Storms Return This Evening Before a Cooler, Drier Pattern Arrives

We have another couple of hot, humid and increasingly unsettled days to get through before a cold front finally brings some meaningful changes to our weather this weekend. Thunderstorms return to the forecast later today and this evening, with at least a few storms potentially becoming strong to severe. Friday then looks increasingly active as the front gets closer, and there are some signs that heavy rainfall could become just as important—if not more important—than the severe weather threat before everything begins shifting southeast this weekend.

One thing that makes today’s forecast a little more complicated is what has been happening well to our west overnight. A mature thunderstorm complex, or MCS, developed over western Kentucky and has been moving east with the stronger flow aloft. That system should continue through the Ohio Valley and largely pass north of our area as it gradually weakens. We aren’t expecting the overnight complex itself to come charging into the mountains and foothills this morning.

Its indirect effects, however, could matter quite a bit later today.

Cloud debris spreading downstream from those storms may limit the amount of sunshine we receive through at least part of the day. That matters because sunshine is one of the ingredients that helps destabilize the atmosphere. Some of the high-resolution guidance has struggled to build much more than 1,000 to 1,200 J/kg of surface-based CAPE for a good portion of the day, which would represent relatively modest instability for late August.

The atmosphere may begin recovering more substantially by mid to late afternoon, however, and that is where today’s forecast becomes more interesting.

A disturbance embedded within the larger-scale flow is expected to move through the Tennessee Valley while the weak frontal boundary to our north gradually gets closer. As that happens, instability could increase considerably late in the day. Some guidance eventually develops roughly 2,000 to 2,500 J/kg of CAPE across portions of the region. At the same time, we’re looking at around 20 to 30 knots of deep-layer wind shear.

Neither of those numbers by itself screams a major severe weather event, but together they are enough to support better thunderstorm organization than we see on a typical August afternoon.

The biggest question is whether storms can actually get going early enough to take advantage of that environment. This may be a later-than-normal round of thunderstorms, potentially developing during the late afternoon and continuing well into the evening. That’s important because we normally expect summertime convection to reach its peak around or shortly after maximum afternoon heating. Today, the forcing needed to initiate more widespread storms may arrive relatively late.

If storms successfully develop while appreciable instability remains in place, some could organize into clusters or short line segments. Damaging straight-line winds would be our primary concern with the strongest storms. With 20 to 30 knots of deep-layer shear available, storms should have enough organization to produce stronger, more concentrated outflow rather than simply collapsing almost directly over where they develop.

That doesn’t mean everyone is going to see severe weather. In fact, the severe threat remains conditional because of the uncertainty surrounding cloud cover, instability and exactly when thunderstorms develop. But where the atmosphere is able to destabilize sufficiently, an isolated damaging wind event is certainly possible through this evening.

There will also be plenty of moisture available. Dewpoints are generally running in the lower and middle 70s, which is a good indication of just how humid the lower atmosphere has become again. Any thunderstorm that develops will be capable of producing torrential rainfall, frequent lightning and locally gusty winds even if it never reaches severe limits.

Unfortunately, the humidity isn’t going anywhere before the front arrives.

The brief improvement we experienced behind Monday night’s front has essentially disappeared. Low-level southwesterly flow has transported warmer and increasingly humid air back into the region, and today will feel every bit like late August.

Across much of the foothills and northwest Piedmont, afternoon heat index values should reach the upper 90s to around 100 degrees. Farther south and east, especially toward the I-77 corridor and Upstate South Carolina, heat indices can reach the lower 100s. The mountains will obviously be somewhat cooler because of elevation, but even there the humidity will make the afternoon feel uncomfortable.

Friday may be just as hot and humid before the front arrives, although Friday’s temperature forecast comes with an interesting complication: thunderstorms.

The approaching cold front should provide a much better focus for thunderstorm development Friday afternoon. If storms become widespread relatively early, clouds and rain could prevent temperatures from reaching their full potential. If convection holds off for several hours, however, temperatures will have more opportunity to climb while dewpoints remain very high. That would allow heat index values to once again approach or exceed 100 degrees across portions of the foothills and Piedmont.

Friday’s thunderstorms themselves may present a somewhat different problem than what we’re watching today.

The atmosphere should once again become fairly unstable, and there will still be enough wind shear for some stronger thunderstorms. But the orientation of the winds aloft may become increasingly parallel to the frontal boundary. When that happens, individual storms can merge into larger clusters while repeatedly developing and moving across some of the same locations.

That’s where our attention begins shifting toward the heavy-rain potential.

Some of the high-resolution guidance is producing pockets of 2 to 3 inches of rainfall over relatively short three- to six-hour periods Friday afternoon and evening. We don’t want to take those exact amounts or locations literally this far out, especially because the models are still trying to determine exactly where the front will settle. But the signal is strong enough that we’re paying attention.

There are even indications in some guidance that a small area of low pressure could develop along the front. If that happens, convergence along the boundary would become stronger and could further concentrate thunderstorms and heavy rainfall.

Right now, the more concerning corridor for heavier rainfall appears to be near and east of I-77, although there is still considerable uncertainty over exactly where the axis ultimately sets up. A relatively small change in the frontal position could shift the heaviest rainfall considerably east or west.

This is one of those situations where the broad pattern is becoming fairly clear while the important smaller-scale details are not.

We know a front is approaching. We know the atmosphere will contain plenty of moisture. We know instability will be available, and we know the wind profile may favor thunderstorms growing into larger clusters and potentially training along the boundary. What we don’t know yet is exactly where that corridor will become established.

That distinction will matter quite a bit Friday.

Some communities could simply receive another round of ordinary summertime thunderstorms, while a narrower corridor could receive repeated heavy downpours over several hours. Rainfall rates beneath the strongest storms could be very high given the amount of moisture available in the atmosphere. If storms repeatedly move across the same locations, localized flooding could become a concern.

Saturday remains somewhat uncertain because it depends heavily on how quickly the front moves.

If the front slows down and lingers across or near our area, another round of showers and thunderstorms would be possible Saturday afternoon. If a surface low develops along the boundary and helps push the front southeast more quickly, much of our area could begin drying out sooner.

Either way, the overall trend during the weekend is toward improvement.

Behind the front, winds will begin bringing in a noticeably different air mass. Temperatures should already be held down somewhat Saturday, and by Sunday the deeper moisture and better thunderstorm chances should finally be displaced southeast of our area.

Sunday currently looks like the beginning of a much quieter stretch of weather.

More importantly, we’ll finally start getting rid of some of these lower and middle 70s dewpoints. That will make a tremendous difference in how the air feels, even where afternoon temperatures remain fairly warm. There is a big difference between upper 80s with oppressive humidity and upper 80s with a more comfortable air mass in place.

That cooler and less humid pattern should carry into at least the beginning of next week.

There is increasing disagreement among the longer-range guidance once we get toward Tuesday and beyond, so confidence decreases concerning exactly how long the dry pattern lasts and when our next meaningful rain chances return. There is better agreement, however, that we don’t immediately return to the kind of excessive heat and humidity we’re dealing with right now.

Most indications keep afternoon temperatures closer to seasonal levels, generally upper 80s to lower 90s outside of the higher elevations through the middle portion of next week.

So we have a fairly active transition period ahead of us before the better weather arrives.

Today starts hot and humid, with the thunderstorm threat increasing later than we would normally expect during a summer afternoon. If enough instability develops before the approaching disturbance arrives, a few storms this evening could organize and produce damaging wind gusts.

Friday may ultimately become the wetter day as the cold front moves into the region. Strong thunderstorms remain possible, but we’ll also be watching closely for repeated storms and localized corridors of heavy rainfall, particularly around the I-77 corridor and points east.

Some showers and thunderstorms could linger into Saturday depending on the speed of the front.

Then we finally turn the page.

By Sunday, the front should be southeast of the area, rain chances drop considerably, humidity begins coming down, and a more seasonable air mass settles across the mountains, foothills and northwest Piedmont. After several more days of August heat, humidity and thunderstorms, that change should be very noticeable heading into the beginning of next week.