(Posted 4:20 p.m., 10/13/2014)
Tornado warnings have started being issued in Hardin, McNairy and Decatur counties west of the Tennessee River.
Stay with 4WARN Weather for updates.
(Posted 3:25pm 10/13/2014)
Just a quick update to let you know that the Storm Prediction Center really hasn’t changed their thinking regarding our threats this evening and tonight — they’re maintaining the Moderate/Slight risk dividing line along the Tennessee River (upper left). The Moderate Risk corresponds with the highest tornado threat, also some tornadic potential exists across the entire Midstate (upper right). Within the Slight Risk area, locations west of I-65 have a higher probability of damaging winds occurring within 25 miles (lower left). All locations have a relatively low risk of large hail (lower right).
Dan Thomas and Lisa Spencer will be on-air this evening during our newscasts and, if necessary, during prime time if/when warnings are issued.
(Posted 1:15pm 10/13/2014)
A few updates to our severe weather outlook, now that things are beginning to come into better focus regarding our potential for nasty weather later today…
It looks like there will be plenty of instability available in the atmosphere to help drive severe storms — as of 1pm, temperatures are in the upper 70s, so we’re on track to reach our forecast highs in the low 80s. The warm temperatures combined with unseasonably high humidity will result in a “buoyant” atmosphere supportive of surface-based thunderstorms (more capable of producing tornadoes, compared to elevated-base thunderstorms). Even as the atmosphere cools after sunset, strong southerly winds will keep plenty of warm and unstable air in place ahead of the storms.
The precise timing of when the storms will arrive remains the least-confident part of the forecast…until the squall line shows more and consistent signs of actually slowing down, I’m maintaining this estimate of severe storm “windows”:
If anything, I’m erring on the early side — partly to account for pre-squall-line storms, and partly so the storms in the main line don’t catch you off-guard. The Nashville office of the National Weather Service is still forecasting a later arrival time — they’ve adjusted the previous version of this map to reflect an earlier time frame, but they’re obviously banking on the squall line slowing down significantly. (To be clear: I’m providing their map not as a “hey, look what THESE guys are saying LOL” but as an example of how reasonable minds are interpreting data in a slightly different manner):
One concerning possibility is that of isolated supercell thunderstorms developing ahead of the main squall line. As I wrote this morning (below), squall lines are typically associated with a straight-line wind threat — but any discrete storms that develop ahead of the line will have a higher tornadic potential. I’m not changing my thinking regarding our relative threat levels — winds and flooding are still the biggest concerns, but the tornado potential definitely isn’t zero. The pre-squall-line storms are the ones developing down in Alabama as of just after 1pm (also, this image gives you an idea of what’s headed our way):
The Storm Prediction Center has expanded their “Moderate Risk” outlook a little farther east (not far enough, in my opinion — I still think I-65 should be the Moderate/Slight dividing line).
I’m still concerned that SPC hasn’t adequately adjusted for the faster-than-expected movement of the entire complex of storms. Their next update is scheduled for 3pm — I’ll post a quick update with some thoughts at that point. We’ll likely have a tornado watch issued for our western counties before then — but the storms themselves will still be in West Tennessee.
This morning’s post is below…
(Posted 9:25am 10/13/2014)
A potentially dangerous and complicated weather pattern is shaping up over Middle Tennessee and southern Kentucky over the next 24 hours. So let’s take a look at what we know, what we think we know, and what we’re still figuring out.
WHAT WE KNOW
1. Strong/severe thunderstorms are moving eastward through Arkansas, Louisiana and Texas this morning.
2. Computer models have been WAY too slow in how they’re depicting the progression of these storms (most are 3+ hours behind). Check out the 9am radar image (left) next to Futurecast’s version of what that radar should look like:
3. Other than isolated showers/storms, the atmosphere over the Midstate will be warming up and becoming progressively more unstable. It will also be quite windy — a Wind Advisory is in effect this afternoon through tonight:
4. The ground is already saturated, due to last week’s and this morning’s rain. Storms moving in later today and tonight will be capable of producing flash flooding in a very short period of time, especially considering that rainfall amounts through early Tuesday are forecast in the 2″-3″ range. A Flash Flood Watch is in effect until 7pm Tuesday:
WHAT WE THINK WE KNOW
1. The environment over Middle Tennessee and southern Kentucky as the storms arrive will be especially favorable for damaging straight-line winds. There’s a lot of wind energy associated with this system, but a majority of it is associated with “speed shear” (change in wind speed as you ascend in the atmosphere) as opposed to “directional shear” (change in wind direction as you go up). That means that while tornadoes are still possible, the greater threat is from thunderstorm wind gusts over 60mph.
2. As the atmosphere warms up, there will be enough instability over our heads for severe storms to maintain their strength. However, it does not look there will be enough instability to support much of a hail threat at this point.
3. The earlier the storms move in, the greater our severe weather potential will be. Which brings us to…
WHAT WE’RE STILL FIGURING OUT
1. When will the storms get here??? Pretty much the million-dollar question, especially considering that “daylight arrival = nastier storms”.
Obviously some pretty major differences. I think it’s likely that the storms will decelerate as they cross the Mississippi River, but they’re already 3 hours ahead of this morning’s computer guidance. Speaking of computer guidance, here’s how Futurecast depicts the radar simulation this evening through tomorrow morning — the hot-off-the-press newest model run is significantly faster that previous versions, and has a more-reasonable estimate of the timing (but I’m thinking it’s still an hour or two too slow):
Despite its timing challenges, I do think Futurecast has a pretty good handle on what the storm structure will look like — a squall line (technically, a quasi-linear convective system) with embedded supercell features. The individual storms that comprise the squall line will be moving generally south-to-north within the line, while the line itself will slow down and move from west-to-east throughout the evening and into tonight. Notice that some re-development of storms is predicted for early Tuesday morning along and east of I-65.
Squall lines are primarily associated with straight-line wind events, but little areas of rotation can spin up within the line and produce isolated tornadoes. The heavy rain and slowing movement of the line could produce flash-flooding throughout the night…especially with the potential re-development of storms Tuesday morning. Overall, I’d rank the threats in this order: straight-line winds, flash flooding, tornadoes, hail:
The Storm Prediction Center has noted the fast forward movement of the storms so far this morning, but their 8am outlook maintained the position of the severe weather “Moderate Risk” to the west of the Tennessee River, with a “Slight Risk” for Middle Tennessee and southern Kentucky:
My concern with SPC’s logic is that the storm line may outrace the Moderate Risk area — I think a better Moderate/Slight dividing line would be roughly along I-65, since that’s how far east the storms could get before the sun goes down (instability levels decrease after sunset). After sunset, and as the storms progress eastward into a less-favorable air mass, the severe threat will diminish…but it won’t go away entirely. SPC’s next update is due to be posted at 11:30am, so we’ll see what they say at that point.
The biggest wild-card in all of this is the possibility that this morning’s squall line will move too far ahead of its upper-level support and fall apart…if that happens, we’d be playing a waiting game as another squall line develops to take its place. (In which case the forecast becomes so complicated that you’ll be able to watch my hairline recede before your very eyes as I try to figure out the forecast.) There are other wild-cards that could also reduce or rearrange our severe weather potential — daytime clouds, non-severe storms developing ahead of the main line, a layer of warm air in the atmosphere that could inhibit storm development…you get the idea.
Either way, plan on staying weather-aware this afternoon and this evening. I’ll be on Twitter regularly (and Facebook irregularly) with updates, and of course we’re on the air with newscasts at noon and beginning at 4:00 this afternoon. Watches and warnings are automatically posted to my Twitter account — and allow me to remind you once again that Facebook is not a good source of time-sensitive information like severe weather alerts.
Above all, don’t freak out. (Easy for me to say after I just dumped a thousand words worth of “this could be bad” on you!) This is NOT a situation where you should plan on sleeping in your safe place with a helmet on and a weather radio clutched in your shaking hands…even in high-risk severe weather scenarios, the odds of something happening in your immediate vicinity are pretty low — but it’s always best to be prepared!
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