Severe Weather Monday: Hail/Wind/Tornado Details, Uncertainties

Today, we’re trying to be less this:

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And more normal.

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Today – High 71°Sunny…then Clouds & Rain Chances Increase

A very nice day. After dark, rain chances arrive.

Monday – Wake 53°, High 76°Severe Weather Possible

Here’s the Where/What/Certainty/When:

Where: Well, us. I probably wouldn’t talk about it unless it involved us, so I’m answering my own dumb question. Both Davidson & Williamson Counties (and all of Middle TN for that matter) remain in the Storm Prediction Center’s “Slight” Risk for severe weather tomorrow:

Storm Prediction Center Oct 5, 2014 0600 UTC Day 2 Convective Outlook - Internet Explorer 2014-10-05 08.33.11

What: In order of likelihood: Hail, Damaging Thunderstorm Winds, Tornado(es)

This won’t be a “squall line” like we had in the wee hours of Friday morning. Rather, we expect supercell thunderstorms along an east-to-west oriented boundary.

Hail is the main concern. Temps aloft will be well below freezing, and updrafts should be sustained long enough to promote hail formation.

Damaging thunderstorm winds are the secondary concern due to the strength of the upper-level wind fields. Those winds transfer to the ground when the downdraft part of the thunderstorm comes crashing down.

As for tornadoes, I quote SPC: “While low-level shear will not be particularly strong…it would appear sufficient for some cells to produce tornadoes. The tornado threat will also depend heavily on actual boundary-layer moisture content during the afternoon.”

When: Rain is possible as early as 9 or 10 PM Sunday. In fact, the RAP model is already showing mid-evening showers.

More showers and a few storms are possible in the morning hours (HRW-A&N has rain/storms by mid-morning; NAM4km model has some around 11 AM).

But, the main severe concern is in the afternoon/early evening hours.

SPC says “conditions are expected to destabilize rapidly during the day due to heating and moisture advection in the low-levels and cooling aloft.” Translation: the transport of warm, wet air into Middle TN during the day, combined with very cold temps overhead, sets up a “destabilizing” atmosphere = cold, drier air sitting on top of warm, wet air. This creates thunderstorms, and if you get enough of it (and other things), severe thunderstorms. Models indicate this destabilization will continue through the day. Around 4 PM to 7 PM, we should see thunderstorms start to pop.

Certainty: The best way to quantify this is in terms of probability of a severe weather event happening within 25 miles of you. In other words, we think something will happen in a large area, but the science isn’t good enough this far out to be more specific than that.

SPC thinks there is a 15% probability of severe weather happening to, or within 25 miles of, us:

Storm Prediction Center Oct 5, 2014 0600 UTC Day 2 Convective Outlook - Internet Explorer 2014-10-05 08.34.50

SPC has decided to wait until lunchtime to decide whether to upgrade this probability from 15% to 30%.

Other uncertainties include:

(1) the actual location of the storm-making boundary. Right now, SPC thinks that boundary will sit on top of us, but the models disagree. The Euro model thinks it’ll set up closer to the TN/MS border; if that happens, our storm chances will decrease.

(2) the amount of moisture the SW winds will carry into Middle TN. Moisture is the “stuff” from which these storms are made. If there is less of it, the storms will just be meh.

Bottom line: This looks like a hail event to me, but the Euro solution keeps the best severe ingredients out of Middle TN and puts them in near the TN/AL border, so this is far from a certain event. I would not be surprised to see SPC shift the above outlook SW of us; however, other models think this severe weather event is ours. All that to say: a good amount of uncertainty remains. Stay tuned. If the models ever reach a consensus, we can start talking like normal people.

Regardless of whether the severe dynamics shift SW of us, rain Monday is likely. Around 1″ is expected.

TuesdayRain May Continue – Wake 60°, High 76°

Officially, NWS-Nashville has another 0.41 falling from 7 AM to 7 PM Tuesday before rain chances drop (but aren’t nil) for about 36 hours.

Total expected rainfall Sunday through Wednesday at 7 AM is around 1.50″:


Extended Forecast:

This website supplements @NashSevereWx on Twitter. No warnings are ever posted here.