Isolated to Widely Scattered Severe Storms Possible This Afternoon

Good Saturday afternoon!

We are watching the weather closely across western North Carolina this afternoon, but I want to start with something that can easily get lost when we start talking about severe weather: thunderstorms are not expected to impact everyone today.

Storm coverage should remain isolated to widely scattered. There will likely be plenty of places that stay dry, and some communities may hardly see much more than clouds in the distance. At the same time, the atmosphere is capable of supporting a few strong to severe thunderstorms, and where those storms do develop, damaging wind will be the primary concern.

This is one of those days where storm coverage and storm intensity are two different things.

We have a cold front approaching the region while a disturbance and ongoing thunderstorm activity move southeast out of Tennessee, Kentucky and southwest Virginia. That activity will approach the mountains first before trying to work southeast toward the foothills and northwest Piedmont.

Ahead of it, we have plenty of heat and humidity available.

Instability has been building with afternoon heating, with around 2,000 to 3,000 J/kg of CAPE possible in parts of the region. You don’t really need to remember that number. What matters is that there is enough energy sitting over the region for a thunderstorm that develops to become strong fairly quickly.

What has my attention a little more today is the potential for strong downdrafts.

DCAPE values are around or above 1,000 J/kg in portions of the area. That’s essentially telling us the atmosphere has a pretty good setup for rain-cooled air inside a thunderstorm to accelerate downward toward the ground.

Think about what happens inside a strong summertime thunderstorm. A tremendous amount of rain and sometimes hail is suspended inside the storm. As some of that precipitation falls into relatively drier air underneath the storm, evaporation cools the surrounding air. That colder air becomes heavier than the warm air around it and starts sinking.

If that descending air accelerates enough, it reaches the ground as a downburst and then spreads outward in every direction.

That’s where we can get our damaging wind today.

A storm may only affect part of a region, but underneath one of those stronger downbursts we could see winds capable of knocking down trees and power lines. And because the trees are still fully leafed out, they have a lot more surface area for the wind to push against than they would during the winter.

There is also enough wind through the middle levels of the atmosphere to keep some of these storms moving southeast rather than simply sitting in one location. If several storms develop close together, their outflow can interact and occasionally help organize a small cluster of stronger storms.

That doesn’t mean we’re expecting a solid line of severe thunderstorms to sweep across western North Carolina.

We’re not.

This looks much more like an isolated to widely scattered storm day where most people don’t experience severe weather, but a handful of communities could get hit pretty hard.

The mountains will play an important role in how things evolve.

The first storms will approach the northern mountains from Tennessee and southwest Virginia. Sometimes thunderstorms coming from that direction struggle as they cross the higher terrain. The mountains can disrupt the circulation and cold pool supporting the storms, and activity can weaken considerably before reaching the foothills.

But we can’t count on that happening today.

The air east of the Blue Ridge has had time to heat and destabilize. If storms remain organized crossing the mountains, they could continue directly into the eastern escarpment and foothills.

Even if the storms themselves weaken over the mountains, we still have to watch the outflow coming out ahead of them.

That cooler air spreading away from thunderstorms creates an outflow boundary, and those boundaries can travel quite a distance after the original storms weaken. When that boundary runs into the hot, humid and unstable air east of the mountains, it can provide enough lift to fire new thunderstorms.

That’s one reason this afternoon may look fairly quiet for a while and then change rather quickly in a particular area.

For our Foothills Weather Network coverage area, I will initially be watching McDowell, Burke, Caldwell, Catawba and Alexander Counties closely as activity comes southeast from the mountains. Farther south, Rutherford, Cleveland, Lincoln and Polk Counties will also need to be watched for additional development or any outflow boundaries moving through later in the afternoon and evening.

Again, this does not mean all of those counties are going to see storms.

That’s probably the most important point I can make today.

You could be sitting in Burke County with sunshine and nothing happening while a strong thunderstorm is knocking trees down somewhere in Caldwell County. Or one side of a county could get hammered while the other side barely gets a drop of rain.

That’s the nature of isolated and scattered summertime convection.

So if you look at radar this afternoon and there aren’t storms everywhere, don’t assume the forecast busted or that the severe threat has disappeared. We are looking at the environment available to the storms that actually manage to develop.

And today that environment is capable of supporting damaging winds.

Another thing we’ll be watching is what the first round of thunderstorms does to the atmosphere.

Thunderstorms consume instability. If a decent cluster comes through early enough and produces widespread clouds and rain-cooled air, it can stabilize the atmosphere and make it much harder for additional severe storms to develop later.

But the opposite can happen too.

If the first storms miss an area and sunshine continues for another couple of hours, instability can continue building. Then an outflow boundary comes through later in the afternoon and suddenly provides the trigger needed to get a storm going.

Those smaller details are difficult to forecast down to individual communities several hours ahead of time, which is why radar and surface observations become so important once we get into the afternoon.

The overall tornado threat appears low. Today’s concern is much more focused on straight-line wind from stronger thunderstorms and localized downbursts.

Heavy rainfall will also occur underneath the stronger storms simply because there is plenty of moisture available. But storms should generally keep moving, which should help limit the overall flooding concern compared with some of the slow-moving thunderstorm setups we’ve dealt with this summer.

As we get deeper into the evening, thunderstorm coverage and intensity should gradually decrease. The cold front will continue working through the region and eventually bring us a much different air mass.

Behind the front, cooler and noticeably less humid air will begin filtering into western North Carolina. That should set us up for a much more comfortable stretch of weather as we move into the new week.

But we have to get through this afternoon and evening first.

For most of us, today may end up being fairly uneventful.

For a few communities, it may not.

That’s really the message.

Thunderstorms will be isolated to widely scattered, but any storm that becomes well established will have enough instability and downdraft energy available to produce strong to potentially damaging winds. We will be watching the storms approaching the mountains, how well they survive the terrain, where outflow boundaries develop and how much instability remains across the foothills and northwest Piedmont through the afternoon.

If you have outdoor plans, there’s no reason to cancel the entire afternoon just because severe thunderstorms are possible. Keep an eye on the sky and radar, and make sure you have a reliable way to receive warnings.

And if a Severe Thunderstorm Warning is issued for your location, take it seriously. A thunderstorm doesn’t have to produce a tornado to cause significant damage. A localized 60 to 70 mph downburst can bring down trees, knock out power and make being outside dangerous very quickly.

We’ll continue watching everything closely through the afternoon and evening and will pass along updates as conditions evolve.