Thursday Evening Storms (A.M. Update)

Two blog posts planned today — this one, with more weather geekiness than you can shake a stick at…and an update this afternoon (hopefully posted by 3pm) to handle any last-minute adjustments to this evening’s severe weather forecast.

Ahead of the front that will cause this evening’s storms, the atmosphere is really getting “squeezed” by the rapid change in air pressure associated with the whole storm system.  This produces very strong winds — sustained winds around 20-30mph through the evening, with gusts possibly over 40mph.  The National Weather Service has issued a Wind Advisory for the entire Midstate:

The main event, of course, is the severe weather event that continues to look likely for Middle Tennessee and southern Kentucky. Before the storms arrive, we look not only at computer-model forecasts, but also at information from weather balloons (to give us a look at conditions in the upper atmosphere).  Here’s this morning’s weather balloon data from Nashville — don’t worry, I’ll explain what it means:BNA_12z_Feb20

Okay, here’s what it means (skip down to the paragraph above the Storm Prediction Center graphic if you’re not interested in the weather nerdiness):
1) The temperature data (red line) indicates an inversion — a layer of warm air (in the black circle, where the red line squiggles to the right) a little over a mile above our heads that will act like a “lid” to suppress any daytime development of storms.  The front headed our way, combined with warmer temperatures near the ground this afternoon and evening, will likely be able to break that lid to produce strong storms this evening.
2) The moisture data (green line) shows a layer of dry air just above the inversion (in the blue circle).  I think the strong winds from the south today will adequately mix the lowest couple of miles of the atmosphere to reduce that dry layer…but if there’s still a dry pocket in place when the storms develop, the evaporation of rain in that dry air could help to further accelerate the downdrafts that produce strong straight-line winds.
3) The wind data (red box, toward the bottom) shows already-impressive levels of wind energy in the atmosphere over our heads, and that wind energy will only increase throughout the day.  “Helicity” lets us quantify wind shear (change of speed and direction as you go up in the atmosphere) — it’s already over 200, and will probably be over 400 by this evening, which is significant.
4) The one factor working for us is the limited instability in the atmosphere.  We measure that with “CAPE” (Convective Available Potential Energy, highlighted in the yellow box at the bottom)…right now it’s zero, but it’s forecast to be “high enough” (500-1000) for at least some potential for storms to grow tall enough to produce tornadoes.  Storms have to grow vertically for a significant tornado threat — more “stretching” accelerates the rotation within the storm.  The low CAPE numbers also indicate low hail potential.

So, now you have an idea of all the factors we’re looking at when we’re putting together these forecasts.

If you slipped the geekiness, welcome back.  The Storm Prediction Center has placed most of the Midstate within a “Moderate Risk” region for severe weather (the red area in the image below), with the remainder of our area still included in the “Slight Risk” region (yellow):

Within 24 hours of a severe weather event, the SPC also breaks down their outlooks into specific threat forecasts — tornadoes, straight-line winds, and hail.  Those maps indicate a significant straight-line wind threat, a low-but-still-there tornado threat, and a low potential for large hail:

In case you’re not into reading the fine print, I’ll turn it into regular print…the percentages on those maps show the probability of that particular phenomenon occurring within 25 miles of any given point.  So, for most of the Midstate, here are the odds of these types of severe weather happening within 25 miles of you this evening:
WIND: 45% chance of 60+mph wind gusts (10% chance of 75+mph gusts)
TORNADO: 5% chance areawide
HAIL: a 5% chance in western Middle TN, less than a 5% chance elsewhere

Don’t like numbers?  Here are the threat levels in chart form:

There are still reasons for optimism, particularly with cloudy skies so far this morning.  Our best hope is for skies to remain cloudy, which would reduce our afternoon temperatures, and thus reduce the amount of energy available to fuel the evening storms.

Okay, we’ve covered what’s likely to happen…but what about the when?  Here are the latest Futurecast simulations of what the radar will look like throughout the evening…right now I lean toward the left-hand images being more accurate:

As I mentioned yesterday, it’s been my experience that computer models tend to slow down the arrival times of storm systems like these.  Factoring that in, here’s the latest estimate of when the storms will arrive:

Within those windows, I would lean toward the earlier arrival times…but until the storms actually start to develop, it’s hard to know for sure.

Like yesterday, our colleagues at the National Weather Service are thinking along very similar lines:

BOTTOM LINE: Plan on staying weather-aware this afternoon and especially this evening.  Of course, as a loyal Channel 4 employee, I recommend that you hunker down and watch the Winter Olympics, and we’ll be there to let you know when the weather becomes threatening.  We’ll also be streaming any weather coverage on and on our mobile app.

This is likely not going to be a good evening for heading out and about…if you’re going to be away from the TV, you can keep up with us on Twitter (@WSMVweather and @PaulHeggenWSMV) and Facebook.  (SOCIAL MEDIA NOTE: Twitter is by far the better resource in situations like this, because it’s a chronological stream.  Facebook filters out posts and presents them out of order, which isn’t very helpful.)

Again, I’ll post an update this afternoon to address any adjustments to the forecast.

About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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3 Responses to Thursday Evening Storms (A.M. Update)

  1. Colleen says:

    I’ve surprised myself with how entertained I am reading these blogs! Never thought I’d be able to stay awake through the recitation of weather analysis details. But whoa! I actually read (and somewhat understood) the “weather nerdiness” section. Good job!

  2. Lorraine says:

    I live in Michigan and just felt like checking out the storm situation y’all are about to get in your state.( mildly weather geeky) I googled TV stations and yours came up first-this blog section is great! Informative + funny, love the graphics and GIF’s. Wish our stations did the same, but they always too busy hyping/ panic inducing and are way too serious by far. ( our spring storms tend not to be so severe) Like the post above, I actually learned a bit, fancy that! Another advantage is no comments to blogs from internet trolls with negative dopey weather comments or faux knowledge. Well Paul Heggen, I shall be checking out future storm related weather in your area because it’s actually interesting and quite fab!

  3. Pingback: #4WARN Weather Alert: severe thunderstorms likely to impact the Midstate this ev… |

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