AFTERNOON UPDATE (POSTED 12:30PM)
The Storm Prediction Center issued their update thunderstorm outlook about an hour ago, and they significantly trimmed back the “Enhanced Risk” area — now only the westernmost counties of the Midstate are included in that, with the rest of us a notch lower in the “Slight Risk”:
Look, the SPC has literally decades of experience on staff, and I don’t criticize them lightly…but I’m not buying this just yet. I get why they did it — the cloud cover stuck around most of the morning, and temperatures and humidity levels have been slow to recover after the morning rain. As of noon, the temperature in Nashville is 67 and the dew point is 57. The temperature would have to climb 8-10 degrees and the dew point 5-7 degrees in order for significant severe weather to occur. But there’s still time for that to happen!
The HRRR model (reference below in this morning’s original post) has a good handle on the current weather setup — in meteorologist-speak, its most recent run “initialized well.” This particular model pushes storms into Nashville by 7:00pm — and that same model shows temperatures warming up 75 and the dew point increasing to 64 before that point. (It depicts a similar increase in both temperatures and humidity for the rest of the Midstate along and west of I-65 — I’m just using Nashville as a convenient and well-populated data point.)
So is that a reasonable scenario? I think it is — skies have at least partially cleared over the western half of the Midstate, and temperatures should warm up to the mid 70s. There’s humid air (60+ degree dew points) just off to our south, and strong southerly winds should pump that up into our neck of the woods this afternoon. That’s more-than-ample “atmospheric recovery.” So I’m sticking with the general timing and potential severity I outlined in this morning’s post. Could I be wrong? Absolutely! I’d LOVE to be wrong today. Everything has to come together almost perfectly for significant severe weather to occur…but the potential is still there. Plan for the worst, hope for the best, as always.
Let’s get right to it: severe thunderstorms are looking increasingly likely late this afternoon and this evening. There are still a few things that can happen to diminish that threat, but the overall picture isn’t looking favorable.
This morning’s rain produced anywhere from 1″-2″ of total accumulation. Fortunately, we didn’t see any severe weather with that activity, but we didn’t really anticipate any. As I wrote in yesterday’s blog post, our HOPE was that the morning rain would take its sweet time moseying through the Midstate, leaving the cloud cover overhead longer, and preventing the atmosphere from warming back up and destabilizing. But it looks like we won’t be that lucky.
As I’m posting this (9:30am), the back edge of the solid cloud cover is approaching from the west, and sun is starting to shine through the clouds along the Tennessee River:
I doubt we’ll see perfectly clear skies at any point today, but even a little sunshine will be enough. Some spotty showers will pop back up as the atmosphere warms up, but those spotty showers won’t be associated with enough cloud cover to keep the atmosphere stable. Futurecast shows breaks in the clouds by noon, with temperatures warming up well into the 70s as the sun rapidly does its thing:
I know I go into some pretty detailed stuff on this blog sometimes, but in this case I can boil it down to a pretty simple relationship. Warmth = bad. That’s it. Every degree matters in a situation like this, and the farther we climb into the 70s, the bigger the powder keg will be for explosive thunderstorm growth this afternoon and evening. If we can find a way to stay cooler (say, around 70 this afternoon), that’s our best-case scenario — but I’m running out of ways to see how that’s going to happen.
The exact evolution of the storm threat this afternoon is still a big question. One of our Futurecast models, the RPM, shows storms developing to our west, tracking into the northwest part of the Midstate by early evening, then marching across the area throughout the evening and early overnight:
A different forecast model, the HRRR (high-resolution rapid refresh) shows a similar solution, but it’s also been trying to develop several clusters of storms traversing the Midstate throughout the afternoon and this evening:
The HRRR updates every hour, so I’m trying to not overreact to any one model run. Bottom line, ANY of these storms could become severe once the atmosphere warms up, but the greatest severe weather potential will be in the 4pm-9pm time frame — that’s when the most-favorable combination of instability and wind shear will be in place. I won’t bother running through the forecast values of all the various severe weather predictive indices…just trust me, they’re more than sufficient for ALL types of severe weather, and ALL of the models agree on that.
Smoothing out the model differences, here’s when I expect the greatest severe weather threat to shape up in different parts of the Midstate:
That’s an estimate, obviously…until the afternoon storms develop and start moving, we won’t be able to pin down specific arrival times.
The Storm Prediction Center has outlined almost the entire Midstate within an “Enhanced Risk” (level 3 of 5) for severe thunderstorms:
In light of the way things are shaping up, and considering the SPC’s definitions of the various threat levels, I wouldn’t be at all surprised if much of the Midstate is upgraded to a “Moderate Risk” (level 4 of 5) later today. The SPC issues updates at 11:30am and 3:00pm, and I’ll share the updated outlooks on social media and at the top of this post when they come out.
Regarding the specific threats, the SPC shows the greatest tornado threat developing along and south of I-40, where there’s a 10% chance of a tornado within 25 miles of any point in that region. The “hatched area” indicates that there’s a 10% chance of strong (EF2+) tornadoes within 25 miles:
Large hail is also a significant threat — the SPC estimates a 30% chance of 1″+ diameter hail for most of the Midstate, with the hatched area representing a 10% chance of 2″+ hail along and west of I-65:
The SPC has only outlined a 15% threat area for damaging wind gusts — this is my one quibble with their forecast. I think that threat is greater than what they’re estimating, but here’s the “official” version:
Finally, the rain associated with this afternoon’s and evening’s storms will fall onto already-saturated ground. The National Weather Service office in Nashville hasn’t issued any Flash Flood Watches, but you should still be aware of the potential for localized flooding problems. Packaging everything up, here’s what you need to know for today:
Now that I’ve said all of the ominous stuff, let me offer some hope — the chance that we dodge the severe weather threat today is very much alive and well. Maybe the cloud cover sticks around longer, maybe the storms fire up TOO quickly (before the atmosphere is ready to make them severe), maybe the placement of the storms doesn’t quite line up with the best severe-weather ingredients, maybe all the forecast models are just flat-out wrong. There are also what Donald Rumsfeld once famously called “unknown unknowns” — the factors that could help us out that we don’t even know about yet.
Plan on staying weather aware throughout the day. On the TV side, severe thunderstorm and tornado watches will be displayed at the bottom of your screen when they’re issued, and they’ll be posted to social media as well (links below). Severe thunderstorm warnings will also show up in the on-screen ticker and on social media, and we may use commercial breaks on TV to keep you updated. Tornado warnings will prompt us to break into programming and stay on until the threat has passed. One of our meteorologists will also be on social media in the event of widespread severe weather, passing along updates and analysis. I’ll be posting a few thoughts on my social media accounts (mostly Twitter) all day to keep you updated regarding how things are unfolding.
Social media links