Forecast Blogs

ETA 7 AM to 9 AM, Give or Take a Few Hours. Damaging Winds Most Likely, Hail & Tornado Also Possible

Quick Look Forecast

Wednesday

What: Damaging Winds (Main Threat), Large Hail, & (a Smaller) Tornado Threat

Storms will form a long line on radar, called a squall line, which will quickly speed through Wednesday morning. They should be packing quite the punch:

  1. Damaging straight-line winds are the most likely severe weather threat.
  2. Large hail is the next most likely hazard.
  3. Less likely, but still possible, is a brief tornado embedded in the squall line.

The probability of any of these happening withing 25 miles of you is 30%.

When: 7 AM to 9 AM, With a Margin of Error +/- a Few Hours

Remember, we’re talking only about Nashville (Davidson Co) and Williamson Co.

Before we get our storms, supercells should move north of us. From midnight to 4 AM, the HRRR model thinks supercells will race out of West Tennessee into Kentucky. These should not impact us.

The main event for us will be a squall line packing the aforementioned risk of damaging straight line winds (most likely), large hail (less likely), and a tornado or two (even less likely, but still possible).

Squall Line ETA is 7 AM to 9 AM, Give or Take a Few Hours.

The latest run of the HRRR model predicts an intense squall line to show up between 8 AM and 9 AM.

Earlier today, this model liked an earlier ETA of 6 AM, but tonight it’s moving closer to 9 AM. Hence the margin of error.

A different model, the NAM3, predicts the squall line will arrive between 7 AM and 8 AM:

These storms will be arriving with plenty of bite, as seen by high values in the Significant Tornado Parameter:

*Keep in mind some of these squall lines like to accelerate and slow down at their own pace. Timing them out is quite difficult.

The Storm Prediction Center will probably issue a Severe Thunderstorm Watch or a Tornado Watch hours before these storms arrive. If  you have a weather radio and a wake-me-up app, the issuance of a Watch may wake you up. I encourage you to have one of these — if the models are wrong and the storms come early, you need something to wake you up to alert you to the danger. We like this.

Remember, a Watch covers a large area. It means “be on alert.” A Warning covers a much smaller area, in a box (a polygon). A warning means to take shelter right away because severe weather is either occurring or imminent.

Other Things to Keep In Mind

  1. Be prepared to respond in the case of a severe weather warning. Know your safe place and since this is a morning event, do not attempt to drive when severe weather is imminent. Those in mobile homes are most vulnerable; if you have a better option, take it.
  2. This will likely be more of a damaging straight-line wind event than anything else. However, tornadoes can develop quickly along notches and bends in a line of storms. Sometimes, a tornado can occur without a tornado warning. A lot of severe thunderstorm warnings have an extra tag at the bottom that says “Tornado…Possible”.
  3. Furthermore, severe thunderstorm winds can be just as strong and do the same or more damage as tornadic winds. This means that we treat every severe thunderstorm and tornado warning with the same level of importance, and you should, too.
  4. Watch the dewpoints. If this forecast is going to bust (meaning, we think it’ll be bad, and it isn’t), one reason may be due to a lack of instability and moisture. All models predicting severe thunderstorms assume we’ll actually have 63°+ dewpoints early in the morning. If we don’t — for example, if the dewpoints are only in the upper 50°s when we wake up — the models will have been way off and this event may be noisy, but not as bad as forecast. When forecasts bust, this is often the reason why. There is some reason to think the line may weaken as it works east.
  5. If this does turn out to be a tornado producing squall line, remember tornadoes can spin up, drop, then lift between radar scans. Depending on the orientation of the storm winds to the radar, the radar may not “see” the winds very well. There may be damaging winds or (God-forbid) a tornado, even if radar is not seeing it very well. Radar can see a lot, but it can’t see everything. Tornadoes don’t care if radar can detect them or not. This is why you should heed all warnings, and take cover if one comes out.
  6. It’s possible a supercell or two could form out ahead of the main squall line, but I think that’s unlikely. I just wanted to mention it.
  7. By no means are we calling for panic, just be prepared, aware, and don’t take any chances. The storm will move through very quickly, lasting no more than an hour, letting you get back to your day soon enough. A storm isn’t a big deal until it happens to you.

Our Twitter, @NashSevereWx, will have the most timely updates tomorrow morning on this event. Warnings are never posted to this website. You’re encouraged to tune in to your favorite local TV station as the storms approach.

After the Storms

Quoting NWS-Nashville:

Even before the storms arrive, winds will become strong and gusty tonight, and after the storms pass, they will remain pretty gusty as well for least an additional 3 to 6 hours. Once the [cold] front passes on Wednesday, winds will switch from the south to the northwest, and temperatures will fall.

Temps should still be in the mid 60°s Wednesday afternoon. The real temp drop will happen late Wednesday night and into the wee hours of Thursday morning.

Finally….

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Categories: Forecast Blogs