Some strong/severe storms moved through the Midstate late last night, producing some 1″ diameter hail along the way…but the main threat of severe weather will take shape over the next 24 hours.
Let’s start with a somewhat tenuous severe threat this afternoon and evening. It looks like scattered storms are going to develop, and some of those could become strong or marginally severe. This Futurecast image shows that the best chance of these scattered afternoon storms will be along and north of I-40, and west of I-65:
Like with the storms last night, the main threats will be large hail (1″+) and damaging winds (60+mph). The severe threat won’t be as high with these storms (compared to tonight) because while there will quite a bit of energy in the atmosphere, there won’t be much to organize these storms. (Organized storms are much more likely to become severe.) Because of that lack of organization, the exact timing (or even their development in the first place) is really hard to pin down — at any point this afternoon and evening, the atmosphere will be able to support thunderstorm activity. The Storm Prediction Center does not anticipate issuing any watches for our area through early afternoon.
There are still a number of questions surrounding tonight’s severe weather potential, mostly relating to the timing of the squall line that will be moving in from the west. My original thinking this morning was that storms could be moving into the Tennessee River valley a little before midnight, but some of the latest computer model guidance suggests that the squall line will only be reaching the Mississippi River at midnight, which means a later arrival time for us, within the time ranges shown here:
Usually as a severe weather event approaches, we’re able to narrow down the time frame as our confidence in the computer models’ output grows, but so far today that hasn’t been the case…thus, still a three-hour window from the earliest possible arrival time to the latest. (One of the models is depicting a scenario with two strong storm lines moving through, so obviously things are still in flux.) The later the storms arrive, the lower their severe potential will be…but that doesn’t mean we’ll be out of the woods. As I wrote yesterday, the main threat with squall-line severe thunderstorms is associated with damaging straight-line winds in excess of 60 mph. We’ll also face a severe threat in the form of large hail, and there may still be enough rotation within the storms embedded in the line to a produce a couple of isolated tornadoes:
Breaking down the specific threats into categories, here’s the SPC’s map for the tornado threat — most of the Midstate is within the 5% risk of a tornado within 25 miles of any one location, 10% in extreme western counties:
Hail, as I mentioned earlier, is also a concern…15% risk of 1″ hail within 25 miles for most of Middle Tennessee and southern Kentucky, but again those westernmost counties have a higher risk, in the 30% category:
Overall, you’ve got the idea by now — the farther west you are, the storms will arrive earlier and they’ll be stronger. The worst weather today and this evening will be in Arkansas and Missouri — they’re staring down the barrel of a significant tornado outbreak, with baseball-size hail thrown in for good measure. Our severe weather threat is more borderline in comparison, but what I wrote yesterday still stands: storms will likely be severe as they move into the Tennessee River valley, not-as-strong-but-still-severe as they approach I-65, and marginally severe as they move into eastern Middle Tennessee.
Coincidentally (NOT ironically), today’s severe weather threat comes on the 40th anniversary of the “Super Outbreak” of April 3-4, 1974. The NWS has put together a thorough look back at that event here, if you’re so inclined.