T’Storms Today; Supercells Forecast Monday

We start with this from our NWS-Nashville:

Today – Rain, Non-Severe Thunderstorms Likely This Afternoon

Last night, I wrote that the Hi-Res NAM weather model thought it would rain this afternoon. Skeptical, I wrote that I’d wake up and see what the morning runs of the HRRR said, because the HRRR usually does a decent job. Well, here it is:

These storms can already be seen on radar (scroll down). Although we are expecting rain, lightning, and some wind this afternoon round of storms, we are not expecting severe weather this afternoon.That’ll come later.

Tonight/Overnight — The Squall Line

Today, supercells are expected to be ongoing in Arkansas and Missouri. Tonight. those supercells should develop into a line of showers on the west side of the Mississippi River, cross the river, the move as a squall line east through West then Middle Tennessee.

The squall line should arrive around midnight. This will be our first chance of severe weather, capable of hail and damaging winds. Note two things about these storms:

1.  The midnight ETA is uncertain, and for that reason, the severe threat may diminish at I-65. “May.”

2.  While we cannot rule out tornadoes in this squall line, damaging winds and hail are the much bigger concern. This will not be the case Monday.

Monday — Concerned About Supercells

Whatever is left over from the overnight squall line is expected to clear out Monday morning. A “lull” in the storm activity is expected late Monday morning, lasting into the afternoon. During this time, the atmosphere is expected to “recharge.”

At the risk of getting too technical, heating at the surface, 60+ degree dewpoints, and impressive wind profiles all the way up the atmosphere, can set the stage for explosive storm development. As the NWS wrote this morning, “the atmosphere’s ability to reload during the lull . . . will weigh heavily on the magnitude of the severe weather we receive later Monday and Monday night.”

NWS thinks “the most likely scenario at this point” for Monday afternoon is:

• Temps approaching 80
• Dewpoints in the 60s
• MUCAPE (technical term) 2,000 j/kg
• 50+ knot wind sheer at 800mb

all should favor supercell formation.

Supercells are storms — usually, but not necessarily, thunderstorms — that contain updrafts that rotate about a vertical axis. Once formed, a supercell may perpetuate itself for an appreciable length of time, even upon encountering an environment that is hostile to the development of new storms.

Back to the NWS-Nashville: “Initial storm mode should favor supercells with fairly high tornado potential, including long tracked and strong tornadoes.“ This has my attention, and it should have your attention.

The severe weather threat will persist through Monday night, even if these supercells again congeal into large clusters.

The following map expresses broad probabilities for various modes of severe weather for Monday:

Note we’re on the northern border of the “worst” outlook area (in purple). Don’t think “nothing will happen here because we aren’t in the worst of it.” Also, know that this outlook will be issued three more times before the supercells are expected, and outlook areas almost always shift.

Anyway, for now, what the map means for Davidson & Williamson Counties is that there is a 30% probability — just under 1 chance in 3 — that a severe weather event (tornado, damaging winds, large hail) will happen 0 to 25 miles from you.

The “hatched” (shaded) area means there’s a 10% “or greater” likelihood that severe weather event happening within 0 to 25 miles of you will be significant. Here, “significant” means 2″ or larger hail, 65 knot or stronger winds, and EF-2 or stronger tornadoes.

Tuesday — Severe Potential, Lots of Uncertainty

The potential for severe weather continues, but the extent of which is highly dependent on the storms we get Monday. The trend, however, is that our threat Tuesday is decreasing, having shifted South. This is reflected by the SPC Day 3 Outlook, which has, I’m happy to report, removed us from its Slight Risk outlook:

We’ll update this website this afternoon.

I wrote this last night, but in case you missed it:

For starters, be sure to have multiple reliable sources to receive Watch and Warning information. This should include an app, a NOAA weather radio, “regular” radio, and/or, of course, local TV (2, 4, 5, and/or 17).

I think your well being is worth heeding NWS Warnings. This means you taking cover in a lowest level, interior room of your house (not a trailer and not a car), with your shoes on, ID in your pocket, charged phone, and helmets for everyone. If you’re in a Tornado Warning polygon (more on this below), do not go to the front door and try to see it — it’s very likely going to be wrapped in rain. You. Will. Not. See. It.

Remember, a Watch means conditions are favorable for the development of a tornado. You should “watch” and otherwise monitor your sources of severe weather information, already prepared to take action if a warning is issued.

A Warning means a tornado is occurring or imminent. It could mean a tornado is detected on radar (but no one has reported it … yet). It could mean one has been spotted or has otherwise been confirmed on the ground. Act anyway.

A Warning always comes with a polygon. Those inside the polygon are in the warned area and should immediately take shelter. Those outside the polygon aren’t warned, but if you’re close to it, take cover anyway.

Example:

You can help the NWS and your community by tweeting your report of occurring severe weather (funnel cloud, wall cloud, tornado, trees down/structure damage, hail the size of a quarter, flash flooding) to NWS-Nashville by including the hashtag #tSpotter. Either geotag your location, or let us know where you are and when you saw it. The NWS will see your report, and it could assist the NWS in issuing a warning that saves someone’s life.

Note: WARNINGS WILL NOT BE POSTED TO THIS WEBSITE.

Finally, don’t forget about the Flash Flooding potential. Do not drive over water-covered roads. It’s cheezy, but true: “Turn Around, Don’t Drown.”

We’ll be up with another post in the morning.

Current Official Hourly Observation (taken at :53 on the hour)