Thursday Night Storms

After three consecutive months of below-average temperatures (via NWS Nashville), warmer than normal temperatures have made themselves at home over the Midstate the past few days.  And the reaction has been, well…
cookie-monster

But it’s springtime, which means that warm spells are often followed by cool spells (next week), and the transition is marked by severe weather possibilities.  That’s exactly our concern for Thursday night into Friday morning.  Before that though, we’ll continue to experience above-normal temperatures today and tomorrow, with highs near 80 degrees both days.  Some showers and storms are possible this evening and overnight, and again Thursday afternoon, as shown by these Futurecast snapshots:
RPM 8P WED
RPM 2P THU

Initially, I didn’t expect these storms to pose much of a severe threat…but dew points have risen into the upper 50s (indicating a favorable level of humidity for stronger storms), and the Storm Prediction Center has included the northwestern part of the Midstate in a “Slight Risk” region for this evening and tonight.
PAUL SEVERE RISK REGION 2

It’s a borderline severe weather scenario, with the yellow-shaded locations indicating where there’s a 15% chance (estimated) of severe straight-line winds or large hail happening within 25 miles of any given point.  For the most part, I think we’ll just see strong-but-not-quite-severe storms — the strongest ones could certainly be loud, producing a lot of lightning and thunder…but lightning and thunder don’t make a storm severe.  Lightning and thunder just make a thunderstorm, well, a thunderstorm.
back-off-man-Im-a-scientist

Our severe weather chances increase significantly Thursday night.  The greatest severe potential will take shape well to our west, in Missouri and Arkansas — that’s where a significant tornado threat will develop Thursday afternoon and evening (they could get baseball-size hail as well).  Those storms will move to the east, and by the time they reach the Mississippi River they’ll merge into a squall line (technically, a “quasi-linear convective system,” or QLCS), which will continue to advance eastward through West Tennessee.

Here’s where the forecast gets tricky.  The squall line will likely still be capable of producing damaging straight-line winds and large hail as it approaches the Tennessee River…but the severe weather threat will fade dramatically as the storms transit the Midstate during the night.  This sequence of Futurecast images shows our two favorite computer models’ version (RPM and BAMS, for the weather-dorks among you) of what the radar will look like through Thursday night:
RPM 12A FRI
RPM 2A FRI
RPM 4A FRI
RPM 6A FRI

Still some timing differences between the two models, but notice that the colors look less “angry” in the later images, showing the weakening trend as the storms progress into eastern Middle Tennessee.  The Storm Prediction Center’s severe-weather outlook for Thursday and Thursday night reveals the same pattern: the greatest severe potential cuts off at the Tennessee River, but the whole Midstate is still included in the “Slight Risk” region, indicating an elevated threat for severe weather (this outlook did not change much at all with the SPC’s afternoon update):
PAUL SEVERE RISK REGION

Smoothing out the differences amongst the various computer models, here are the arrival-time windows for the strongest storms Thursday night:
PAUL STORM TIMING

Squall lines are typically associated with damaging straight-line winds, and at this point that looks like our major cause for concern.  There will be enough instability in the atmosphere for hail to develop, but whether that hail could grow to severe criteria (1″ in diameter) is still questionable.  The tornado threat for Middle Tennessee and southern Kentucky will be low…not zero, but low.  Whenever storms develop, we’re always watching carefully for signs of rotation.
PAUL SEVERE RISK GRAPH 2

Overall, my expectations at this point is that storms will be severe as they move into the Tennessee River valley, marginally severe as they approach I-65 (meaning there will be warnings, but probably not much damage), and strong-but-not-severe as they move into eastern Middle Tennessee.  Plan on having a source of weather information handy before you head to bed Thursday night.  We’re still 36 hours from the storms even heading toward the Mississippi River though, so there’s a lot of time for the atmosphere to throw a curveball.
curveball

So if all of that has you doing this…
what
…here’s a shameless plug: you can join us Thursday evening for our severe-weather-awareness program, the “4WARN Weather Alert Tour” — we’ll be in Metro Nashville this week, at the Northside Church of Christ.  Lots of severe weather safety information, some interactive learning, trivia games…good stuff!  Doors open at 6pm if you want to come early and say “hi” (or ask questions about the severe weather later that night), the show starts at 7pm.

And of course you can find us on Twitter and Facebook as well!  (You can follow or like me on social media too, if you’re interested in my random brain droppings whenever I’m not talkin’ weather.)

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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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