Why Friday’s Heavy Rain Could Become a Bigger Story Than the Severe Weather

Much of the attention today is understandably focused on the possibility of strong to severe thunderstorms. But as I look ahead to Friday, another threat may ultimately become the bigger weather story across portions of North Carolina: heavy rainfall.

A cold front moving into the region will provide a focus for numerous showers and thunderstorms Friday into Friday night. The atmosphere will also contain an unusually large amount of moisture, while storm movement could become slow enough for thunderstorms to repeatedly affect the same areas.

Put those ingredients together, and the concern shifts from simply asking whether a thunderstorm becomes severe to asking how much rain falls and how quickly it falls.

Why Friday Is Different

Friday's setup won't necessarily resemble a typical summer afternoon when thunderstorms develop, produce a quick downpour and then move along.

The approaching cold front will begin settling south across North Carolina. As it slows, thunderstorms could repeatedly develop along and near the boundary.

Atmospheric moisture will also be very high. Forecast precipitable-water values across portions of the region could reach approximately 2 to 2.3 inches, giving thunderstorms plenty of moisture to work with.

Meanwhile, weaker steering winds near the front could cause some storms to move slowly.

That combination creates an environment favorable for backbuilding, merging and training thunderstorms. The Weather Prediction Center has placed portions of Virginia and the Carolinas under a Slight Risk for excessive rainfall Friday into Friday night, with guidance depicting locally substantial rainfall where storms persist.

What Does "Training" Mean?

Training thunderstorms occur when multiple storms repeatedly move across approximately the same location.

Think about train cars traveling down the same section of track. One storm passes, another follows it, and then another may develop behind that one.

A single thunderstorm producing an inch of rain may not cause significant problems in many locations.

But if several thunderstorms each dump heavy rain over the same neighborhood during a period of only a few hours, rainfall totals can increase very quickly.

That's why where the storms set up Friday could matter as much as how strong they become.

Several Inches Could Fall in Some Areas

The greatest heavy-rain concern currently appears to be across portions of central and eastern North Carolina.

Current guidance supports the possibility of 2 to 4 inches or more from portions of the Triangle south and east toward the Interstate 95 corridor. Some guidance farther across the Carolinas has depicted localized totals reaching the 3-to-5-inch range where storms repeatedly develop.

Those numbers should not be interpreted as everyone receiving several inches of rain.

Thunderstorm rainfall is rarely distributed evenly.

One community could receive less than an inch while another location relatively nearby receives several inches because thunderstorms repeatedly pass over it.

That's one reason rainfall forecasts involving stalled fronts and thunderstorms can change considerably even within 24 hours of an event.

What About Rowan County?

For Rowan County and the western Piedmont, the exact placement of Friday's frontal boundary will be extremely important.

The highest flash-flooding probabilities currently appear to be farther east, but that doesn't eliminate the potential for locally heavy rainfall around Rowan County.

Guidance for the western Piedmont has also indicated the potential for repeated thunderstorms Friday into Friday night. If storms train over the same portion of Rowan County, localized rainfall totals could climb considerably above the general forecast.

Areas near and east of the Interstate 77 corridor will be particularly important to watch as the forecast evolves.

A relatively small shift in the front could move the axis of heaviest rainfall farther west or east.

Davidson County and the Triad

Davidson County and the Triad currently appear to be near the western edge of the greater heavy-rain setup.

Even there, an inch or more of rain could occur during the event.

If the front slows farther west than currently anticipated, heavier thunderstorms could also spend more time across the Triad.

If it progresses southeast more quickly, the heaviest rain would likely shift toward the Triangle and eastern North Carolina.

That frontal position is one of the most important pieces of Friday's forecast that I'll be watching.

Why Flash Flooding Is Possible Even After Dry Weather

One factor working in our favor is that portions of the region have experienced relatively dry conditions recently.

Drier soils can initially absorb more rainfall than already saturated ground, which could reduce the overall flooding threat.

But there's a limit.

Thunderstorms can produce rainfall faster than the ground, storm drains and drainage systems can handle it. The Raleigh forecast discussion notes that relatively dry antecedent conditions are somewhat mitigating the threat, but very heavy rainfall over urban areas could still result in flash flooding.

Urban areas are particularly vulnerable because pavement, parking lots and buildings prevent water from soaking into the ground.

If several inches fall within only a few hours, flooding can develop regardless of how dry it was beforehand.

Severe Storms Aren't Off the Table

Heavy rain may become Friday's biggest concern, but I'm not dismissing the severe-weather potential.

The atmosphere south of the front should remain quite unstable.

Thunderstorms developing within that warm and humid air could produce wet microbursts, where a tremendous amount of precipitation and descending air accelerates toward the ground before spreading outward as strong winds.

That means isolated damaging wind gusts remain possible Friday afternoon and evening.

The difference is that the environment could increasingly favor clusters of storms, slow movement and repeated rainfall rather than a widespread organized severe-weather event.

So Friday could become one of those days when the storms don't necessarily have to be severe to cause significant problems.

Friday Night Could Be Important

Another reason I'm watching this closely is that thunderstorms won't necessarily end around sunset.

Rain and storms could continue well into Friday evening and Friday night as the front moves through the region.

Nighttime flooding creates additional concerns because flooded roads and rapidly rising water are much more difficult to see.

Never attempt to drive through a flooded roadway. It is impossible to determine how deep the water is or whether the road underneath has been damaged or washed away.

A Small Shift Could Make a Big Difference

There is still uncertainty about exactly where Friday's heaviest rainfall corridor develops.

In this type of setup, moving the frontal boundary 30 to 50 miles can dramatically change rainfall totals for an individual county.

That means today's rainfall maps should not be viewed as precise predictions of where the heaviest rain will fall.

Instead, I'll be watching trends:

Where does the front stall?

Where do thunderstorms initially develop?

How quickly are storms moving?

Do new storms repeatedly form upstream?

Does one corridor begin receiving multiple rounds of heavy rainfall?

Those answers will become increasingly important as we move through Friday.

The Bottom Line

Thursday's severe-weather potential deserves attention, but Friday's heavy-rain setup may ultimately become the more significant story for portions of North Carolina.

Repeated thunderstorms could produce several inches of rain in localized areas, with the greatest concern currently centered farther east from portions of the eastern Piedmont and Triangle toward the Interstate 95 corridor. The Triad and western Piedmont aren't completely out of the picture, especially if the frontal boundary sets up farther west.

For Rowan County, I'm not expecting everyone to receive several inches of rain. But the environment will be capable of producing locally significant rainfall wherever thunderstorms repeatedly track.

That's why I'll be paying particularly close attention to the placement of the front and thunderstorm development over the next 24 hours.

And once the front finally moves through, there's a reward waiting on the other side: cooler and more comfortable weather for the weekend and beginning of next week.

I'll continue tracking Friday's setup and provide updates through Rowan County Weather as the higher-resolution guidance gives us a clearer picture of where the heaviest rainfall is most likely to develop.