April 27: Severe Storms Possible Today

WEATHER

Yesterday, I wrote about how the atmosphere was nearly ideally set up to produce severe weather in Tornado Alley — and severe thunderstorms developed as-expected from Nebraska southward into Texas (but fortunately, without the long-track tornadoes).  We’re facing a severe weather threat here in Midstate today, but the good news is that the atmospheric setup isn’t as favorable as it was yesterday.

What we DO have is warm and humid (and thus unstable) air across the region, with southwesterly winds throughout the day to lock that warm and muggy air in place.  An approaching cold front, along with the rain-cooled air being pushed out by collapsing thunderstorms to our west, will act as a weak-but-strong-enough trigger to initiate the strongest storms this afternoon.
WEDNESDAY SETUP
What we DON’T have overhead today is wind shear.  Wind shear is a key ingredient because it helps to tilt thunderstorm updrafts (so the storm doesn’t choke off it’s own supply of unstable air), and wind shear also can rotate storms to produce a greater tornado threat.  The lack of wind shear today means that tornadoes are unlikely, and that while severe weather is still possible, a widespread severe weather outbreak isn’t as likely.

But there’s still enough instability hanging around to produce a straight-line wind and hail threat.  60+mph winds and 1″+ diameter hail will be the primary concerns with the strongest thunderstorms this afternoon and evening:
PAUL SEVERE RISK GRAPH
The tornado threat isn’t zero, but it is quite low.

The timeframe for the strongest storms will be from mid-afternoon through the evening, but that doesn’t mean we’ll be completely dry between now and then either.  Most of the Midstate will stay dry enough for temperatures to warm up (and become even more unstable):
PAUL RPM 4KM Temperature
Spotty showers and thundershowers through early afternoon will be most likely in western and northwestern Middle Tennessee, and in the western half of southern Kentucky:
BAMS 11A WED BAMS 1P WED
The strong/severe storms will develop in West Tennessee this afternoon, then march west-to-east across the Midstate this evening.  Here’s the BAMS model’s version of what the radar will look like…but keep in mind, this is a simulation.
BAMS 3P WED BAMS 5P WED BAMS 7P WED BAMS 9P WED BAMS 11P WED
Other forecast models are a little faster or a little slower — so, smoothing out those model differences, here’s your timing window for the strongest storms:
PAUL STORM TIMING

The later storm arrival times for the eastern half of the Midstate means that the severe weather threat is lower in that part of the area.  The Storm Prediction Center has outlined a “Slight Risk” (level 2 of 5) for severe thunderstorms west of I-65, with a “Marginal Risk” (level 1 of 5) for the rest of the Midstate:
PAUL SEVERE RISK REGION

Some other pieces of data support the “stronger west, not-as-strong east” conclusion.  The SPC’s short-range ensemble forecast model shows a 30-50% chance of thunderstorms with at least some severe characteristics this evening, with the higher chances to the west:
SREF_prob_combined_supercell__f015
And the analog forecast (comparing this pattern to similar historical patterns) points to a higher probability of 5+ severe weather reports in generally the same area:
SVRall_N5_110km_nam212F036

The rain will mostly be off to our east by Thursday morning, but a few lingering showers will be possible through midday.  Decreasing clouds will allow temperatures to warm up to the mid 80s, and we’ll top 80 degrees again Friday.  Dry weather will prevail during the day Friday, but scattered showers will be possible Friday night into Saturday morning:
WSMV 7 Day AM
I’m still thinking that Saturday morning looks showery rather than stormy — hopefully that’s “good enough” for folks running the Country Music Marathon.  The best chance of rain (and thunderstorms) this weekend will move in Saturday night and Sunday.  It looks like the best chance of severe weather will stay down to our south in that timeframe.

 

LINKS

Today’s lack of nerd-links is brought to you by pollen.  And sinus drainage.  Aaaaaand the vague out-of-body sensation induced by “non-drowsy” medication.  (HEAVY use of sarcastic quotes there.)

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About paulheggen33

Morning meteorologist for WSMV-TV in Nashville.
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