Severe Threat Has Shifted a Bit West (uh-oh); Yet Big Uncertainties Remain.

Posted on Tuesday, April 4, 2017 @ 1:54 pm by David

We will approach 80° today as southern winds pump warm, moist air into Middle Tennessee.

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

Rising Concern Over Wednesday’s Severe Weather Potential

The lunchtime update from the Storm Prediction Center said the word “outbreak.” That’s not good. Mostly, their “outbreak” reference refers to the area to our south and southeast, but they are not discounting that here.

For Davidson & Williamson Counties, alert readers recall the big forecast uncertainty, the big question about this event: Will storms form in West Tennessee, intensify as they approach us, slam into us, then race away? (We do not want this).

Or, will they form atop or east of us, and wait to intensify after they have raced east and further away from us? (If there is to be severe weather, this is best for us).

This morning, forecasters at the Storm Prediction Center in Norman, Oklahoma put the significant severe (the shaded area) just east of I-65, suggesting the answer to that question is that the real concern is actually east of Nashville. Here’s their map from this morning.

This was updated at lunchtime. Notice where the “significant severe” area is now. It has moved west, and now covers all of Davidson and almost all of Williamson County (that’s not to say the risk for Fairview is meaningfully reduced when compared to Brentwood, Nashville, or Franklin):

We are included in the red-shaded area, which means for us that there is a 30% probability of damaging winds, large hail, and/or a tornado happening to, or within 25 miles, of us Wednesday.

In addition, we are 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.

I know, right.

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.

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 about last week? Y’all said we were going to get severe weather, all we got was rain?

First, wrong. Last week we examined the potential and discussed the uncertainties; if you thought we predicted severe weather, please read more carefully. Fortunately, last week’s uncertainties resolved in favor of nothing.

If anyone says tomorrow’s storms “will” or “won’t” happen, run away. He or she is a guessing hedgehog. Forecasts are expressions of probabilities amidst uncertainties, it’s not a “yes or no” binary thing.

We face two big uncertainties tomorrow, which could prevent a storm event from happening:

1. Will the storms form to our west and blast us, or will they form on or east of us and run away from us? We still don’t know the answer to that question; however, concern is rising that they will form west of us, speed quickly east, and nail us.

2. The same thing that happened last week could happen again: morning storms form to our south, soak up and cut off our supply of storm food, as illustrated here:

This is entirely possible. This is what we want. And, we don’t know right now whether that will happen. Some models have these southern storms developing in the above-circled area, which may cut off the storm food for East Tennessee, but not necessarily cut off our supply. If this event doesn’t happen, this will probably be why. Stay tuned.

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.