April 5: Severe Weather Possible Today

1:15PM UPDATE: As I’m typing this, we’re in “wait and see” mode, which is the most-frustrating part of any forecast.  Here’s a look at where we stand in terms of severe-weather ingredients…
1) Warmth.  Yup.  Already 81° in Nashville as of 1pm.
2) Humidity.  Dew points are already around 60°, which is “good enough.”  Anything higher just puts us in bonus territory.  But remember what I said this morning: “the strong southerly winds we’ll experience today [could] grab some drier air from farther up in the atmosphere and bring it down to ground level.”  Fingers crossed!
3) Wind energy.  Borderline for now, but expected to increase as the heart of the parent storm system gets closer.
4) Trigger mechanism.  That’s what we’re waiting on.  The dividing line between muggy air over the Midstate and drier air out west (what we call a “dry line”) is edging toward the Tennessee River, as is an area of faster winds high up in the atmosphere.  In conjunction, those should spark the first storms within an hour of when I hit “publish” on this.  In fact, I’m seeing the first few blips appearing on the radar…

Also within an hour of when I hit publish, I expect the Storm Prediction Center to issue a Tornado Watch for a large portion of the Channel 4 viewing area.  That will be posted to social media as soon as it’s issued.

Our overall threat levels have changed very little since this morning.  Here are the latest SPC threat graphics — an update is due out at 3pm, and will also be posted to social media:

My overall estimate of timing/probabilities hasn’t changed much:

Also unchanged: my assessment of the big-picture threats.  “Forecast confidence” is a little higher though, since the humidity is in place and the first storms will be firing up soon:

Same advice — stay weather-aware this afternoon and this evening!

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Well, this seems familiar — severe thunderstorms will be possible across the Midstate this afternoon and early this evening, but we’re once again looking at a lot of uncertainty regarding the overall setup.

Before the storms develop, we’ll see warm and windy conditions, with highs in the upper 70s:

I’ll be honest: I stared at a blank computer screen for a LONG time before starting to write this, just because I struggled with how to best communicate the uncertainty in today’s forecast.  Part of that is “recency bias” — last week’s storm threat fizzled out, so the natural instinct is to be hesitant to ring the bell so loudly about a severe weather threat.

But one of the worst mistakes you can make it to say “nothing happened last week, so nothing will happen today.”  Every storm system is different — last week’s storm system has nothing to do with today’s.  The problem today is this: the worst-case scenario is very bad, but the best-case scenario (i.e. another “fizzle”) is still quite possible!  So let’s start with the worst-case setup, and then we’ll get into the factors that could prevent it.

WORST-CASE: Storms pop up by noon along (or even west of) the Tennessee River, intensifying rapidly and becoming “supercells” (those are bad) before they reach I-65 by late afternoon.  Numerous severe storms then continue moving through eastern Middle Tennessee, moving off the Cumberland Plateau by 8pm.  This radar simulation is closest to that worst-case scenario:

If that happens, all types of severe weather will be possible along and east of I-65 — large hail, tornadoes, damaging straight-line winds and frequent lightning.  The Storm Prediction Center has tilted their forecast toward this setup, indicating an “Enhanced Risk” (level 3 of 5) for severe weather along I-65, then a “Moderate Risk” (level 4 of 5) in eastern Middle Tennessee:

The forecast sounding (weather balloon data) from one weather model looks very ominous (I’ll explain all of what you’re seeing right below the image):

That plot indicates a very unstable atmosphere — a dew point this afternoon in the mid 60s would provide ample storm fuel (“CAPE,” circled in red).  This model also shows very low cloud bases (less than 1km above the ground, circled in white) which along with substantial wind shear (circled in light blue) indicates a significant tornado threat.  The severe weather statistics we use (circled in yellow) are maxed out — a Supercell Composite Parameter over 12 and a Significant Tornado Parameter around 4 are both very bad.  To emphasize…that’s projected data, from a forecast model that has a tendency to overdo severe weather situations.  It’s rare for everything to come together so perfectly for a severe weather outbreak.  Which brings us to…

BEST-CASE: The humidity remains lower.  It’s as simple as that.  If the dew point stays in the 50s to around 60°, the storms would have a harder time getting going, they wouldn’t intensify as rapidly, the bases of the storm clouds would be farther off the ground, and there wouldn’t be as many storms overall.  Sounds great, right?

There are a couple of ways it could happen — one is that storms in Alabama and Mississippi this morning block the flow of moisture into the Midstate.  The other is that the strong southerly winds we’ll experience today will grab some drier air from farther up in the atmosphere and bring it down to ground level.  Our RPM model shows dew points this afternoon that are more favorable for us:

That wouldn’t eliminate the severe weather potential, but it would reduce it!  It’s amazing what just a five-degree difference in the dew point would mean.  The RPM’s radar simulation still shows some storms, but they’re not as intense and they’re not as widespread:

The uncertainty between those two scenarios is why I indicated my forecast confidence as “medium” on the SPC map above.  (Last week I would have said “medium-low” for Thursday’s threat.)  I can’t even guarantee rain in your neighborhood today, much less if you’ll get a severe thunderstorm nearby.  If you want something etched in stone, go to Mount Sinai and be prepared to wait a while.  Meteorology is a science of statistics and probabilities — right now the numbers say “pay attention, but don’t panic.”

Speaking of numbers, let’s get into the details of the SPC’s outlook.  They’re estimating a 15% chance (that’s high) of a tornado within 25 miles of any point in the red area on this map…and the “hatched” area indicates a 10% chance of an EF-2+ tornado:

SPC also estimates a 45% chance of 1″+ diameter hail within 25 miles of any spot in the red area, and a 10% chance in the hatched area of 2″+ hail:

Finally, the damaging wind (60+mph) threat:

All of the best-case/worst-case stuff, all of the computer models, and all of the statistics in the world aren’t much help if I don’t tell you what I actually think.  If you’re going to get strong/severe storms, here’s when I think they’ll occur (along with the odds of them occurring in the first place):

If you live near the Tennessee River, I wouldn’t sweat this one too much.  That doesn’t mean your severe weather threat is ZERO, but it means the odds are in your favor.  It’s basically a coin-flip for I-65, which sounds like a cop-out — but that’s the way the numbers are shaking out right now.  We’re going to be right on the dividing line, so don’t cancel anything at this point — just plan on staying weather-aware this afternoon.  Folks in eastern Middle Tennessee have the greatest risk, and that’s where you should be thinking about a Plan B for anything outdoors this evening:

There are several ways to stay “weather-aware”…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.

Once the storms move out this evening, we’ll be stuck with some lingering showers on Thursday, with highs only reaching the upper 50s.  Mostly sunny Friday through the weekend, and warming back up — highs near 60° on Friday, in the upper 60s Saturday, then the upper 70s Sunday.

Patchy frost will be the concern Saturday morning — if you’ve already done some early-spring gardening, be ready to cover things up or bring the sensitive plants inside.

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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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