2:30PM UPDATE: Storms have fired up to the northwest of Nashville, and we’ve already had a few severe thunderstorm warnings issued for southern Kentucky. A severe thunderstorm watch is in effect until 10:00pm for the northern counties of Middle Tennessee, and all of southern Kentucky.
This likely won’t be the last watch issued. As the focus for severe weather moves deeper into the Midstate this evening and tonight, I’ll be on-air with updates throughout our evening newscasts, and I’ll do some cut-ins to regular programming if the situation warrants (I’ll try to stick to the commercial breaks!). If you see anything worth sharing — cool clouds, damage, etc. — post it to my Twitter timeline or Facebook. Here’s hoping the storms stay non-severe and you don’t have any pictures to share!
11:30AM UPDATE: The new SPC outlook has trimmed back the “Enhanced Risk” to areas north of I-40, with the “Slight Risk” remaining for the rest of the Midstate:
They’ve also shifted the 5% tornado-risk area farther to the north, leaving northern Middle Tennessee and southern Kentucky with a 2% risk (the areas along and north of the outflow boundary I reference below):
While those are both pieces of good news, it still leaves us with a elevated threat of severe thunderstorms from late this afternoon through the first half of tonight. Overall, my analysis in this morning’s post doesn’t really require any significant revision.
POSTED 8:45am: Compared to where things stood yesterday, our severe weather chances for the next 24 hours have increased significantly. The Storm Prediction Center has included most of the Midstate in an “Enhanced Risk” (level 3 of 5) of severe weather:
In yesterday’s post, I showed the outlook with only a “Marginal Risk” for most of us, with the “Slight Risk” farther off to the northwest. Why the changes? Thunderstorms in Kansas, Missouri and Illinois last night moved a little farther south than anticipated — that essentially re-arranges the atmospheric setup for today’s storms. There are some other factors at play that I’ll touch on farther down in this post.
Here’s what we know (or what we THINK we know): storms will develop to our northwest, and move into our area late this afternoon and this evening. Those storms will move into a hot and muggy air mass, allowing them to intensify even more. The main severe weather threat will likely be damaging straight-line winds, but other types of severe weather can’t be crossed off the list at this point.
Here’s what we won’t know until the storms develop: Exactly where will they develop? Exactly what time will they develop? How fast will they move once they start heading our way? (Actually, we have a reasonable estimate on that, but the evolution of the whole complex of storms could adjust that estimate.) Will scattered storms that pop up over the Midstate in the heat and humidity this afternoon (ahead of the main line) further re-arrange where the greatest severe threat will take shape? All of these variables will determine where and when the worst weather will occur. That’s why it’s impossible to tell you exactly when and exactly where it will be storming this evening and tonight. That said, we can give you a (hopefully) good estimate.
We’ll start with some computer model data. This is the HRRR (High Resolution Rapid Refresh, for those of you who want to win at trivia next time weather is a subject) model’s radar simulation for this afternoon and this evening. Keep in mind, as always, that model data isn’t perfect…but it’s a good starting point.
Notice a few things in that sequence:
1) Scattered non-severe storms will likely develop this afternoon, ahead of the main line of stronger storms.
2) The main line of stronger storms will develop off to our northwest, so we should be able to see it coming (i.e. it won’t develop right over our heads).
3) Once the first line of storms moves through, additional waves of thunderstorms will be developing to our west, which will follow up the first wave.
There are other computer models that disagree with the timing and placement…in fact, they all say something at least slightly different. Smoothing out the differences between all of the data, here’s my estimate of when severe storms will arrive in different parts of the Midstate:
Yes, I’m aware that those time windows are annoyingly non-specific. In general, the best bet is the middle of the time windows, but earlier or later arrivals times will be possible, depending when and where the storms fire up to our northwest. So how do we figure that out? Here’s a regional radar snapshot from this morning:
The storms in Missouri will slowly collapse this morning — as they do so, they’ll push out a wave of rain-cooled air that will advance to the southeast. The leading edge of that rain-cooled air is what we call an “outflow boundary,” and it will basically act like a mini-cold front as it runs into hotter and muggier air. So it will be the focusing mechanism for storms to fire up, mostly likely in southeast Missouri and southern Illinois (where I’ve drawn the ???), most likely in the early afternoon. But here’s the tricky part: any adjustment to that “most likely” when and where will have an impact on when the storms arrive in Middle Tennessee and southern Kentucky. That’s the frustrating part of being a meteorologist — the atmosphere will do what it wants to do, and we’re just along for the ride trying to read its mind along the way.
Once the storms DO move in, we’re mainly concerned about their potential to produce damaging winds. That’s because the air mass that’s been hanging around all week is obviously very warm and very muggy…and thus, very unstable. That instability fuels explosive updrafts within the storms, and the corresponding downdrafts will lead to wind gusts over 60 mph…possibly up to 75 mph. The Storm Prediction Center has outlined a 30% risk of damaging winds within 25 miles of any point in the orange-shaded area on this map:
Those strong updrafts also support the potential for large hail — the strong upward motion will keep growing hailstones elevated within the storm long enough for them to grow large enough to survive the trip to the ground (they’ll melt a bit on the way), and do some damage when they hit. SPC’s estimate is a 15% risk of 1+” diameter hail within 25 miles of any point in the yellow-shaded area:
The tornado threat isn’t the primary concern. (If it was, I definitely would have led with that.) But we can’t cross it off the list either, despite relative weak wind shear in the atmosphere overhead. What’s up with that? Check out the radar snapshot from just after 2:30am today:
See that faint, ragged, little blue line ahead of the storms in southern Kentucky? Outflow boundary! It probably won’t be sufficient to initiate thunderstorms in the Midstate, but outflow boundaries don’t just indicate temperature changes — they also show subtle changes in wind direction. As the storms move in and encounter that boundary, some small-scale rotations could develop…which means our tornado threat isn’t zero. In fact, the SPC shows the greatest tornado threat along and north of that boundary, a 5% risk within 25 miles of any point in the yellow-shaded area:
Now, that’s not a huge threat. Significant tornado outbreaks are usually preceded by a 10% or 15% risk area when the SPC crunches the numbers. We just can’t cross “isolated tornadoes” off the list of potential weather hazards this evening, even though the wind and hail threat is more likely to be the cause for concern.
Potential is one thing, but how likely is it that we’ll actually get severe weather around here? The SPC’s SREF (short-range ensemble forecast) model thinks the greatest severe threat will take shape to our northwest…problem is, it’s counting on a farther-northwest area of development, and that the storms won’t even form until late afternoon.
The “analog” forecast method, comparing this weather pattern to similar patterns in the past, yields a troubling 50-60% likelihood of at least one severe weather report in the Midstate, and a 10-20% chance of 10 or more severe reports (what we could call an “outbreak”):
Finally, there’s the localized flooding threat. Even once the strongest storms have moved through or weakened, those additional waves of storms out west will be the follow-up insult to injury. More heavy rain falling onto already-saturated could cause some high-water concerns on area roads — river flooding won’t be a concern, but be aware if you’re driving around tonight that a few roads could be flooded. (A very similar scenario to what we saw late Tuesday night.)
To sum everything up:
Basically, we’re in for a stormy night. The last wave of heavier storms should be moving to the east as the sun comes up Saturday morning, with lingering showers winding down from west to east by early Saturday afternoon. Sunday is looking spectacular: low humidity and lots of sunshine, with morning temperatures in the low 60s warming up to the mid 80s by afternoon.
If any watches or warnings are issued, the on-air ticker (at the bottom of the TV screen) will be updated on Channel 4, and we’ll break into programming if a tornado warning is issued. ALL severe thunderstorm and tornado watches and warnings are automatically posted on my twitter feed (they’re the tweets that start with “iNWS Alert – New Event”). Social media links are below. Dan Thomas will be on at noon with an update, and I’ll be back for all of our evening newscasts. Double shifts are fun! Right?!?
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