Thursday Evening Storms (P.M. Update)

As promised, here’s your afternoon severe-weather update.  There really aren’t too many changes to the forecast situation, so I’ve done a copy-and-paste of this morning’s post, and made some relevant edits.  So…as you’re reading the subsequent paragraphs, a lot of what you read will seem very familiar.  But rest assured, the information is either updated or still valid.  Enjoy!

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 over 40mph (the highest wind gust so far in Nashville has been 43mph, at 2pm).  The National Weather Service has issued a Wind Advisory for the entire Midstate through this evening:

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 below the Doctor Horrible “Science” meme 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 warm 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.  (It’s well over 1000 in Little Rock this afternoon, which is “upstream” and a good indicator of how our atmosphere will behave.)  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.

I had hoped the afternoon weather balloon data would be available by the time we published this update, but as of 3pm it’s still not there.  Once that data becomes available, I’ll post some quick analysis on Twitter to let you know if anything has changed.

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).  No real change to this since this morning:

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 in the Midstate

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

As of mid-afternoon, the first watches have been issued by the Storm Prediction Center…a Tornado Watch is now in effect for roughly the western one-third of the area (this watch doesn’t include the Nashville metro):

Important: THIS WON’T BE THE ONLY WATCH ISSUED.  It’s just the first.  Remember, a “watch” means that conditions are favorable for severe weather, a “warning” means that severe weather is actually happening.

My hope this morning was that cloudy skies would prevail, keeping temperatures cooler and reducing our damaging-storm potential.  But alas, those hopes have been dashed — with breaks in the cloud cover allowing the sun to shine through, the temperature in Nashville has already reached a record-tying 78 (at 2pm).  The warmer air at ground level will have a better chance of breaking the “lid” on the atmosphere…and it increases our tornado potential as a result.

Okay, we’ve covered what’s likely to happen…but what about the when?  Here’s the latest regional radar image (complete with fronts!) as of 3pm:

And 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 — that particular model matches up quite well with the 3pm radar image:

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:

Like yesterday, our colleagues at the National Weather Service are thinking along not-identical-but-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.  (By the way, if we’re in NBC’s Olympic coverage on Channel 4, we could also be doing non-stop weather coverage on the live-stream, so check both!)

This is 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.)

About paulheggen33

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

  1. Dorothy Hartzog says:

    Thanks, Paul – great job!

  2. Leon Rader says:

    Thanks for your professional approach, Paul. We watch you every morning.

  3. Joan Clark Mann says:

    Thank you. Will be tuned in.

  4. Sheila Marie Wright says:

    Your blogs are a wonderful way to keep us up to speed on in-coming weather. I always look forward to the way you present weather info. Well done, kind sir!

  5. rachel carter says:

    wish it wasn’t dark when the storms arrive. always much scarier when you live alone. columbia tn.

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