April 25: Severe Weather Update

5PM UPDATE: A severe thunderstorm watch has been posted for the remainder of southern Kentucky, in effect until 11pm.  No watches on the Tennessee side of the border.

1PM UPDATE: A tornado watch has been issued for Calloway, Trigg, Christian and Todd counties in southern Kentucky, in effect until 8:00 this evening.

Developing storms are still around and west of St. Louis, but they’ll track along the warm front I talk about below…we’ll keep a careful eye on those storms as they approach!

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(Originally posted 11:30am Saturday)

Some good news and some bad news for today…first, the good news!  Overall, our severe weather potential has diminished and shifted farther to the north.

Now, the bad news: our severe weather threat still isn’t zero, and the farther north you go into Kentucky, the greater the severe potential will be later today (primarily this evening).

Let’s get to the details!

24 hours ago, I was reasonably confident that at least the northern half of Middle Tennessee and all of southern Kentucky would see a significant severe weather threat this afternoon and evening.  But beginning late yesterday morning, my reaction to every new batch of model data has been this:

Here’s the deal…all week, in this space and on TV, I’ve been talking about how “conditional” our severe weather threat would be — that is, storms would likely be severe IF they developed in the first place.  The environment conducive for severe thunderstorms really hasn’t changed much…if anything, the severe weather parameters have looked even more impressive over the last 24 hours.  But the likelihood of any “trigger” developing to initiate thunderstorm development has consistently looked a lot less impressive, especially on the Tennessee side of the border.  Basically, the best environment for severe storms and the best environment for any storms to develop are in different places on the map.

Here’s the regional radar view as of late this morning, with the surface analysis (fronts and low/high pressure areas) drawn as well.

The area of low pressure in Missouri is the best “trigger” for thunderstorms, and it will ride along the warm front stretched west-to-east.  The warm front itself will be drifting farther north…which is why the severe weather threat has shifted farther north.  Imagine a train (that’s the low pressure) confined to a path along the tracks (that’s the warm front), but the train tracks themselves aren’t stationary.  We have to predict where the low will be, where the warm front will be, and where any storms triggered by the low will be.  Now you’re starting to get a feeling for why weather forecasting is a wee bit complicated.

Even though the outlook has significantly improved for much of Middle Tennessee, it’s not all great news.  Southern Kentucky will still be under the gun for severe storms as that low-pressure center moves in this evening.  The latest outlook from the Storm Prediction Center shows how quickly the severe threat escalates as you go north:

That’s the regional view…on a local level, I’m estimating our storm chances (this is for any storm — regardless of severity) late this afternoon and this evening to shape up like this:

As usual, Metro Nashville finds itself right on the dividing line.

Areas to the south of I-4o aren’t completely in the clear — it’s a slight chance of storms overall, but if a thunderstorm manages to pop up it will be like lighting a match in a gasoline refinery.  Northern Middle Tennessee is in that annoying middle ground — by the time storms make it in this evening, the severe weather ingredients won’t be as favorable, but we’ll still be on the lookout for strong to severe storms…especially if they move in faster.  Southern Kentucky, it’s not a guarantee of severe storms for you either — you’ve got the higher storm chances, but the environment over your heads isn’t quite as favorable for those storms to be severe (it’s still looking like it will be “good enough” though).  Overall, the specific severe weather threats shape up like this:

The timing of the greatest threat has been adjusted somewhat as well.  We’ll be on the lookout for any storms to pop up as the atmosphere warms this afternoon, but overall the odds of that are relatively low.  It’s a good idea for you to stay weather-aware just in case, but there’s no real reason for alarm at this point.

This evening, storms will develop to our northwest and move into southern Kentucky and northern Middle Tennessee.  One of the few computer models to handle this whole scenario in a somewhat-reasonable way shows this pattern this evening (notice the dramatic weakening after 10pm):

Is that going to be perfect?  NOPE.

The storms could be faster, slower, weaker, stronger, you name it.  But it at least gives us an estimate of how things will play out, and that particular model seems to have its head in the right place.

BOTTOM LINE: I’d love to be able to give you a yes-or-no answer on a county-by-county basis regarding whether it will storm, and whether those storms will be severe.  But we deal in probabilities, not certainties.  I wouldn’t cancel any plans this evening, but I would plan on staying weather-aware as you’re out and about…or sitting on the couch watching the Predators game.  Just remember:

• storms are still possible this afternoon and this evening
• storm chances increase the farther north you go
• ANY storm that develops could become severe, even if you’re in a low-storm-chance part of the Midstate

While things have been consistently trending in an encouraging direction, it’s still a situation that could change very quickly.  I’ll be monitoring things throughout the afternoon — if watches are issued (indicating a higher potential for severe weather), I’ll post that information on social media and at the top of this post.  If warnings are issued (indicating severe weather is actually happening), they’re posted automatically to my Twitter feed and at the bottom of your screen on Channel 4…as always, I encourage you to NOT use Facebook as a severe-weather source.

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Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville