Severe storms possible Tuesday night into predawn Wednesday, but the threat isn't guaranteed!
If you have been following our forecast information since about the middle of last week, you will know that we have been watching Tuesday night into Wednesday morning for the potential for strong thunderstorms in or near our area. Since our weather across the local area for Monday and Monday night will be quiet, and weather after Wednesday morning through the weekend will be low impact (showers possible late week and the weekend), this blog post is going to focus specifically on the severe storm risk to our area coming up Tuesday night into early Wednesday morning.
First off, areas to our west across north and western Mississippi, northeast Louisiana, and southeast Arkansas have been upgraded to an uncommon Level 4 of 5 "Moderate Risk" of severe storms for Tuesday and Tuesday night. The discussion portion of the outlook states that a "regional tornado outbreak" is possible in those areas with the potential for long-tracked strong tornadoes. That is where the worst of the severe weather threat is expected to line up for Tuesday and Tuesday night. The last time a Level 4/5 "Moderate Risk" was issued the day before an event in the month of November for anywhere in the United States was November 2013. Before that, the last such occurrence was November 2006. It is very uncommon to get a "Day 2" (day before the event) Moderate Risk in the month of November. This is a big deal type of threat for those areas involved.
But what about our local area here in the Tennessee Valley? The same overnight outlook from the Storm Prediction Center office of the National Weather Service expanded both the Level 2 of 5 and the Level 3 of 5 risk zones more into our local area. A somewhat uncommon Level 3 of 5 risk of severe storms now includes portions of our area near and west of the Natchez Trace Parkway. In our local viewing area, this includes Wayne and Hardin Counties in Tennessee, Tishomingo County in Mississippi, and western portions of Lauderdale, Colbert, and Franklin Counties in Alabama. East of there, the "standard" Level 2 of 5 risk of severe storms now includes the remainder of our viewing area across southern middle Tennessee and northwest Alabama with the exception of areas east of I-65 in Morgan County, Alabama. But even there, there is now a Level 1 of 5 risk in place. These risk levels include all "threat types", meaning that severe storms would be capable of damaging winds of 58+ mph, hail to quarter size, and even tornadoes. That is the official outlook in place. Going forward in the outlook, we will talk more in detail about why the risk isn't exactly a guarantee that severe storms WILL happen in our area...
The individual probabilities associated with that outlook show elevated potential for damaging straight-line winds and tornadoes over those areas in the Level 3 of 5 risk. The outlook actually includes the potential for a strong tornado or two (EF2 or greater) near and west of the Natchez Trace Parkway in that Level 3 risk area, although the threat for that is higher back to the west. Again, this is the official outlook from the NWS Storm Prediction Center. We are showing this because this is official information and we want to be fully transparent with legitimate and respectable weather information, and the potential for this to evolve like this is there. It's just maybe not as clear cut and set in stone as the outlook product would have you to believe at first glance...
We've been looking at the large scale setup for this storm system for days, and on the large scale, many of the players that you'd look for in order to see a significant severe weather event are indeed on the board. However, we are now starting to get into the time frame where this threat is beginning to get into the higher resolution model data that is designed to help guide us with how the smaller scale features will evolve, the features of a storm system that will ultimately make the final decision of how the event will unfold.
The first model we're going to look at is our in-house 3 kilometer "Futurecast" model from Baron Weather in Huntsville. Futurecast starts our Tuesday with a mostly sunny to partly cloudy sky and temperatures generally in the mid 40s. The more unstable air is off to the south, but it does move northward through the day. By the midday hour, temperatures are climbing into the mid 50s with low-level cloud cover becoming widespread. That is something we will have to watch. Often times in the cooler months of the year, when we have these events where the higher moisture moves back into the threat area in the last few hours before the event instead of being there ahead of time, that moisture often brings thick low-level cloud cover with it. If that is the case, we won't be able to warm as much going into Tuesday afternoon, and that would help hold back on how unstable our atmosphere can become.
By early to mid afternoon, as the deeper moisture starts to approach our area with a northward moving warm front, what we call "elevated" thunderstorms begin to develop over north Mississippi and west Tennessee and then move across our area during the afternoon hours. These storms are "elevated" because they are forming in the warm, unstable air that is moving up and overtop the cooler and stable air at the ground level that's north of the warm front. Elevated storms can still sometimes be severe, especially with a threat of hail (although the threat of hail on Tuesday isn't high) and sometimes gusty straight-line winds, but with the flow of warm, moist air into the storm being rooted a few thousand feet above ground level instead of at the ground, it's extremely hard to get a tornado threat from elevated storms. Another thing these storms forming north of the warm front would do is put rain falling into a cooler and drier air mass over the area, and when this happens, it often reinforces the cooler and stable air and holds the warm front back from moving northward as quickly.
As we work into the evening hours, Futurecast has strong thunderstorms over central to northeast Mississippi approaching, but the rain-cooled air from the afternoon storms sets up a boundary that keeps the stronger storms to our south, down over west central into central Alabama. Meanwhile, locally heavy rain and embedded non-severe storms move into our area of north Alabama and up into southern Tennessee into the evening and overnight. IF this Futurecast model idea is right, this would keep the main risk of severe and especially tornadic storms off to the south of our viewing area, and our local area would mainly be dealing with the risk of locally heavy rain and isolated flooding... and maybe an isolated risk of small hail or gusty winds during the afternoon with those elevated storms north of the warm front. There are other models that we don't ingest into our TV graphics system that do something at least somewhat similar to this.
Another reliable high-resolution model we use for thunderstorm forecasting is the HRRR, the High-Resolution Rapid Refresh model. It is run hourly every day, and four times a day, the model runs go out to 48 hours. The overnight run of the HRRR goes out to Midnight Tuesday night, 12 AM Wednesday. The HRRR starts Tuesday morning similar to the Baron Futurecast model... a mix of sun and clouds over us with temperatures into the 40s. However, as the higher moisture content air begins to work northward in the low-levels during the day, the HRRR has drier air aloft that allows some holes in the cloud cover. This allows for us to have sun breaks which, in turn, warms us up into at least the mid to upper 60s during the afternoon... if not near 70 degrees in a few spots. There are still scattered storms during the afternoon, but they are much more isolated in nature. The other concern is that they have a somewhat more unstable surface-level environment to work with, especially back over northern Mississippi. However, the richer low-level moisture still hasn't moved into our immediate area during the afternoon, and as some of those scattered storms form over southern Tennessee and northwest Alabama late in the day, they kind of do the same thing that the Baron Futurecast was doing, and they reinforce cooler and stable air over southern Tennessee and north of the Tennessee River in north Alabama as we head into the evening.
After sunset, the HRRR has intense long-track supercell storms approaching from north central Mississippi, but as they move into northwest Alabama, they encounter a more stable air mass and begin to lose their identity as individual supercells. Having said that, the HRRR shows just enough instability at the surface near and south of the Tennessee River that any storms moving in would still have some degree of a tornado risk, just not NEARLY as much as back in Mississippi. North of the Tennessee River, the air is cooler and more stable, and although there is heavy rain and strong thunderstorms with gusty winds or maybe small hail, the tornado risk is much much lower if the HRRR is correct. Both models show these thunderstorm complexes shifting east of our area during the predawn hours of early Wednesday morning and the actual cold front moving through near or before daybreak.
The parameter above is Updraft Helicity from both the Baron Futurecast and the HRRR. Updraft helicity is the amount of wind rotation from the environment that the storm's updraft (its engine that takes in warm, moist air) is able to ingest based on low unstable the air is and how high that wind shear is in the environment. Storms with higher updraft helicity have an updraft that is rotating more strongly and have a higher chance of being severe or tornadic. You can definitely see that the Baron Futurecast is keeping those rotating storms to the south of our area, from central Mississippi into central Alabama. However, the HRRR brings rotating storms into northeast Mississippi and areas of north Alabama west of I-65 and near and south of the TN River, matching the idea that there's a little more juice and instability to work with in north Alabama on the HRRR. However, even then, it's very easy to see that the most violent storms are back into Mississippi.
With all of this information in mind, what are MY thoughts about the threat in the local area? The map to the left explains how, at least as of now, I think the event may unfold Tuesday night in our area. Locations in yellow, extending as far north as southern middle Tennessee, do not need to ignore this threat. Even if storms are elevated, there could be gusty winds and small hail, and there may even be a couple of severe thunderstorm warnings. IF afternoon and early evening storms aren't as widespread as some of the higher-resolution models are suggesting and we get 64-65 dewpoints up into the area before overnight storms move in, there might even be a tornado threat in these areas. HOWEVER, based on the latest data we have available, it is very possible that the tornado threat (and possibly even the severe threat overall) may stay to the south of southern middle Tennessee.
The areas outlined in orange, northwest Alabama and northeast Mississippi... this is where I think there is the potential for enough surface-based instability to be in place for a severe storm threat (including a possible tornado risk) during the overnight hours of Tuesday night into the predawn of Wednesday. HOWEVER, if a solution closer to the Baron Futurecast is right and these areas get locked in the rain-cooled stable air mass, the risk of severe storms and tornadoes would likely stay off to the south, down over central Alabama.
The areas in red on the map are where I think the ingredients do line up for the potential for long-tracked individual supercells that may produce strong (EF2-EF3) or even violent (greater than EF3) type tornadoes that stay on the ground for a long period of time. That type of risk gets close to our southwestern coverage areas, but it doesn't quite make it. I think the biggest risk of that type of threat is back over Mississippi.
These thoughts are all still subject to change in one direction or the other. Should we see more evidence in the data that the unstable air can move into northwest Alabama and/or southern middle Tennessee, the tornado threat for those areas would increase because of that. But for right now, the high-resolution model data isn't trending in that direction. If anything, the data right now currently fits the idea that this may be yet another event that is displaced a little south and east of where larger scale ingredients would suggest the greatest threat should be. We've seen that SEVERAL times over the last few years with events, and this may be another one of those. This is why we stress that it is those smaller scale features that have the final say in how a severe weather event ultimately behaves and evolves.
Regardless of how this all unfolds in the local area, it is important that you take time over the next 24 to 36 hours while the weather is quiet to prepare for the potential for severe weather. Make sure you have a safety plan of where you are going to go to seek shelter if a warning has to be issued for your location. Make sure you have multiple reliable ways of getting a warning, including one that will wake you up in the middle of the night. Above are several graphics with information on things like the difference between warnings and watches, tornado safety/shelter guidelines, as well as information on how you can get updated information from us.
We will have the Tennessee Valley Weather Center staffed for the duration of the threat, and we will be ready to provide LIVE coverage should warnings get issued in our viewing area, including LIVE NON-STOP coverage on all of our platforms should any county in our viewing area come under a Tornado Warning.