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Significant Severe Event Possible Thursday; 2 Waves of Storms! Risks, Timing, Probabilities, & Uncertainties

Quiet Weather Today as Thursday’s Storm System Sweeps Our Way

Partly cloudy skies this morning will gradually become mostly cloudy by late afternoon/early evening. Temperatures today will approach 80º!

Significant Severe Event Possible Thursday


The Storm Prediction Center has upgraded a majority of our area to an “Enhanced Risk” (3 out of 5) for seeing severe thunderstorms tomorrow. A small portion of Davidson and Williamson Counties, mainly west of I-65, are now in a “Moderate Risk” (4 out of 5).

How does this make David feel…?

He may be saying profanity. (David’s Note: over the past 5 years, only two events were graded “Moderate” for us the day before the event. Usually, that designation is reserved for the day of the event. This day-before designation is an indication of forecast confidence from the Storm Prediction Center. Hence the profanity. Don’t worry mom, it is a righteous profanity.)

All that said, there are meaningful uncertainties about the severity of the forecast.  Those are described below.

Before discussing the storms, you should know that non-storm winds are expected to gust over 40 MPH from noon to 9 PM Thursday. This prompted NWS to issue a Wind Advisory:

Two Waves of Storms

There will be two rounds of showers and storms tomorrow.

1. First Wave: In the Morning

The first round is expected in the morning, the storms that are leftover from severe weather expected overnight in Arkansas, but it is not the main concern.

ETA for this first wave is 9AM to Noon

The risk/threat is low, but not zero. Damaging winds are the main concern. These storms should be weakening on approach.

After these storms — or maybe just rain — pass, there should be a lull in activity. If the clouds clear and the sun comes out, watch out for the second wave.

2. Second Wave: The Afternoon/Early Evening Main Event

The second round is expected by late afternoon/early evening. This is the main severe event of concern.

Latest NAM3 Loop for Thursday from 1 PM to 10 PM:

Current “best” ETA: 4PM-8PM. Remember, this may change. Stay tuned today, tonight, and tomorrow.


Potential impacts include:

  • Damaging winds, 70 mph or greater
  • Very large hail
  • Isolated tornadoes, including a few strong tornadoes

The more sunlight we receive after the first wave Thursday, the worse the storms will be during the second wave. Without that heating, the storms won’t be as bad; some models think the atmosphere will not “recover” in time to make the storms all that bad. Other models disagree, and think the sun will come out and these storms will be a big deal. This is the main uncertainty about this forecast.

Storms will initially be “supercells,” which are individual cells of storms that are very intense. These may quickly congeal and form into a line as they approach I-65.

Update from David: What We Hope Will Happen

We hope the storms lose their fuel, and therefore their power, when they get to us.

Every indication is that there will be plenty of fuel to feed storm strength in west Tennessee, where the risk is greater than it is here.

We’re hoping the storm fuel will lower once the storms get here, which should reduce our risk. New mid-day models are showing this reduction in storm fuel, which is an encouraging sign:

NAM3 model:

GFS model:

SREF model:

This may turn out to be wrong, but at least this is an encouraging sign. I think it also explains why the Storm Prediction Center lowers the threat level from 4 of 5 to 3 of 5 at the I-65 corridor. Although it reduces my blood pressure a bit, I wish the storm food would reduce further west of us, long before it gets here, rather than as it’s arriving.  It’s going to be a close call. Even if the tornado concern reduces, the damaging wind and hail threat will remain.


The Storm Prediction Center has us right on the line between its 30% and 45% probability of severe weather (hail, damaging winds, and/or tornadoes) within 25 miles. What does this even mean? SPC explains:

The probabilities that you see on the graphics represent the probability of one or more events occurring within 25 miles of a point during the outlook period. This definition is used as the probability of severe weather at a given point is quite small. How many times have you experienced a tornado in your neighborhood? For most people, the answer is never. Now think of how many times severe weather has occurred within 25 miles of your location. It’s probably safe to say that you can think of some close-by severe weather events. You should be able to imagine that the probability of having severe weather occur within such an area is much larger than the probability of having it occur specifically within any one neighborhood.

How should you interpret probabilistic values? The smallest values represent areas where the most uncertainty exists and correspondingly where the smallest expected coverage of storm reports exists. The higher the probabilities, the greater the perceived threat, and the greater the expected coverage of that hazard being forecast. The highest probabilities are generally reserved for more significant severe weather events and are used infrequently, if at all, during the year.

Also notice the “significant severe” area, the shaded/dashed area, referred to as “hatched.” This area appears pushed up to I-65, where it stops:

This indicates the probability of significant severe is 10% or greater tomorrow. A “significant severe” event involves:

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

Remember the uncertainties. Wave 2, the main event, can go a few different ways. NWS-Nashville expressed it like this:


Review your severe weather safety plans.

  • If you live in a mobile home, locate a sturdier structure well in advance of these storms. Some local churches and other public entities are opening their doors tomorrow for shelter during the potential severe weather.
  • Predators game…have tickets to the game? With the game starting around the time storms may hit, do not get caught in your car when they arrive. Also, if storms are ongoing at the time of your departure from the Arena, wait it out and let them pass.

Have multiple outlets to receive weather information from tomorrow, including @NashSevereWx on Twitter.

New to severe weather? Click for more information.

This is a bigger risk event than what we usually see. The threat is real, but may happen to someone else. The potential impact is large. Because you might be that “someone else,” we think precautions are easy, effective, and worth it.

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