The Heat’s Days are Numbered. Fall Arrives This Week and Mother Nature Aligns With Timing

We have a few more days of late-summer weather ahead before a noticeable change arrives in the mountains, foothills, and northwest Piedmont later this week. The Fall Equinox arrives Tuesday, and so will a fall weather pattern.

This morning, the weather still looks very much like summer. A broad upper-level ridge sits to our west, and a weak upper low sits near Florida and the eastern Gulf. These features are bringing deep moisture into the Southeast. Meanwhile, the strong ridge that caused our recent heat is slowly weakening. As it does, we can expect more afternoon thunderstorms now and a bigger pattern change by midweek.

Today, we expect another round of scattered afternoon showers and thunderstorms. Not everyone will see rain, and many places may stay dry, but the mountains are likely to see storms first before they drift south and southeast toward the foothills and Piedmont later in the day. This is a typical setup for us when moisture is plentiful, and winds are light.

The severe weather threat today: The risk of severe weather today is not very high, but a strong storm is still possible. Instability could reach about 1,500 to 2,000 J/kg, and dry air near the ground could lead to an isolated downburst. While these conditions extend south toward the coverage area, the main severe threat stays north of most of our area today. Any stronger storm that forms could bring gusty winds, frequent lightning, and heavy rain for a short time.

On Monday afternoon and evening, we expect more widespread storms as the atmosphere becomes more unstable and the ridge weakens further. More surface-based instability and enough downdraft energy will support damaging wind gusts. The SPC has placed part of the North Carolina mountains under a Slight Risk for severe thunderstorms, and the national WPC also highlights a continued risk for isolated flash flooding for parts of the central and southern Appalachians and Southeast.

Not every thunderstorm on Monday will be severe, and not everyone will see storms. However, the atmosphere will be more likely to produce strong downbursts than today. Plenty of low-level moisture, moderate to strong instability, and enough dry air higher up will help push rain-cooled air down quickly. When this happens, a thunderstorm can cause a sudden burst of wind as its core collapses. Damaging wind gusts will be the main severe weather concern with the strongest storms.

Heavy rain is another important part of this forecast, just like the wind threat.

There’s much more moisture in the air than usual for this time of year, and storms are expected to move slowly. Warm-cloud depths will increase, and storms will stay fairly slow. The WPC has expanded a Marginal Risk for excessive rainfall in our area early this week. This doesn’t mean we expect widespread flash flooding, but a slow-moving storm could drop a lot of rain quickly in a small area.

This is especially important in the mountains and along the eastern escarpment, where the terrain can increase rainfall. If storms keep forming over the same ridge, drainage, or community, local flooding of roads, low-lying areas, and small streams could happen quickly. Most of us probably will not see flooding, but in this pattern, rainfall amounts can vary a lot over just a few miles.

We also need to highlight the continued heat. The heat is especially marked west of the mountains across the Tennessee Valley, where heat index values near or above 100 degrees remain possible through Monday. For our foothill and northwest Piedmont counties, the bigger story is simply that it will remain warm and muggy before temperatures begin to ease Tuesday as cloud and rain coverage increase.

Tuesday will be the day when the weather starts to change.

A backdoor cold front will start moving south through the Carolinas as low pressure forms off the Mid-Atlantic coast. At the same time, high pressure will build over Ontario and Quebec. Together, these changes will bring cooler northeast winds down the eastern side of the Appalachians. We’ll still see showers and thunderstorms on Tuesday, and heavy rain is still possible, but the atmosphere will already be changing.

By Wednesday, the weather could feel completely different.

Northeast winds should set up a cold-air damming wedge east of the mountains. Locally, we call this ‘the wedge,’ and when it forms, it can quickly lower temperatures across the foothills and Piedmont. Cool, dense air moves south along the eastern side of the Blue Ridge, and clouds and light rain help keep near-ground air even cooler.

Currently, we expect temperatures in the second half of the week to be about 8 to 10 degrees below normal. That’s a big shift from what we’ve had lately. Instead of hot, humid afternoons, many places could see clouds, northeast winds, and temperatures that feel more like early fall on Wednesday and Thursday.

There’s still some uncertainty about how strong and long-lasting the wedge will be.

Later in the week, the larger weather pattern will become more pronounced as an upper low strengthens over the eastern U.S. The WPC notes that a more blocked pattern is developing, with a deep trough in the east and surface low pressure development near the Mid-Atlantic coast. This should help keep our area cooler through at least the end of the week.

To summarize, two main weather stories are unfolding over the next few days.

Today will bring scattered thunderstorms with lots of dry breaks in between. Some storms could be strong, but the main severe weather risk looks set for Monday afternoon and evening, especially in the mountains and nearby foothills. Damaging wind gusts will be the main threat, and slow-moving storms with deep moisture could also cause localized flash flooding under the heaviest downpours.

Then the pattern changes.

By Tuesday, the front will start moving through, and by Wednesday we should feel cooler northeast winds and see high pressure building to our north. Temperatures will drop noticeably compared to recent days, clouds will become more common, and the middle to end of the week should finally start to look and feel more like September in the foothills.

We’ll be watching Monday closely, as it still looks like the day with the highest chance for strong thunderstorms. As with most late-summer patterns, most places won’t see severe weather, but just one strong downburst can cause problems in your area. Watch the sky Monday afternoon, make sure you can get warnings, and be ready for brief but possibly intense rain wherever storms develop.