Heavy Rain vs. Severe Weather: Why Thursday and Friday Could Be More About Water
A more active weather pattern is taking shape across the North Carolina Piedmont and surrounding region for the second half of the week.
Tuesday provides one more generally dry and relatively comfortable day across much of the region, but humidity begins returning Wednesday as deeper moisture moves back into the Carolinas. Shower and thunderstorm chances will increase Wednesday into Wednesday night before becoming considerably more widespread Thursday and Friday.
This setup could bring thunderstorms from the Charlotte metro and southern Piedmont north through Rowan, Cabarrus and Davie counties and into the Triad and central North Carolina.
There will be some potential for strong thunderstorms, particularly Thursday afternoon and evening. However, the more widespread concern with this particular pattern may ultimately be something different:
Heavy rainfall.
A Regional Change Begins Wednesday
The relatively dry air currently across the Piedmont won't last much longer.
As high pressure shifts away, winds will become more southerly and begin transporting moisture northward across the Carolinas. At the same time, an upper-level trough will deepen to our west while a warm front lifts north through the region.
Wednesday will essentially serve as the transition day.
Humidity will increase, and scattered showers and thunderstorms will begin developing across portions of the western Carolinas and Piedmont during the afternoon. Storm coverage should increase Wednesday night as the warm front continues moving north.
The more significant change arrives Thursday.
Thursday Could Be the Most Active Day
Thursday currently appears to have the ingredients for a much more active day across portions of the Carolinas.
Atmospheric moisture will increase substantially, with precipitable water values potentially approaching 2 inches in portions of the region.
At the same time, there should be at least moderate instability along with enough wind shear to support organized thunderstorms.
That means a few strong to possibly severe thunderstorms could develop Thursday afternoon and evening, particularly where enough sunshine occurs between periods of cloud cover and rain.
Damaging wind gusts would be one of the primary concerns with stronger thunderstorms.
Exactly how widespread any severe-weather threat becomes will depend heavily on how much instability develops. Extensive morning clouds and showers could limit instability, while more sunshine would allow the atmosphere to become more unstable.
But even if instability ends up limiting the severe-weather threat, another concern would remain.
There's A Lot of Moisture Available
One of the most important ingredients in this setup is the amount of moisture expected in the atmosphere.
Precipitable water values could approach 2 inches across parts of the region Thursday.
That does not mean two inches of rain will automatically fall.
Precipitable water measures the total amount of water vapor contained in a column of the atmosphere. Higher values tell us that thunderstorms have considerably more moisture available to convert into rainfall.
In an environment like the one expected later this week, thunderstorms can become very efficient rain producers.
A storm doesn't have to contain damaging winds, large hail or a tornado to produce an impressive amount of rainfall in a short period of time.
Severe Thunderstorm or Flooding Thunderstorm?
This distinction will be important Thursday and Friday.
People understandably pay close attention to Severe Thunderstorm and Tornado Warnings. But thunderstorms that never become severe can still create dangerous conditions.
A slow-moving thunderstorm producing torrential rainfall could overwhelm storm drains, flood low-lying locations and put water across roads without ever meeting the criteria for a Severe Thunderstorm Warning.
That's one reason the heavy-rain threat may ultimately affect more locations than the severe-weather threat with this particular setup.
Some communities may experience nothing more than ordinary showers and thunderstorms. Others could experience torrential rainfall capable of producing localized flooding.
Repeated Rounds Could Increase the Flooding Concern
Another part of this pattern I'm watching is the potential for repeated rounds of thunderstorms.
The atmosphere won't simply become humid for a couple of hours Thursday afternoon and then immediately dry out.
A stalled or slow-moving boundary combined with persistent moisture transport could provide opportunities for multiple rounds of showers and thunderstorms from Thursday into Friday.
That becomes important if storms begin traveling repeatedly across the same locations.
Meteorologists call this training.
Think about railroad cars following one another along the same track. With training thunderstorms, one storm moves through and produces heavy rain. Another follows approximately the same path. Then another may develop behind it.
Each individual thunderstorm might not create a flooding problem.
The cumulative rainfall can.
Where Could the Heaviest Rain Fall?
It's too early to pinpoint exactly where the greatest rainfall totals will occur.
The setup could affect a broad portion of the region, including areas from the Charlotte metro and southern Piedmont through Rowan, Cabarrus and Davie counties, into Davidson County, the Triad and portions of central North Carolina.
However, rainfall will not be evenly distributed.
One community could receive multiple rounds of thunderstorms while another location 10 or 15 miles away receives considerably less rain.
The eventual position of the warm front, approaching cold front and any smaller-scale boundaries left behind by previous thunderstorms will help determine where storms repeatedly develop.
Those details usually become clearer as we get closer and can begin analyzing radar trends.
Friday Is Becoming More Interesting
Thursday isn't the only day worth watching.
Forecast guidance has been trending wetter for Friday, and precipitation probabilities have increased across portions of the region.
A cold front approaching from the northwest will provide another source of lift while abundant moisture remains in place.
That could lead to another round of showers and thunderstorms Friday afternoon and evening.
Friday could become particularly important in locations that receive substantial rainfall Thursday.
If the ground becomes increasingly wet and streams and drainage systems are already responding to Thursday's rainfall, another round of heavy thunderstorms Friday could produce flooding more quickly.
This is why the two-day rainfall picture may ultimately matter more than focusing on either Thursday or Friday individually.
What About Saturday?
The cold front should gradually push eastward before slowing or stalling near or east of the region.
That should allow somewhat drier air to begin filtering into portions of the western Piedmont.
However, shower and thunderstorm chances won't immediately disappear.
Scattered afternoon thunderstorms remain possible Saturday, particularly farther east, before the overall pattern begins trending drier.
Exactly how quickly the front moves will determine how quickly storm chances decrease.
Don't Focus Only on the Severe-Weather Outlook
One message I want to emphasize heading into Thursday and Friday is that the absence of a significant severe-weather risk does not necessarily mean the weather won't cause problems.
Severe-weather outlooks primarily communicate the potential for hazards such as damaging winds, large hail and tornadoes.
Flooding is a different hazard.
You can have a day with a relatively low severe-weather threat and still experience thunderstorms producing torrential rainfall and significant localized flooding.
That's why I'll be watching both sides of this setup.
Can thunderstorms become strong or severe?
Yes, particularly Thursday.
Could heavy rainfall become the more widespread concern?
That's increasingly possible.
Turn Around, Don't Drown
If repeated thunderstorms develop later this week, flooded roads could become one of the most significant hazards.
Never assume you know how deep water is covering a roadway.
Heavy rainfall can wash away pavement underneath floodwater, and flowing water can be considerably stronger than it appears.
If you encounter a flooded roadway, turn around and find another route.
This becomes particularly important at night, when judging the depth and movement of water becomes even more difficult.
What I'm Watching Across the Region
Several factors will determine exactly how Thursday and Friday unfold:
- How quickly moisture increases Wednesday and Wednesday night
- The eventual position of the warm front
- How much sunshine develops Thursday
- How unstable the atmosphere becomes
- Whether thunderstorms organize enough to produce damaging winds
- Where the highest atmospheric moisture sets up
- Whether thunderstorms repeatedly move across the same locations
- How much rain falls Thursday before additional storms develop Friday
- The speed and position of Friday's approaching cold front
- Whether the excessive-rainfall and flooding threat increases in later forecasts
Those details will become increasingly important over the next 24 to 48 hours.
Bottom Line
A more humid and unsettled weather pattern is returning to the Carolinas and North Carolina Piedmont beginning Wednesday, with Thursday and Friday currently looking like the most active days.
A few strong to possibly severe thunderstorms could develop, particularly Thursday afternoon and evening.
But severe weather may not be the entire story.
With abundant atmospheric moisture and the potential for repeated rounds of thunderstorms, torrential rainfall and localized flooding could become the more widespread concern across portions of the region Thursday and Friday.
Not everyone will receive heavy rainfall, and it remains too early to identify exactly where the highest totals will occur.
For now, this is a setup worth watching rather than one to be alarmed about.
I'll continue monitoring the severe-weather potential, rainfall projections and flooding threat across the Charlotte metro, Piedmont, Triad and central North Carolina as we get closer to Thursday and Friday.
Stay with Rowan County Weather and Regional Forecast for updates as the forecast becomes clearer.