The latest update overnight from the Storm Prediction Center for Wednesday has expanded the Level 2 of 5 risk of severe storms northward to include a large portion of Tennessee, all the way northward to the TN/KY line. It remains in place south of there all the way down into south central AL/MS. What is important to note is that the outlook forecaster mentioned that they had considered upgrading to a "significant tornado driven" Level 3 of 5 risk across our immediate area, and that that upgrade may happen in future updates. Trends in high-resolution model guidance have been concerning, and are pointing toward higher confidence in the potential for supercells with a tornado threat.
Above is output from the SREF (Short-Range Ensemble Forecast) system. The SREF is a system of high-resolution models that are put together into an ensemble system and then compared, contrasted, and averaged together to get a set of possible outcomes for weather systems. This particular product is called the "Significant Tornado Ingredients" product, and it shows the probability of significant (EF2 or greater) tornadoes based on the environmental conditions averaged together from all of those high-resolution models in the SREF ensemble system. The SREF historically has a slow timing bias, but it often does a good job keying in on the placement and intensity of environments. As you can see, the SREF system here is really targeting an area favorable for significant tornado formation from north Mississippi, through north Alabama, and into middle Tennessee. This is the same area that the Storm Prediction Center is most concerned about and is considering an upgrade to Level 3 of 5 risk, for the potential for significant tornadoes (EF2 or greater intensity).
Here is the latest run of the high-resolution Baron Futurecast model. You can scroll back and forth through the images to see the overall timeline. This gives an idea of the timing and the evolution of the storm threat for Wednesday. I have taken the future radar/clouds product from the model and overlaid model rotation tracks on top of it to create the "StormTracker" product. This will not only show where the model thinks storms will be, but it will show which ones the model thinks will or will not be rotating. Please note that a rotating storm does NOT automatically mean a tornado, but rotating storms are the ones that produce a tornado if a tornado is going to develop. Also, please don't get hung up in the exact specific timing or location of a specific storm. This is only to show an overall idea of the evolution and timing of the threat, and an idea of the behavior of the storms the model forecasts.
There may be a few showers or even a rumble of thunder overnight Tuesday night across portions of north Alabama or middle Tennessee, but severe weather is not expected. This may fill in a little more over portions of middle Tennessee toward daybreak before shifting northward. Locally heavy rainfall will be possible, but again, severe storms during the morning are not expected (although you never say never, and we will have eyes on radar in case of a surprise). As we head through the morning, midday, and even afternoon, we look to get a few breaks in the clouds from time to time. That will warm temperatures into the low to mid 70s, with dewpoints climbing into the mid and upper 60s. With cooler air in the mid and upper levels, the atmosphere will grow pretty significantly unstable by late December standards.
By midday and early afternoon, Futurecast is showing a few random showers and storms popping up in the open warm sector. We will have to begin watching these because the atmosphere will be unstable enough for severe weather, and wind shear will be increasing, but that early on the stronger low-level shear and stronger lift hasn't quite arrived. By the mid to late afternoon strong to severe storms start developing over west Tennessee and north Mississippi and begin tracking east-northeast toward the area. We think it's around 3:00pm or so that the main threat window begins to open across our viewing area, from west to east. Then, from that timeframe through the evening, Futurecast here is showing multiple discrete (individual and separated) rotating severe thunderstorms tracking across northern Alabama, southern middle Tennessee, and north Mississippi. These would be supercell thunderstorms with an elevated tornado threat. In setups like this where you have very subtle lifting mechanisms, it's true that you don't have a "classic" setup for severe weather on paper, but that subtle lift means that storms are more isolated and off on their own, and you don't get as many competing for each other or bumping into each other and lining up. You're more likely to get a supercell storm mode in situations like this, and they're more likely to stay supercellular for a longer time. With this in mind, and the environment in place, our concern for a tornado risk is increasing. And while there won't be a high number of storms to make this an "outbreak" or anything, that says nothing of the intensity... and we cannot rule out a strong tornado or two (EF2 or greater intensity). This is also the concern that the Storm Prediction Center expressed in their outlook, and why they mentioned our area may be upgraded to a Level 3 of 5 risk in future outlook updates. Storms will also be capable of producing wind gusts of 50-70 mph and quarter to half dollar size hail, and if we get multiple storms tracking across an area, isolated flooding is also a concern. This continues through the evening, with the threat shifting south and east of our viewing area counties and diminishing with time after 10:00-11:00pm.
A quick note about the weekend system... We will be talking about that in our weathercasts today on social media and on the Tennessee Valley Weather Channel. There is still an areawide severe weather threat Saturday and Saturday night, but the magnitude is still uncertain, and there are more questions than answers at this point. Another 24-36 hours, and we should start getting a better first idea of how the Saturday threat will evolve.
With either system, it is important to be prepared and informed ahead of time. Take the time today while the weather is still quiet to review your safety plans, or formulate one if you don't have one, and make sure you have multiple reliable ways of hearing watches and warnings. Waiting until the last minute to try to come up with an on-the-fly plan can put your life in danger! This is the case with any severe weather threat, not just this one. Plan now if you haven't already.
Be sure to stay in touch with weather information as we approach this Wednesday threat and the weekend threat. And we will continue to fine tune the forecast and provide updates as details continue to work out.