Weather is quiet across the Tennessee Valley early on this Sunday morning, but we are watching developments off to the north in the Upper Midwest ahead of a cold front that will move our way over the next 24 hours. Severe weather has been ongoing across portions of Iowa, Illinois, and northeast Missouri in association with this system. This storm system will eventually bring showers and thunderstorms to our area later this morning and afternoon and then again this evening, in two rounds, and some of these storms have a good chance of being strong to severe across our area.
The overnight update from the NWS Storm Prediction Center has upgraded a large part of our viewing area (as well as regions from Arkansas and Mississippi to the Ohio Valley) to a Level 3 of 5 risk of severe storms for today and tonight (Sunday). In our local area, the primary risks are the potential for multiple storms to have damaging winds of 50-70 mph as well as the potential for a few storms to have hail. The tornado risk is low but is not completely zero. Flooding isn’t expected to be a big issue, but there could be some localized street flooding or flash flooding in areas that get heavier rainfall amounts.
There’s a good bit of uncertainty with how everything will evolve later today and tonight, especially with the second round during the evening. That round’s intensity and timing will both depend a lot on exactly how the first round of storms during the late morning to early afternoon behaves and how much air mass recovery there can be behind it (if any) during the afternoon hours. Our in-house Futurecast model shows things quiet across the area until around 10-11am when a complex of storms from the north starts moving into our northern and northeastern viewing area counties. It then continues southeastward toward northeast Alabama by midday, with much of the original complex missing us to the east. However, outflow from that complex sparks new strong storms over southern middle Tennessee during the midday to early afternoon hours, and these move southward through southern middle Tennessee into northwest Alabama in the Noon to 4pm timeframe IF this model is correct. This doesn’t leave a lot of time for air mass recovery before the evening round of storms starts to approach from the west around 6-7pm. IF this model is right, this would allow storms to weaken as they move into the area during the early to mid evening. There would probably be some initial severe storms with damaging winds and hail, but IF this idea is correct, storms would be weakening with time as they move west to east across the area through the evening hours.
The HRRR model, another one of the main high resolution computer forecast models we use, has some similar ideas (especially with the first round) but ultimately ends up with a different outcome. Like the Baron Futurecast model, the evening run of the HRRR starts moving storms into our northern and northeastern counties around 10-11am, with the main complex moving southeastward toward northeast Alabama by midday and the early afternoon. It should be noted that recent runs of the HRRR model since this one have also kept trending east with this first complex. Another important difference is that, unlike the Baron Futurecast, the HRRR does not develop storms along the outflow to the west of the complex over southern middle Tennessee. This allows much of northwest Alabama to be mainly quiet for the afternoon hours before the second round approaches during the evening. The other big difference between the HRRR model here and the Baron Futurecast is that the HRRR model is a few hours later with the evening round of storms, arriving in the area during the 8-9pm timeframe instead of around 6-7pm or a touch before. This would give more time for air mass recovery ahead of it, allowing the second round of storms to be stronger as it moves across the area during the evening hours. Another interesting difference is that this run of the HRRR model tracks the second complex of storms in the evening a bit farther south, mainly focusing over northwest Alabama.
The big takeaways from the model data above are as follows:
- There isn’t a lot of confidence in the exact evolution of everything, but there is a basic idea in the potential for two rounds of storms. The first round looks to threaten some or all of the area between the late morning and mid afternoon, generally between 10am and 3-4pm from north to south. Both the timing and exact placement of the potential second round of storms in the evening completely hinges on exactly how the first round behaves, but the second evening round looks to threaten part or all of the area in roughly the 6pm-11pm timeframe. It WON’T storm that whole time window, but we have to provide wiggle room for the different solutions that are on the board.
- Both rounds of storms have the potential to be severe with damaging straight-line winds of 50-70 mph and hail of quarter size or larger. However, it is the late morning to afternoon first round that has the better shot at having an atmosphere that is more unstable and not worked over. The intensity of the evening round depends on how much recovery time the air mass has after the first round moves out and before that evening round moves in. If the first round sticks around for longer or it covers a larger part of the area, that would help to lower the severe threat with the second round for the evening. It would not completely remove the severe threat altogether, but it would help reduce it.
Storms move out later on tonight and we transition toward quieter weather as we begin the new work week on Monday. We will still have nearly daily rain and storm chances on the board for the week ahead, but it will be much more in the way of the isolated afternoon/early evening pop-up variety that we usually see during the summer months. After today and tonight, no signs of big widespread rain for at least the next several days.
One big change that will start to happen this week is we make a solid shift toward warmer temperatures. We have already seen a preview of that this weekend with highs in the low to mid 90s, but more consistent and steady summer heat is coming this week. Behind this front on Monday, we have a brief slight cool down with daytime highs hovering in the upper 80s to lower 90s, but then we trend hotter as the week continues. The upper-level ridge of high pressure that’s been parked over the Plains starts building into the Lower Mississippi Valley and toward the Southeast by late week. Daytime highs start climbing consistently into the lower to middle 90s for Tuesday and Wednesday, and by the latter part of the week and the coming weekend, we are in the mid to upper 90s at minimum. There is some raw model guidance that takes us into the triple digits for daytime air temp highs by Friday and Saturday, and it’s very possible we may have to shift our official forecast in that direction as we get closer. Heat index values look to hang in the mid 90s through Wednesday, but as high temps soar and humidity builds by late week, those heat index values climb well above 100 degrees, possibly above 105 degrees, by Thursday, Friday, and Saturday. It is likely we will meet at least Heat Advisory criteria during this time, and I can’t completely rule out some areas getting near Excessive Heat Warning criteria. Summer weather has taken its time getting here, but it definitely looks to be on the way!
