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Rainy weekend: How much rain that might actually mean, and when the pattern will break

We’re in the midst of what National Weather Service meteorologists were referring to for several days leading up to it as a “wet pattern” in East Tennessee, with likely rain chances every day, and that pattern continues through the weekend, with the NWS forecast calling for a 50% chance of thunderstorms on Saturday and an 80% chance of storms on Sunday.

The NWS has issued a hazardous weather outlook for Saturday, saying that a few isolated storms may become strong with gusty winds and torrential downpours that lead to localized flooding.

Basically, that’s the same forecast that has been in place since Wednesday.

The pattern to this point: So far, though, it’s hardly been a “wet” pattern for most of Scott County. Despite the near constant threat of showers and thunderstorms, we’ve been primarily dry. Official data from the NWS has recorded 0.17″ of rain in Oneida for the past week, and a broader perspective from radar estimates show that areas south of Interstate 40 have been the clear winners in the rainfall department over the past several days.

Radar estimates show that less than half an inch of rain has fallen across most of Scott County over the past 72 hours, as of 7 p.m. Friday, July 9, 2021.

That’s not necessarily surprising. As I wrote in our last post on this subject, leading models were showing relatively scant rainfall totals for our region, and the ECMWF in particular (perhaps the world’s leading weather model) was showing only around half an inch of rain through this evening. That’s not to say that there haven’t been torrential downfalls associated with this weather pattern; there certainly have. It’s just that they have occurred well to our south.

That’s how it goes with convective precipitation. It’s impossible to pinpoint in advance where the heaviest rains will set up. A glance at USGS’s streamflow gauges for Scott County shows that Clear Fork did not rise at all over the past few days, while New River rose a few inches. We can take away from that that the greatest rain that fell locally fell over the New River basin (generally areas like Huntsville, Winona and Norma), while less rain fell over the Clear Fork basin (generally areas like Robbins, Elgin and Glenmary). That’s backed up by the radar estimate above.

The pattern going forward: With a pattern like this, the past does not necessarily indicate the future. With at least a 50/50 chance of thunderstorms continuing through the weekend, the fact that we’ve been relatively dry to this point doesn’t mean we’ll continue to be relatively dry. A weak impulse is expected to move into the Cumberland Plateau region late tonight, which will enhance rain chances from late tonight through the day tomorrow.

Still, it’s impossible to pinpoint exactly where rain will fall, or how much. Diurnal heating will play a heavy role in the development of convective activity. If scattered thunderstorms haven’t developed before then, they’ll likely begin to be a threat by around lunchtime Saturday, and then continue throughout the day, with it being literally impossible to say which areas will be struck.

Interestingly, for those headed to Music on Main Saturday evening, while earlier runs of the HRRR short-range model had shown scattered storms clearing the region by the start of the festival, the latest run of that particular model is showing a more organized line of storms that will be weakening as it drops south out of Kentucky and crosses the northern plateau tomorrow evening. The NAM model, on the other hand, is not showing this feature, and continues to keep tomorrow evening dry for the northern plateau region.

The GFS model’s projected rainfall for Saturday, July 10, 2021 is very similar to what we’ve seen the past three days, with the greatest rainfall south of Interstate 40 and a sort of donut hole over the northern Cumberland Plateau region. But it’s nearly impossible to pinpoint convective precipitation in advance.

After Saturday: Even as Saturday ends, there’s the potential that a more organized cluster of thunderstorms will be advancing towards the Cumberland Plateau, which could help fuel widespread thunderstorm activity at some point on Sunday. It’ll be interesting to watch and see how this feature plays out. There’s at least a chance that this MCS, as it’s termed in meteorological circles, will weaken and play itself out before it advances this Far East.

For the foreseeable future, we’ll remain in a pattern very similar to the one we’ve been in for the past several days. Hot, humid weather will be in place, with relatively moist air and instability. This is a classic summertime pattern that brings daily chances for scattered thunderstorms. These thunderstorms are enhanced by the heating of the afternoon, and by the terrain differences of the mountains. Any thunderstorm that develops carries an attendant threat for wind gusts and torrential downpours.

As for more organized drivers of thunderstorms, there’s potential for a weak system to slide through late next week that could enhance rain chances.

The Bottom Line: Scattered thunderstorms will be the rule of thumb every single day for at least the next week. That hasn’t amounted to much rain for Scott County since this pattern set in on Wednesday, with only .17″ recorded in Oneida, and what little rain has fallen being over the New River Basin as opposed to the Clear Fork Basin. But that doesn’t mean things won’t change moving forward. It’s impossible to predict very far in advance whether rain will spoil Music on Main in Oneida Saturday evening, just as it’s impossible to predict in advance where thunderstorms will set up each day after that. But there’s a threat of scattered storms every day, and any storm that develops could produce strong wind gusts and torrential rainfall — very typical summer weather.

Eye to the Sky is a weather blog of the Independent Herald, written primarily by IH publisher Ben Garrett. Views expressed here are those of the authors and should not be considered substitute for official advisories, watches or warnings from the National Weather Service. For the latest, most up-to-date forecast information, see weather.gov/mrx.
Ben Garrett
Ben Garrett is Independent Herald editor. Contact him at bgarrett@ihoneida.com. Follow him on Twitter, @benwgarrett.

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