June 4-5-6 Severe Threat

CMA Fest kicks off in Nashville tomorrow, so as downtown swells with the arrival of thousands of visitors, what better way to welcome those visitors than with a few consecutive days of severe weather chances?

The first and greatest severe weather threat will arrive this evening and tonight.  In the meantime, the daylight hours will be hot and MUGGY — the heat index will likely climb to the mid 90s this afternoon, so stay hydrated and take it easy.

Thunderstorms are expected to develop by late this afternoon along the Ohio River, and they’ll head generally north-to-south, which will bring them into the Midstate this evening and tonight.  Trying to pin down the exact arrival time is challenging, to say the least — we can’t know exactly when and exactly where until the storms actually develop (and then we can track them).

That said, we can at least give you a hopefully-good estimate of when storms are mostly likely.  This series of Futurecast images represents one computer’s estimate of where the storms will be throughout this evening (this particular computer model seems to have the best handle on how things will evolve):

So that’s what one computer thinks…factoring some other model information, here are the best-estimate storm arrival time “windows”:

Those four-hour windows are longer than what we’d like to be able to give you, but the reality is that the specific times will be dependent not only on when the storms develop, but also on how fast they move once they’re off and running.  I’m not terribly concerned about the storm threat in Nashville as the pre-CMT Award festivities are happening, but it’s certainly possible that there will be strong/severe storms as the show is letting out.

I’ve talked (well, “typed” I guess) in this space before about the “recipe” for severe thunderstorms.  But it’s been a while, so let’s revisit it.

The ingredients for severe thunderstorms are always the same: moisture, unstable air, a lifting mechanism to get things started, and wind energy in the environment.  The “how bad” is determined by the ratio of those ingredients.  If all of the ingredients are ideal, then significant severe weather is possible…but more often, one or more of those ingredients are barely present, while others are there in abundance.  (Think of trying to make chocolate chip cookies with five pounds of flour and only a tablespoon of sugar.)  Here’s how the ingredients will break down this evening and tonight:

Moisture: Walk outside.  Notice how gross it feels.  It’s humid.  Yup, we’ve got enough moisture.

Unstable air: Big-time.  Temperatures near the ground will be around 90 degrees this afternoon, while temperatures farther up in the atmosphere will be significantly colder — a high difference between those readings means that explosive thunderstorm growth will be possible (and large hail becomes more likely).

Lifting mechanism: Thunderstorms are fueled by rising air, but something needs to start that rising motion…you can have all the warm, unstable air in the world in place and still not get storms, if there’s nothing around to get the party started.  This is the big wild-card with this evening’s forecast — storms this morning in Missouri, Illinois and Indiana left behind an “outflow boundary” (kind of a mini-cold front) that will spark the development of storms we expect to occur late this afternoon in the Ohio River valley.  Once those storms develop, they can pretty much create their own lift as they move to the south/southeast.

Wind energy: This tends to be the limiting factor regarding our severe weather potential this time of year.  In the summer months, upper-level winds are usually pretty weak — thunderstorms that develop in that type of environment can still be strong, but are short-lived and don’t rotate much.  But tonight, while the wind-energy levels aren’t going to be as high as they can be in spring, they’ll still be sufficient for strong long-lived storms, and even the possibility of a couple of isolated tornadoes.

Too much detail?  Here’s the bottom line: the tornado potential isn’t our main cause for concern — straight-line winds (gusts over 60 mph) and large hail (1″ diameter or larger) will be the main threats:

The Storm Prediction Center and Nashville’s National Weather Service office are thinking along the same lines…the SPC has included most of the Midstate in a “Slight Risk” area, indicating an elevated threat of severe weather:

Breaking those threats down specifically, the SPC thinks there’s a 2-5% chance of a tornado within 25 miles of any one point in our area…

…and a 15-30% chance of 60+mph wind gusts…

…and a 15-30% chance of 1″+ diameter hail.

Here’s the NWS’ take on tonight’s scenario:

Once tonight’s storms move to the south and dissipate, we’ll have several hours of calm weather through midday Thursday.  But by Thursday afternoon, it looks like storms will re-fire in southern Middle Tennessee, as shown by this snapshot from Futurecast:

The same scenario will repeat itself on Friday, and parts of the Midstate are included in the SPC’s outlook for both Thursday and Friday:

The severe weather potential for Thursday and Friday isn’t as widespread and isn’t as high, but we’ll still be carefully watching any storms that develop those days for straight-line winds and large hail.

No, you don’t need to head to your “safe place” and hide there until Saturday.  Just plan on staying weather-aware the next few days — we’ll let you know if any watches or warnings are issued, both on Channel 4 and online.

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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