Strong Storms Possible Monday as an Active Summer Pattern Continues

We are heading into another very August-like stretch of weather across the western North Carolina mountains, foothills, and northwest Piedmont, with plenty of humidity, warm afternoons, and repeated opportunities for thunderstorms.

Monday, however, deserves a little more attention than the typical summertime afternoon.

The Storm Prediction Center has placed portions of the region in a Slight Risk for severe thunderstorms Monday, as the atmosphere becomes favorable for scattered thunderstorms capable of producing damaging wind gusts.

This isn’t a setup where we expect severe weather everywhere. But compared with an ordinary August afternoon, the environment should give thunderstorms a better opportunity to organize, move, and produce stronger wind gusts. There is a 1 in 7 risk of any area within 25 miles receiving damaging winds.

A strong ridge that has been influencing the Southeast will weaken somewhat Monday as a disturbance in the mid-levels of the atmosphere moves across the region.

That disturbance is the main contributing factor because winds several thousand feet above us will become stronger.

Around 5,000 feet, winds are expected to be roughly 15 to 20 knots, while winds near 18,000 feet increase to around 30 knots. That provides more flow through the atmosphere than we sometimes have during a typical midsummer thunderstorm day.

At the surface, a trough and subtle wind shift extending from North Carolina and southern Virginia back toward Tennessee should provide an additional focus for thunderstorm development. Models likely do not have a good handle on this development yet. We are expecting convection to develop earlier in the day, likely by 3:00 pm.

Once thunderstorms form, the stronger flow aloft should encourage them to move generally southeastward, potentially carrying storms out of the mountains and toward portions of the foothills and northwest Piedmont.

That’s an important difference from days when thunderstorms form over the mountains, move very little, and collapse near where they developed.

Monday’s atmosphere will also be loaded with moisture. Precipitable water values are expected to approach 2 inches. Precipitable water doesn’t mean two inches of rain will automatically fall at your house. Instead, think of it as a measurement of how much moisture exists through the depth of the atmosphere. For our region, a value around two inches represents a very moisture-rich environment.

Combine that moisture with strong August heating and we develop an atmosphere capable of producing thunderstorms containing a tremendous amount of water.

And that brings us to Monday’s primary severe-weather concern.

As temperatures climb Monday afternoon, the lowest portion of the atmosphere should become deeply mixed. When thunderstorms develop in that environment, heavy precipitation descending through the storm can help accelerate air toward the ground. That creates the potential for wet microbursts and powerful thunderstorm outflow winds.

Imagine a column of rain-cooled air rushing toward the ground and then spreading outward when it reaches the surface. That’s essentially what we’re concerned about.

Most thunderstorms won’t necessarily produce severe winds, but the environment appears capable of supporting a few wind gusts exceeding 50 knots—or roughly 58 mph. That is severe-thunderstorm criteria. And isolated gusts could certainly be stronger.

Winds of that magnitude are capable of bringing down trees and power lines. Across the mountains and foothills, where trees frequently border roads and power infrastructure, even a relatively localized severe thunderstorm can create significant problems. The combination of extremely heavy rainfall and sudden strong winds can also make driving conditions deteriorate very quickly.

It’s important to put the SPC Slight Risk into perspective. A Slight Risk does not mean the storms themselves will be “slightly severe,” and it doesn’t mean severe weather is expected everywhere.

It means we have a sufficiently favorable environment for organized severe thunderstorms to become a legitimate concern within the outlook area.

For most people, Monday will probably contain plenty of dry weather. Some locations may not receive a thunderstorm at all.

But if you happen to be underneath one of the stronger storms, conditions can become severe very quickly. That’s why we pay considerably more attention when the SPC moves an area into a Slight Risk rather than the more routine General Thunderstorm or Marginal Risk categories.

The main hazard Monday is damaging straight-line wind.

Thunderstorm development should initially be influenced by the terrain and the surface trough during the afternoon. The mountains will again provide a natural focus for development, but Monday’s stronger winds aloft mean we shouldn’t assume storms stay confined to the higher terrain.

Once storms become established, their expected southeastward movement could bring them from the mountains into the foothills and eventually portions of the northwest Piedmont.

That puts areas around Marion, Morganton, Lenoir, Rutherfordton, Hickory and surrounding communities in a position where we’ll need to watch both storms developing locally and storms approaching from farther northwest.

Exactly where the strongest storms travel will depend on where they initially develop and how individual storm outflow boundaries interact.

Those details often don’t become clear until thunderstorms actually begin forming.

With precipitable water approaching two inches, any stronger thunderstorm will also be capable of producing extremely heavy rainfall. Rainfall rates underneath these storms could become impressive for short periods.

The somewhat faster storm motion Monday should help compared with a situation where thunderstorms remain nearly stationary. However, thunderstorms can still repeatedly develop along outflow boundaries, and localized areas could receive multiple rounds.

That leaves us with a risk of isolated nuisance flooding and potentially more significant localized flooding where storms repeatedly affect the same location. Mountain valleys, poor-drainage locations, low-water crossings and small streams are always worth watching closely in this type of moisture-rich environment.

This isn’t currently a widespread flooding setup. It’s much more about what happens underneath individual thunderstorms, very localized.

The atmosphere should begin stabilizing after sunset as we lose daytime heating. Thunderstorms may continue into the evening as they move southeastward, but the overall severe-weather threat should gradually decrease.

Current expectations have most thunderstorm activity diminishing after approximately 11 PM Monday evening. As always, the timing can shift depending on how quickly storms develop during the afternoon and whether any organized clusters manage to persist longer than expected.

Monday isn’t the end of our thunderstorm opportunities.

The moist and unstable air mass should remain across the southern Appalachians and surrounding foothills through at least the middle of the week. Tuesday and Wednesday should bring additional opportunities for afternoon and evening thunderstorms.

These days may look somewhat more like our familiar summertime pattern: relatively quiet mornings, increasing heat and instability through midday, followed by scattered thunderstorms developing during the afternoon.

The mountains frequently provide the initial spark. Once storms develop, their outflow boundaries can travel away from the mountains and initiate new thunderstorms across the foothills and northwest Piedmont.

That’s why summertime thunderstorm forecasting becomes so dependent on what happens earlier in the day. A single storm over one county can produce an outflow boundary that becomes the trigger for another storm 20 or 30 miles away.

Outside of thunderstorms, it will remain warm and humid.

The higher elevations naturally get some relief from the heat because of elevation, while temperatures become progressively warmer as we descend through the foothills and toward the northwest Piedmont.

Humidity will remain high throughout the region.

For communities across the lower foothills and northwest Piedmont, that means another stretch of uncomfortable August afternoons. The humidity will also prevent temperatures from cooling particularly efficiently overnight, especially outside the mountain valleys.

For our coverage area, however, the thunderstorms are the bigger weather story Monday rather than the more significant heat occurring farther south and east.

Our attention then turns toward Thursday, Friday and next weekend. Forecast guidance continues to indicate that a trough will develop over the eastern United States as the subtropical ridge gradually retreats. That could become important for us.

Several smaller disturbances moving through the larger-scale trough could provide additional lift, while a slow-moving cold front eventually approaches the southern Appalachians.

If those pieces come together, thunderstorm coverage could become more widespread later Thursday and Friday and potentially continue into the weekend.

Instead of depending almost entirely on afternoon heating and the mountains to generate thunderstorms, we may begin adding stronger large-scale forcing again to an already warm and moisture-rich atmosphere.

There is still plenty of uncertainty regarding the timing of those individual disturbances and the eventual front, so we’ll refine that portion of the forecast as we get closer. There is also some encouraging news on the other side of that transition.

If the eastern trough develops as expected, temperatures could eventually trend back toward normal and perhaps even somewhat below normal by next weekend.

The Bottom Line

Monday is not just another routine afternoon thunderstorm day.

The combination of abundant moisture, strong afternoon instability, a surface trough and somewhat stronger winds through the atmosphere has prompted the Storm Prediction Center to place portions of the region in a Slight Risk for severe thunderstorms.

Thunderstorms should develop during the afternoon, initially favoring the mountains and nearby areas before moving generally southeastward toward portions of the foothills and northwest Piedmont.

Damaging straight-line winds are our primary concern. The environment could support wet microbursts and a few severe wind gusts exceeding 50 knots, or about 58 mph. Torrential rainfall will also accompany the stronger storms, creating a localized flooding concern.

Storms should gradually diminish Monday evening, with most activity expected to fade by around 11 PM.