Severe Weather Possible Later Today

Posted on Tuesday, March 21, 2017 @ 10:58 am by Brendan Schaper

Severe Storm Potential Increasing, But with a Few Big Uncertainties

The Storm Prediction Center has upgraded much of our area, except for those north of I-40, to an enhanced risk for severe weather. This does not appear to be a Tornado Day; instead, the risk is mainly damaging straight line winds, with a side of large hail, a brief period of torrential rainfall, and frequent lightning.

Temperatures will rise into the mid-upper 70s with a noticeable increase in cloud cover. Instability (“storm food”) will definitely be on the increase this afternoon, which could better fuel the storms that arrive by evening.

Prime time for severe storms – 4PM to 6PM

Model uncertainty exists — mainly in the fashion of “where the worst of the storms will hit?”

Here is one take, from the HRRR model:

The latest loop of the HRRR model gives a general idea of arrival time and intensity of these thunderstorms. There has been a trend from this particular model to send the worst of the storms to south of I-40. If this guidance is correct, folks in Williamson County would see a greater probability of severe thunderstorms this evening than those in Davidson County.

However…and herein lies the uncertainty…the NAM3 model does not think the threat is mainly south of I-40:

NAM3 model thinks the storms will go south *and north* of I-40. Timing generally same as HRRR model: 4-6 PM. pic.twitter.com/do9v34z3ez

— NashSevereWx (@NashSevereWx) March 21, 2017

The NAM3 model would suggest that everyone, north and south of I-40, may get a dose of strong/severe storms.

So which model’s guidance do you buy in to? This is what makes forecasting, or predicting the future, such a challenging process. The HRRR model has consistently kept the strongest storm dynamics south of I-40, which tracks the “Enhanced” area drawn by SPC. David thinks we will all get some rain, but the worst of the storms should be south of I-40. This may be wrong, so stay tuned. It’s really too close to “call.”

Bottom line

-Storms develop west-northwest of us this afternoon

-Storms, some severe, ETA: 4-6PM

-Main concerns are damaging winds and large hail. The probability of damaging winds within 25 miles of us is 30%. This high probability is why SPC upgraded us to “enhanced.” The large hail probability within 25 miles of us is 15%.

-Tornadoes are not expected. These storms are not expected to be “surface based” or otherwise capable of producing a tornado.

Stay weather aware this afternoon/evening, as this forecast will change in some way, form, or fashion. Updates will be made frequently @NashSevereWx on Twitter, while major updates to the forecast will appear here at NashSevereWx.com.

Rest of the Week: Briefly Cooler, More Storms Possible Saturday, Then An Unsettled Pattern to End March

Dry weather will persist until Saturday, when another potent system develops over the midwest and moves our direction. Showers and thunderstorms look likely on Saturday, and there is concern that some of these could be strong or severe. While the Storm Prediction Center has not yet highlighted our area for severe weather potential, we will be keeping a close eye on this as the weekend approaches. If there is severe weather Saturday, it will likely be after noon.

In fact, the entire rest of March looks to be off-and-on, rainy and stormy. For example, check out the GFS model running Saturday through Tuesday: