Significant Severe Storm Event Possible Thursday: Uncertainties, Timing, Details, Threats.

Posted on Wednesday, March 29, 2017 @ 9:30 pm by David

Hey y’all.

First, everyone take a deep breath. Like any good forecast, below you will find important details, nuance, and uncertainties.

Resist any tendency to panic at the details that may scare you, or ignore the things which may limit the threat.

Tomorrow’s threat to you, personally, is low; but the potential impact is high. Your safety is worth some preparation. Your health is worth responding appropriately to warnings. Storms don’t care about what happened, or didn’t happen, “last time.” Storms do not obey anyone’s gut feeling. They do their thing. So be smart, be responsible, and cool.

This is not the worst case scenario. I’ve seen more dangerous forecasts for Middle Tennessee than this one. However, it has been since Festivus 2015 (December 23, 2015) that I’ve seen one this concerning for us. We dodged a pretty big bullet then. Let’s hope we dodge another one tomorrow.

Don’t just listen to us. Consult multiple, reliable, local sources for your severe weather information, especially local TV meteorologists on 2, 4, 5, and 17.

Overview

We will go to sleep tonight sitting on the line of the Storm Prediction Center’s “Enhanced Risk” (3 out of 5) and the rare “Moderate Risk” (4 out of 5) damaging winds, large hail, and tornadoes Thursday:

Forecast details are below. A long time ago I made this chart. It’s not perfect. You might, very reasonably, consider canceling stuff mid/late afternoon-evening, depending on your risk tolerance and the need for you to be doing whatever it is you’re planning to cancel.

As we wrote this morning, there are meaningful uncertainties about the severity of the forecast. This could turn out to be not that big of a deal, or it could be an event that brings low impacts. Or, it could be a very big day for those near you, but not you. Or, something bad could happen specifically to you. I wish we could tell you exactly what will happen, whether your flight will be cancelled, whether you should cancel practice, whether schools should close. All decisions are personal and multifactorial. They depend on your risk tolerance and the utility of the activity in question.

The Wind Advisory

Non-storm winds are expected to gust over 40 MPH from noon to 9 PM Thursday. This prompted NWS to issue a Wind Advisory:

First Wave of Storms in the Morning

There will be two rounds of showers and storms tomorrow.

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.

In fact, the HRRR model is starting to come into range, and it thinks these storms will weaken considerably.

Second Wave of Storms — Partially Dependent on Whether the Sun Comes Out after the First Wave

The second round of storms is the “main event” with the concerning severe weather potential.

How bad they will be will depend in part on what happens after the first wave.

If the first wave clears out early, and the sun comes out, the second round of storms will be strong, perhaps exceptionally strong. In this scenario “we could be looking at a greater severe risk with robust storms developing earlier in the afternoon and lasting into the evening.” If this “maximum potential is realized, we could have several supercells developing by mid afternoon with tornadic potential. (NWS-Nashville). Do. Not. Want.
If we are cloudy late into the morning and early afternoon, “then the severe weather risk will be reduced. Even so, most models still indicate a significant potential for strong to severe storms by late Thursday.” In this scenario, “storms may be more consolidated in the late afternoon and evening hours within clusters, but still damaging and still with some tornado potential.” (NWS-Nashville)

(9:30 PM Update: The latest runs of the HRRR show (1) the First Wave being very weak, which is a bad sign for later in the day, and (2) clearing/sunshine occurring around 1 PM tomorrow, also a bad sign:

Do. NOT. Want.)

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

This is what we are talking about:

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

Tomorrow morning, SPC will provide specific probabilities for hail, damaging winds, and tornadoes.

Why are we on the line between 3 of 5 and 4 of 5? I think it’s because the threat is higher to our west where the supply of storm fuel is expected to be uninterrupted.

When the storms get to us from West Tennessee, they may lose some (not all) of their fuel, and therefore some (not all) their power.

Earlier today, at least three models predicted this reduction in storm fuel tomorrow, which is an encouraging sign.

The more recent models are little more mixed.

NAM3 thinks there will be plenty of storm fuel when the storms get here.

The GFS model predicts a reduction in storm fuel when the storms reach I-65:

The SREF model also predicts a reduction in storm fuel upon arrival:

This suggests we may see horrible storms coming at us from the southwest, with them weakening slightly — at the last minute before they arrive. This is what happened December 23, 2015. I’m hoping, but not necessarily expecting, a repeat.

Official Probabilities

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:

Tomorrow, after the first wave, we want cloudy skies, mid 50°s dewpoints, and a loss of storm fuel as the line nears.

But not matter what, this setup deserves your attention.

Prepare:

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.