Low Probability, High Impact (Winds, Hail, maybe a Tornado) Threat Tomorrow

Posted on Tuesday, April 4, 2017 @ 8:44 pm by David

Storms are brewing tomorrow. They could be significant, damaging storms, but the forecast has uncertainties.

For a detailed breakdown from Charlie Neese, watch the Periscope he did at 8:30 PM tonight.

Rising Concern Over Wednesday’s Severe Weather Potential

The lunchtime update from the Storm Prediction Center said the word “outbreak.” That’s not good. SPC mostly refers to the area to our south and southeast, and although our risk here isn’t as high as it is elsewhere, it still poses a very real, but conditional and uncertain, threat. Another update is not due from the SPC until after we go to bed

SPC has put us inside its “significant severe” risk area, covering both of our counties.

This means we have a 30% probability of damaging winds, large hail, and/or a tornado happening to, or within 25 miles, of us Wednesday.

We are also included in the shaded-in area indicating a 10% probability of a “significant severe” event within 25 miles of us. A “significant severe” storm produces any/all of the following:

A tornado that produces EF2 or greater damage.
Wind speeds of 75 mph (65 knots) or greater.
Hail 2 inch in diameter or larger.

Q. What kind of storms are we talking about?

Supercells producing, in order of likelihood: (1) Damaging winds, (2) large hail, (3) tornado, and (4) flooding.

Rule nothing out.

The tornado threat appears higher the further east we go, but we could still see a tornado or two.

Storms will be scattered throughout Middle Tennessee. It’s possible some of you may not even see rain. Others may see a supercell thunderstorm.

Q: What’s the ETA?

The big range for all of Middle Tennessee is noon to 9 PM. For us, 3 PM to 7 PM.

THIS MAY BE A LATE AFTERNOON/EARLY EVENING RUSH HOUR EVENT

- If the NAM3 model is correct, and these storms arrive around 4 PM…well, obviously, if you’re planning to be out and about, on a bus, wherever tomorrow, you will be impacted. Monitor the weather tomorrow.
- If you are on the road, which hopefully you will not be, and storms are impending, do not take shelter under an overpass! Underpasses amplify winds for those caught underneath, creating a wind tunnel effect. There have been numerous fatalities for those who sought shelter there.
- Those in mobile homes should ride out warned storms in a site-built structure. Your mobile home is unsafe. A few days ago a mother and her child were killed in a mobile home in Louisiana.

Q: What might happen to keep the storms from happening?

First, storms may form on top of or east of us, then race east and away from us. That’s the best scenario. However, if they form west of us, they may rapidly intensify and end up on us very, very quickly.

Second, we could see a repeat of what happened last week: morning storms form to our south, and soak up and cut off our supply of storm food, as illustrated here:

This is entirely possible. However, latest model runs suggest those storms in Alabama in the morning will move away and maybe not cut off our supply of that storm food.

We won’t really start to have meaningful certainty until we see the satellite photos after sunrise and the storms pop up on radar around lunch tomorrow.

Q: If they come, how long will they last?

If the worst happens, an hour, likely less. Storms will be fueled by a very fast system, so even if we do see storms, they won’t last long in one given area. They’ll be very fast movers, which means you will need to be ready to react.

Keep an eye to the sky and another to your smartphone…follow us @NashSevereWx on Twitter for updates on the forecast for tomorrow and as storms (maybe) roll through the area Wednesday afternoon/early evening.

Finally, I leave you with a quote from the Storm Prediction Center discussing our area:

A band of supercells appear likely per latest model guidance given the strength of low and mid-level winds coupled with weak to locally moderate instability. Large to very large hail and damaging winds appear to be the main threats, with a few tornadoes also possible. Given the forecast coverage of convection, it is not out of the question that higher severe probabilities may need to be considered in a later outlook update.

The few of you who are know-it-alls and are planning to criticize schools/businesses tomorrow night for dismissal decisions, let’s hear from you now, without the benefit of hindsight.

Also, please share this with your grandparents. I’m sure they will appreciate the current state of weather forecasting, even though it isn’t perfect.

Extended Outlook

Prepare for cooler temps on Thursday and Friday with afternoon highs in the upper 50s! There’s an old tale that says to not count out a hard freeze in April, and we may get close to it on Friday night.