May 14 Severe Threat

You didn’t think we’d get through the whole month of May without at least some potential for severe weather, did you?  The good news is that as far as peak-storm-season severe weather threats go, this one is pretty marginal…the bad news is that there is still at least that marginal threat for some strong/severe storms on Wednesday.

WEDNESDAY MORNING:
I expect widespread showers and thunderstorms to be in place across much of the Midstate early Wednesday morning (moving in already after midnight tonight, actually).  A snapshot of Futurecast at 6am Wednesday gives you an idea of what to expect:
BAMS 6A WED

These are not expected to become severe, but they’ll produce some cloud-to-ground lightning and heavy downpours.  Be prepared for a wet commute Wednesday morning.

MIDDAY WEDNESDAY:
Here’s the wild-card…how much of a break do we get after the morning rain, and how long will it last?  The longer the break lasts, the more the atmosphere will be able to re-charge and produce stronger storms later in the day.  At this point, I’m thinking that enough cloud cover will linger overhead to keep temperatures in the upper 70s for Wednesday afternoon highs.  That’s still warm enough for a minimal severe-storm chance in the late afternoon and evening — warmer than that, and our severe weather chances go up…cooler, and they’ll go down significantly.

LATE WEDNESDAY AFTERNOON, WEDNESDAY EVENING:
Storms will re-develop as a slow-moving cold front finally edges into Middle Tennessee.  I estimate the time-frame for the strongest storms to be from 2pm until 10pm…within that range, I’d narrow it down even more to 4pm to 8pm…one of our Futurecast computers has zeroed in on that time frame as well:
BAMS 4P WED
BAMS 6P WED
BAMS 8P WED

(Full disclosure: the other in-house computer model we use produces stronger storms already by 2pm, but I’m just not buying that scenario at this point…that’s why you’re not getting the side-by-side Futurecast images this time around.)

The Storm Prediction Center has placed the majority of Middle Tennessee and southern Kentucky in their “Slight Risk” outlook for tomorrow, indicating an elevated threat for severe weather:
PAUL SEVERE RISK REGION

Specifically, this “Slight Risk” means there’s a 15% chance of severe weather (60+mph wind, 1″-diameter hail, or a tornado) within 25 miles of any one location.  The 15% probability is the lowest that will trigger a threat outlook from the SPC, which means they’re also thinking that this is a marginally favorable setup for severe weather.

So what does “marginally favorable” mean?  It means that SOME of the ingredients for severe weather will be present, but not ALL of them…and that there are significant questions surrounding the degree to which the atmosphere will re-charge for the afternoon/evening storms.  The one ingredient that will almost certainly be missing is wind shear (the change in wind direction and speed as you go up in the atmosphere) — with low wind shear, our tornado threat will be very low (you can never say ZERO around here in the spring).  There will be enough “buoyancy” for storms to rapidly grow, if we get that break around midday — that means our hail and straight-line wind threats will be the primary concern.  In chart form, here are the risk categories for each type of severe weather tomorrow:
PAUL SEVERE RISK GRAPH 2

So what does this “conditional” threat of severe weather mean for you?  As always, it means DON’T PANIC.  There’s a very good chance we’ll get nothing more than “garden-variety” storms throughout the day, with some slightly stronger storms in the late afternoon and evening.  The worst-case scenario would be for numerous storms producing hail more than 1″ in diameter, and damaging straight-line winds — tree branches and power lines would be at greatest risk, along with sheds, barns and other out-buildings.

Just plan on staying weather-aware throughout Wednesday afternoon and evening, and we’ll keep you updated!  We’ll be…um…WATCHING.
HomerSuspicious

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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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