March 30: Severe Storms Possible Today

3:35PM UPDATE

So here we are in late afternoon…where are the storms?  They’re out west!

That is a sad little line of storms, but notice there’s also some activity down around Birmingham.  While the storms in West Tennessee will be moving in from the west, the storms in Alabama will be moving up from the south.  That makes for a complicated picture this evening, but the good news is that the severe weather threat will remain limited…not ZERO, but limited.  The HRRR model seems to have sorted out the complexities — here’s what it’s showing this evening (the loop runs 4pm through 2am):

If you do better with still maps instead of loops, here’s your window for the strongest storms this evening:

The Storm Prediction Center has trimmed back their “Enhanced Risk” region even further, to the point where the entire Midstate is now in a “Slight Risk” (level 2 of 5):

Breaking things down by category, here are your severe weather threats this evening:

The limiting factor is exactly what I thought it would be this morning (see the initial post below) — not enough humidity.  Dew points have been running in the 50s all day, which just isn’t enough moisture for a significant severe weather threat.  That’s the good news.  The not-so-good news (not going to call it “bad”) is that just enough moisture might sneak up from the south ahead of the line of storms, which leaves us with at least some severe thunderstorm potential this evening.  Stay weather-aware, just in case, and be ready to move yourself inside whenever the storms arrive in your neighborhood.

Tomorrow I’ll write up a post looking back on the lead-up to this threat — why were we so concerned 24-48 hours beforehand, what can we do differently in the future, what have we learned…that kind of stuff.  But for this evening, while things are looking significantly better, we’re not completely out of the woods yet.

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10:20AM UPDATE

Just a quick status check on what I typed up a couple of hours ago…we’ve seen some clearing overhead, and temperatures have warmed up to the low 70s as of 10am.  Dew points are still in the 50s, which is sufficient-but-not-ideal for severe thunderstorms. The HRRR model has backed off of its super-dry forecast, instead holding moisture levels close to steady through this afternoon.  Could be better, could be worse.  The other model data continues to be all over the place, confusing, and generally frustrating.

My basic position in the post below was “it will be warm enough, but I’m not convinced it will be humid enough for widespread severe weather” — right now I stand by that assessment.  But let me say this for what feels like the millionth time: lower risk doesn’t mean ZERO risk.  Plan on staying weather-aware this afternoon and this evening!  More updates to come…

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PUBLISHED 8:05AM: Before we get into details about today’s severe weather potential, let me repeat something I said on social media yesterday evening: The weather will do what the weather wants to do.  I’ve seen storm systems more impressive than this amount to nothing, and I’ve seen storm systems much less impressive produce widespread severe weather.  Plan for the worst, and hope for the best.  Now, let’s get into the details of this complicated forecast…

Before the severe thunderstorm threat develops this afternoon, we could see a few showers (maybe even a storm) west of I-65 this morning.  That activity will fizzle pretty quickly.  Breaks in the clouds around midday will allow temperatures to warm up to the upper 70s:

That’s certainly warm enough to support severe thunderstorms, but warmth is just one of the necessary parts if you were trying to “assemble” a severe thunderstorm outbreak.  Wind shear, a trigger mechanism, and humidity are the other key factors:

Big question mark by humidity there — dew points this morning are only in the 50s, which is very borderline for severe weather.  Futurecast (the RPM model again) shows dew points staying in the 50s most of the day, with a little bump up to around 60 (look toward the Tennessee River) right as the storms are moving in:

That’s one version of one model.  Other models show a lot more moisture — the NAM model has dew points well above 60:

Others show a lot less — the HRRR model reduces dew points even more this afternoon:

Our best-case scenario is that thunderstorms along the Gulf Coast this morning help to block the flow of moisture up into the Midstate, AND strong southerly winds mix some drier down to the ground from farther up in the atmosphere.  Fingers crossed!  At this point my confidence in that scenario is “cautiously optimistic.”

That said, at this point the Storm Prediction Center’s ensemble model (a blend of 25+ different models) still indicates a 50-70% chance of storms with severe characteristics developing in the Midstate by late afternoon and early this evening:

In the spirit of “plan for the worst, hope for the best,” I’m going to lean this afternoon’s discussion towards the more-severe side of the spectrum.  The better chance of strong storms will develop in West Tennessee and move into the Midstate by late afternoon and this evening.  That’s shown on our RPM model’s radar simulation:

Other models are, of course, a little slower or a little faster (and again, the atmosphere will do whatever it wants to do), so here’s a general estimate of when the strongest storms should arrive:

All types of severe weather will be possible: tornadoes, large hail, and damaging winds:

The SPC has outlined the northwestern quarter of the Midstate (roughly) in an “Enhanced Risk” (level 3 of 5) of severe weather, with the rest of us in a “Slight Risk” (level 2 of 5):

That’s a reduction from yesterday’s “Moderate Risk,” and even a reduction from their outlook from early this morning — but it doesn’t mean the worst-case scenario looks much better!  It just means the balance between the odds of the worst-case and best-case scenarios has tipped a bit more in our favor. Here’s the translation of the various risks:

To add some statistics into the mix, SPC estimates a 5% chance of a tornado within 25 miles of any point in the yellow-shaded area:

Along with a 30% chance of damaging wind (60+mph) or large hail (1+” diameter) with 25 miles of any point in these orange-shaded areas, 15% in yellow:

An important note: even on HIGH-risk days (which today isn’t), the odds of seeing severe weather in your neighborhood are low.  But when you open the geographic window just a little wider, we get the percentages seen above.

To repeat my point from earlier: the specific severe threat level will depend on how much the humidity increases by this afternoon, so there a very solid chance we’ll avoid the worst-case scenario!  Just plan on staying weather-aware throughout the day — follow us on Twitter (Facebook is essentially useless for time-sensitive information), and download the 4WARN mobile app and set it up to alert you if warnings are issued in your area.  That’s not me pushing the company line — it’s a good way to stay up-to-date, especially if you’re going to be out and about.  You can download the mobile app at this link.  Once it’s on your phone/tablet, go to “Settings” under the drop-down menu:
4warn2
Then turn on the Severe Weather Alerts:
4warn1

We’ll obviously be monitoring the hour-by-hour trends as data comes in throughout the day — I’ll update this post if the situation changes one way or the other, and I’ll be sharing some thoughts on social media throughout the day as well.

Cooler with a few showers Friday, then the weather looks nice this weekend!

More showers and storms likely Sunday night and Monday, but with that system the severe threat looks mercifully low.  Next Wednesday’s system might be stronger, but that’s a long way off right now.

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Social media links

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

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Daphne DeLoren, NWS Nashville

Instagram: PaulHeggenWSMV

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About paulheggen33

Morning meteorologist for WSMV-TV in Nashville.
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2 Responses to March 30: Severe Storms Possible Today

  1. Amy says:

    Thank you so much for these updates and working hard to keep us informed.

  2. Pingback: March 31: What Happened Yesterday? | 4WARN Weather Blog

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