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Rain & Storms Friday Night – Saturday: 2 Rounds

With the Country Music Marathon, Franklin Main Street Festival, early wedding season, ball games, etc., it’s a busy time for everyone. We’re in the middle of severe storm season, and we may have something to worry about. Not sounding any alarms, but it’s time to raise awareness, and worth putting up this extra update today.

First things first.

Current Temps and Radar

It’s going to be clear and chilly tonight.  We’ll hit 52° by 10 PM, on our way to a Friday morning wee-hour low of 39°. A Frost Advisory has been issued for the plateau, but not for us. Coats and jackets will be needed in the morning.

Friday & Saturday

We’ll start Friday off dry. The dew point when we wake up will be 33°, but that’ll almost double by noon Saturday as a warm, humid airmass works in from the south.

Rain & Thunderstorms are likely Friday night through Saturday morning. We may see rain arrive shortly after noon, but right now we think light rain will start early Friday night, picking up in coverage and intensity through midnight and into the overnight/wee hours of Saturday morning.

Almost 0.5″ is forecast to fall through 7 AM Saturday, and NWS says “confidence is good that there will be at least some thunderstorms in the Metro [Nashville] by daybreak Saturday.

Rain is one thing, but lightning is something entirely different. Obey race officials and respect the lightning. These storms should be strong, and maybe even severe.

When the CMM starts, the dewpoint will be an April-typical 56°; by 10 AM, however, the dewpoint will be past 60° and climbing. Don’t let Friday morning’s weather fool you; It’s going to be humid.

Rain and thunderstorms should end at some point late Saturday morning/around noon…however, this is only Round 1. Weather model guidance suggests a break in the action, which may last a handful of hours.

During those early Saturday afternoon hours when the rain should switch off, mid level winds will increase and dry out the mid/upper atmosphere, setting the stage for a second round of storms beginning mid/late afternoon.

This Second Round has a better chance for strong and severe thunderstorms. These storms should be in the form of classic line segments, and maybe even supercells, delivering:

  • Heavy rain; another 0.35″, on top of what we expect to have seen through noon Saturday.
  • Frequent lightning.
  • Large Hail. This is probably the biggest concern.
  • Damaging straight-line winds, courtesy of the forward motion of the storms running between 35 and 50 knots.
  • A tornado or two.

Saturday at 4 PM is roughly the peak time for storm concern. This is because weather model forecast soundings look like this (I drew yellow boxes around the numbers that support a severe-friendly environment):

These are not end-of-the-world numbers, but they represent a potentially strong weather event worth paying attention to.

Of particular interest is the Significant Hail analog. It shows several historical large-hail-producing events where that actual environment looked a lot like the one this model thinks we will see Saturday afternoon. This model thinks 2″ hail is possible.

Other forecast soundings are roughly like this one, although the NAM4 sounding for 4 PM looks pretty ominous: it shows 3200 j/kg of CAPE (“stuff” that makes storms) and Lifted Indicies of -12, which supports the hail concern. We think hail is the most likely item in the Bag-O-Severe Weather.

The tornado threat is not zero, but if I wanted to see a tornado Saturday, I’d probably want to see more of a southeast wind at the surface, lower LCL levels, and a little more shear. Plus, if the more aggressive, hail-loving NAM4 model is correct, there is some good news: LCL levels and shear are not very tornado friendly. Tornadoes are a concern, but they’re just not our biggest concern.

Then there is the European model. The latest run (12z) develops a negatively titled 996mb surface low over the Ohio River Valley, putting us in the warm sector of the storm with 2,000 CAPE. No. Thank. You.  That’s a good recipe for severe weather.

All that said, our skill in nailing any of this isn’t that good. We’re as sure as we can be that it’s going to rain and storm Saturday. The intensity and timing of the rain and storms is less certain and will be revised as we approach the event.

So as you go about your weekend, stay weather aware. Consult multiple reliable local severe weather sources, and heed all warnings.

This website supplements @NashSevereWx on Twitter, which you can find here.

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