Why Friday’s Heavy Rain Threat Is Getting My Attention Across the Region
Friday is becoming the weather day that has my attention this week across a large part of western and central North Carolina.
While showers and thunderstorms will begin returning before then, forecast confidence is increasing that Friday will bring some of the greatest coverage of showers and thunderstorms across the region.
But it's not necessarily the severe thunderstorm potential that stands out to me.
It's the amount of moisture available to these storms and the potential for locally heavy rainfall and localized flooding.
Friday Is Trending Wetter
One of the more noticeable changes in the forecast over the last couple of updates has been the increase in rain and thunderstorm chances for Friday.
A cold front will approach the Carolinas from the west while an upper-level disturbance moves across the region. Together, these features should provide considerably more support for widespread showers and thunderstorms than we'll have earlier in the week.
Across much of the western Piedmont, including Rowan, Davie and Cabarrus counties, rain chances have increased substantially for Friday.
Farther east into the central Piedmont, including Davidson County and surrounding areas, confidence is also increasing that Thursday afternoon through Saturday will represent the most unsettled portion of the week.
The exact timing and placement of the heaviest thunderstorms will become clearer as we get closer, but Friday is increasingly looking like the most active weather day of the week for a large part of the region.
There's Going to Be a Lot of Moisture Available
This is one of the biggest reasons I'm paying attention.
Atmospheric moisture will increase substantially across the Carolinas over the next couple of days.
One measurement meteorologists use to evaluate the amount of moisture available in the atmosphere is called precipitable water, often shortened to PWAT.
Think of PWAT as an indication of how much water vapor is available through the atmosphere for storms to work with.
Across portions of western North Carolina, values are expected to climb into roughly the 1.5 to 2-inch range.
Across central North Carolina, atmospheric moisture could become even greater in some locations, with values potentially exceeding 2 inches.
That doesn't mean everyone receives 1.5 or 2 inches of rain.
Instead, it tells us thunderstorms will have access to a very moisture-rich atmosphere.
When thunderstorms develop in that kind of environment, they can become extremely efficient rain producers.
Some Storms Could Produce Torrential Downpours
Anyone who has experienced a slow-moving summer thunderstorm knows how quickly rainfall can add up.
A storm doesn't necessarily need to last for hours to create problems.
With the amount of moisture expected in the atmosphere Thursday and Friday, some thunderstorms could produce very high rainfall rates over relatively short periods of time.
Visibility could quickly become poor beneath the heaviest storms, water could begin collecting on roads and localized flooding could develop in poor-drainage locations.
The biggest question is exactly where those heavier thunderstorms develop.
Not everyone will receive the same amount of rain.
One community could receive relatively little while another location just a few miles away experiences a torrential downpour.
Slow-Moving Storms Could Make a Difference
Storm movement will also be important.
When thunderstorms move quickly, even a very heavy downpour may only affect a particular location briefly.
Slow the storm down, however, and considerably more rain can fall over the same area.
Relatively slow steering flow could allow some thunderstorms to linger long enough to produce significant localized rainfall totals.
And there's another scenario I'll be watching: training thunderstorms.
That's when multiple storms repeatedly move across the same general location, almost like train cars traveling along the same track.
One thunderstorm might not create flooding.
But if another storm follows it — and then another follows that one — rainfall totals can increase very quickly.
Central North Carolina could be particularly susceptible to this type of setup if storms begin repeatedly developing and moving across the same areas.
You Don't Need Severe Weather for Flooding
This is one of the most important points about Friday's forecast.
A thunderstorm does not have to become severe to become dangerous.
When we talk about severe thunderstorms, we're generally focused on hazards such as damaging winds and large hail.
Heavy rainfall is different.
A storm could remain below severe thunderstorm criteria while producing an exceptional amount of rain.
That's why Friday could become a day when some thunderstorms don't look particularly intimidating from a traditional severe-weather standpoint but still create significant problems from water.
A slow-moving thunderstorm producing torrential rainfall over Salisbury, Mocksville, Lexington, Concord, Charlotte, Winston-Salem, Greensboro or another community across the region could create localized flooding without ever producing damaging winds or large hail.
That's why I'll be paying just as much attention to rainfall rates and storm movement as I am to storm intensity.
Is Flash Flooding Expected?
At this point, I would not characterize widespread flash flooding as expected.
The better way to describe the concern right now is a localized heavy-rain and flooding threat.
Parts of the region are already being highlighted for the potential for excessive rainfall, and that outlook could evolve as confidence increases in exactly where the heaviest rain will occur.
That distinction matters.
It does not mean everyone across western and central North Carolina is going to experience flooding Friday.
It means the atmospheric ingredients may be present for some locations to receive enough rain quickly enough to cause problems.
Urban areas, poor-drainage locations, low-lying roads, small creeks and locations experiencing repeated thunderstorms will be particularly important to watch.
Thursday's Rain Could Matter Too
Friday isn't happening in isolation.
Showers and thunderstorms will already be possible Thursday across much of the region.
If a particular location receives heavy rainfall Thursday and then gets hit by additional thunderstorms Friday, the ground may have less ability to absorb the additional water.
That could make those locations more susceptible to runoff and localized flooding.
On the other hand, locations that receive little rain Thursday may be able to handle Friday's rainfall somewhat better.
That's another reason the exact flooding threat won't become clear simply by looking at Friday's forecast alone.
I'll also be watching where Thursday's heaviest rainfall occurs.
What Could Change Between Now and Friday?
There are still important details that won't become clear until we get closer.
Exactly where thunderstorms develop will matter tremendously.
If storms remain more scattered and move steadily, the flooding threat could remain limited.
If thunderstorms become widespread, move slowly or repeatedly track across the same locations, the threat would increase.
Higher-resolution weather models will become increasingly useful as Friday approaches and should provide a better idea of where thunderstorms may concentrate.
I'll also be watching whether excessive-rainfall outlooks are adjusted as newer forecast data arrives.
What About Severe Weather?
The overall severe-weather threat currently appears lower than the heavy-rain threat across much of the region, but I wouldn't call it zero.
The atmosphere will become increasingly moist while wind shear appears relatively weak across portions of the Carolinas.
That isn't particularly supportive of a widespread organized severe-weather event.
However, summertime thunderstorms can still produce isolated strong or damaging wind gusts, especially where storms become locally intense.
That part of the forecast will continue to be monitored.
Right now, though, heavy rainfall is what has my attention most.
Friday Is a Day to Watch — Not Panic About
There is no reason to panic about Friday's forecast.
But there is enough coming together that residents across the region should pay attention to forecast updates.
The combination of:
Increasing atmospheric moisture
An approaching cold front
Upper-level disturbances
More widespread thunderstorm development
Potentially slow storm movement
The possibility of repeated thunderstorms
creates an environment where some storms could become very efficient rain producers.
The biggest question isn't whether everyone will receive heavy rainfall.
They won't.
The question is where the heaviest storms repeatedly develop and move.
Those locations could experience significantly more rainfall than surrounding communities.
What I'm Watching Across the Region
Between now and Friday, I'll be watching:
- How much atmospheric moisture ultimately moves into western and central North Carolina
- Whether Friday's thunderstorm coverage continues trending upward
- Where Thursday's heaviest rainfall occurs
- The expected speed and direction of Friday's storms
- Whether thunderstorms repeatedly track across the same areas
- Any changes to excessive-rainfall outlooks
- Whether the severe-weather potential increases
- Which parts of the region appear most likely to experience repeated heavy downpours
Friday doesn't currently look like a classic widespread severe-weather setup.
Instead, it could become one of those days when water is the bigger weather story across parts of North Carolina.
I'll continue monitoring the latest data across the region and provide updates as Friday's forecast comes into better focus.
— Rowan County Weather | Regional Forecasts