be in a position to hear watches and warnings if they are issued. What makes that even more important is that most of this will be happening before sunrise on Monday morning, when a lot of you all will be asleep. You need a way to hear warnings that will wake you out of your sleep in the overnight. We will have the weather center staffed and will be ready to provide live updates on all of our platforms should warnings be issued.