Significant Rainfall Returns Late Friday Night, Flash Flooding Possible This Weekend

Lead Forecaster Daniel Crawley

Good Thursday afternoon to everyone, today is a nice day across the Western Carolinas with a mix of sun and clouds but over the next 24 hours things will begin to change quickly as the region is in the crosshairs of another heavy rain event.

Global computer model guidance several days ago indicated that an upper low would close off across Louisiana and would scoop a lot of gulf moisture. That process is starting to happen right now according to water vapor imagery. Now that we are within 48 hours we can begin to put the details together on how this will all evolve across the region.

Due to the fact that our region picked up a significant amount of rainfall this past weekend, the ground is still fairly saturated. With the potential rainfall expected this weekend we could see flash flood issues arise starting on Saturday and possibly lingering into Sunday.

The Weather Prediction Center has placed the Western Carolinas in a Slight Risk on its Day 3 Excessive Rainfall Outlook.

For our area the three biggest factors that go into heavy rain events are…

  1.  Synoptic Lift
  2. Upslope
  3.  Moisture Source

All three of these factors look to be sufficient enough to produce significant rainfall as we go into the weekend.

The first factor, synoptic lift, is occurring as a result of the upper low developing across Louisiana, it will have multiple lobes of lift to move into Western North Carolina, the first lobe being late Friday night and into Saturday and the second being during the day on Sunday

The second factor is upslope component, which is a huge player in rain events along the Blue Ridge Mountains. And that is going to be the case with this event as well.

Looking at the winds at around 5000 ft, today, northeast winds has advected fairly dry air into Western North Carolina. By tomorrow morning with high pressure sliding off the Mid-Atlantic Coastline, winds at 5000 ft will turn easterly and slowly increase during the day.

850 winds (Friday 8 am)

By Friday evening, those winds are now southeast and blowing at about 10-20 knots. This kind of setup will help develop shower activity right up against the mountains by evening. This will keep temperatures down tomorrow in the 70’s.

850mb winds (Friday 8 pm)

The upslope component increases dramatically Friday night and it maxes out at over 40 knots for a period of time Saturday morning. This should help maximize rainfall potential in those favored areas.

850 mb winds (Sat 8 am)

And then by Saturday evening, the winds at 5000 ft begin to veer to South-Southwest. While its still a very moist flow, the upslope effects are not at their peak,

The third factor of course is having a good moisture source and with there being a strong Gulf and Atlantic tap, moisture will definitely be there through the entire weekend.

Precipitable Water Values (Sat 8 am)

Residents along the Blue Ridge Escarpment in Western North Carolina needs to be Weather Aware prepare for significant rainfall and some flash flooding on Saturday and possibly into Sunday. Even those just east of the escarpment should prepare for training of cells that could lay down a heavy amount of rain.

Rainfall amounts through Sunday evening look to be in the 1-3 inch range for everyone, with amounts of 3-5 inches in upslope regions.

Timeline of Events…

Friday: Clouds increasing and cool, few upslope showers developing…

Friday Night: Showers becoming more widespread especially after midnight…

Saturday: Max upslope and lift expected during the morning. Flash Flood issues may arise…

Saturday Night: Heavy Rains continue, off and on. Focus of main might shift from the Blue Ridge to the US 321 Cooridor.

Sunday: SSW winds at the surface and aloft to bring waves of energy, heavy rain showers into the region.

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