After three beautiful days to wrap up last week, we’ve settled right back into a much more active weather pattern across the western Carolinas and northeast Georgia. Sunshine and comfortable conditions have given way to an atmosphere loaded with tropical moisture, and that means we’re once again watching for daily rounds of showers and thunderstorms through much of the upcoming week.
Unfortunately, we already saw what this pattern is capable of.
Late Saturday night, significant flash flooding developed in portions of Surry County, where repeated thunderstorms dumped several inches of rain in a short period of time. Roads were flooded, water quickly overwhelmed drainage systems, and some areas experienced serious flash flooding with roads washed out and water rescues necessary as storms repeatedly tracked over the same locations.
While that amount of rain certainly made headlines, it’s important to keep one thing in perspective.
Events like this do very little to erase an ongoing drought.
When several inches of rain fall in just an hour or two, much of that water simply runs off into creeks and rivers before the ground has time to absorb it. Instead of slowly soaking into the soil and replenishing groundwater, it becomes runoff that creates flooding problems downstream. Even though parts of Surry County picked up tremendous rainfall, that localized event will play only a very small role in improving the drought conditions that have gradually developed across our region since late January.
Over the next several days, we’ll continue walking a fine line between beneficial rainfall and localized flooding. Most communities will receive welcome rain that helps gardens, farms, and streams, but a few unlucky locations could once again see thunderstorms repeatedly move over the same area, producing flooding despite many neighboring towns seeing far less rain.
The overall weather pattern has become almost “stuck.”
A stalled frontal boundary remains draped across the southern Appalachians while an upper-level trough sits overhead. Neither feature has much incentive to move, so each afternoon the atmosphere can recharge with heat and humidity before thunderstorms begin developing once again. In fact, Tuesday could end up being the wettest day of the week as a mid-level low-pressure system tries to close off somewhere just to our west. That means southeasterly flow in the Blue Ridge.
Think of the atmosphere like a sponge that’s already soaked with water. It doesn’t take much additional lift to squeeze that sponge and produce heavy rainfall. Every afternoon, daytime heating provides that lift, while small disturbances rotating around the upper-level trough help thunderstorms become more numerous.
The atmosphere is also carrying an incredible amount of moisture. Precipitable water values—essentially a measure of how much moisture is available for rain—are running between 2.0 and 2.25 inches, which is very high for early August. That means any thunderstorm that develops will be capable of producing torrential rainfall in a very short amount of time.
We’re expecting at least one widespread round of showers and thunderstorms to develop this afternoon (Sunday), with another round possible this evening and into tonight.
Not every storm will produce flooding. In fact, many communities may simply receive a brief downpour before the storm moves on.
The concern is where storms begin moving over the same locations repeatedly.
We have talked about this recently. Thunderstorm training is one of the biggest contributors to flash flooding. Imagine train cars traveling down the same railroad track. Instead of one storm passing through and ending the rain, another develops behind it and follows nearly the exact same path. Before long, rainfall totals begin climbing rapidly.
Current high-resolution forecast models continue to indicate that narrow corridors could receive 2 to 4 inches of rain in just a few hours this evening. The challenge is that those heavier rainfall bands are impossible to pinpoint this far in advance. One neighborhood may receive only a half-inch of rain, while another community just five miles away could experience several inches and flash flooding.
Areas along the Blue Ridge Escarpment remain especially vulnerable because they’ve already received repeated rounds of heavy rainfall over the past several days. If another corridor of training thunderstorms develops there, additional flash flood warnings could become necessary. Additionally, terrain and riverbeds that have changed since Helene, nearly two years ago, could allow for water to rise quicker than most are used to. On the other hand, some areas may take longer for the water to rise as the stream bed has spread out more than pre-Helene.
Flooding remains the primary concern, but we can’t completely ignore the possibility of an isolated severe thunderstorm.
Today’s atmosphere has become moderately unstable this afternoon, and while wind shear isn’t particularly strong, there is enough organization for a few storms to produce damaging wind gusts.
One factor we’re watching closely is a layer of somewhat drier air moving into the middle levels of the atmosphere later today. While that may sound like it would reduce rainfall, it can actually enhance downburst potential. As rain falls through that drier layer, some of it evaporates, cooling the air and causing it to accelerate toward the ground. If that stronger downward rush of air reaches the surface, localized damaging wind gusts can occur.
This isn’t expected to be a widespread severe weather event, but a few stronger storms could produce downed tree limbs, isolated power outages, or pockets of wind damage.
Looking beyond today, not much changes.
The stalled front will remain nearby while the upper-level low becomes nearly stationary over the Tennessee Valley and western Carolinas. That means each day will feature another opportunity for scattered to numerous showers and thunderstorms.
Tuesday and Wednesday currently appear to offer the greatest potential for additional heavy rainfall, as the deepest moisture remains in place and storms may continue beyond the normal afternoon heating cycle due to that aformentioned mid and upper-level low-pressure system. That prolonged activity increases the chance that storms could repeatedly move across the same areas, leading to additional localized flash flooding.
By late week, the upper-level system should finally begin weakening and drifting away. While afternoon thunderstorms won’t disappear—they rarely do in August—we should gradually return to a more typical summertime pattern with somewhat lower humidity aloft, less widespread heavy rainfall, and a reduced flood threat.
Today’s Summary…
We’ve transitioned from last week’s stretch of pleasant weather back into a prolonged period of unsettled conditions.
Most of us will welcome the rainfall, especially after months of gradually worsening drought conditions. However, this will not be the kind of slow, soaking rain that significantly improves long-term drought. Instead, much of the rain will fall in intense bursts, creating localized flooding while offering only limited benefit to groundwater and deeper soil moisture.
As we move through the next several days, expect scattered showers and thunderstorms each afternoon and evening. Most communities will simply receive beneficial rainfall, but a few locations could once again experience significant flash flooding if thunderstorms repeatedly move over the same area.
We’ll continue monitoring the latest trends and provide updates if it appears any particular part of our region becomes more susceptible to flooding or stronger storms.
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