Thursday Evening Storms

It’s looking increasingly likely that severe weather will impact the Midstate late Thursday.  There are still some uncertainties regarding the exact timing of the storms, and how severe they’ll be.  Let’s start with what our two “Futurecast” computers are showing for Thursday afternoon and evening:
RPM 4P THU
RPM 6P THU
RPM 8P THU
RPM 10P THU
RPM 12A FRI

Those are the most-recent radar simulations as I’m writing this early Wednesday afternoon.  Both computers have been flip-flopping regarding earlier/later timing of the thunderstorms, which is part of the forecasting challenge.  With these types of squall-line storms (a solid line of strong to severe thunderstorms), the computers tend to underestimate their speed.  It’s more common for the storms to develop, and then race out ahead of the front that’s causing them.  So while I think the radar will look very similar to the images above, I think the arrival times will be closer to this:
PAUL STORM TIMING

The Storm Prediction Center still has us in their “Slight Risk” area for severe weather.  The red-shaded area indicates a 30% or greater chance of severe weather happening within 25 miles of a given point, while the purple area (including most spots along and north of I-40) shows the locations with a 10% chance of “significant” severe weather (74+mph wind gusts or an EF2 tornado):
PAUL SEVERE RISK REGION

The straight-line wind threat is of greatest concern, as it always is with squall-line storms.  Wind gusts of over 60mph can cause just as much damage as weak tornadoes…and with the amount of wind energy we expect in the atmosphere overhead tomorrow, I certainly wouldn’t rule out some 70+mph wind gusts.  Within squall lines, little areas of rotation can spin up and produce brief tornadoes as well:
PAUL SEVERE RISK GRAPH 2

The Nashville office of the National Weather Service put out these graphics, so they’re pretty much on the same page (just a few subtle differences):
NWS_Feb20_timing

The “analog” guidance I wrote about yesterday (comparing this storm system to previous events to determine our chances of severe weather) still points to a heightened severe threat as well:
CIPS_FEB20

Like I said at the top, there are still plenty of uncertainties about exactly how this event will shape up.  Among them:
1) How much sunshine will we see on Thursday afternoon?
More sunshine = Warmer temperatures = More instability = Stronger storms
2) How much warm air will exist above the ground, in the atmosphere above our heads?
A ”cap” of warm air overhead would help reduce the vertical growth of storms.
3) How fast will the storms move in?
My timing estimate is above.  Earlier arrival = Stronger Storms.
4) Will the storms grow enough for a higher tornado potential?
“Taller” storms accelerate rotation as they stretch, increasing our tornado chances.
5) Will any storms develop ahead of the main squall line?
Pre-squall-line storms would be more likely to have supercell characteristics.

We’ll keep you posted on-air and online regarding the latest details of this severe weather threat.  The good news is, it’s still over 24 hours away — still time for things to change!

And if severe weather DOES happen, please DON’T do this:
SEVERE_DONT

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About paulheggen33

Morning meteorologist for WSMV-TV in Nashville.
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2 Responses to Thursday Evening Storms

  1. Pingback: Thursday will be a 4WARN Weather Alert day: Paul Heggen WSMV just updated the #4… | NashVegas.com

  2. I LOVE your post. It gives information but has a very human side to your comments

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