Flash Flooding in North Carolina, August 16, 2021

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
349 AM EDT Mon Aug 16 2021

.SYNOPSIS...
A surface front will remain nearly stationary across the area for
the next few days. The remnants of Fred is expected to generate
heavy rainfall from the Gulf coast to the Tennessee River Valley
Monday night into mid week. Expect cooler than normal temperatures
into mid week. Abundant tropical moisture will continue across the
region through much of the week leading to areas of heavy rainfall
and isolated strong thunderstorms.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 333 am EDT Monday: Still dealing with persistent showers
with heavy rain and embedded thunderstorms caught in the southerly
upslope flow into the Balsam Mtns at this early hour. Will bump the
precip probs way up in those areas for the rest of the pre-dawn
hours. Elsewhere...probably not much action...but a shower could
form at any time.

As for today...eventually a lull of sorts will set in during the
morning hours over the mtns, unless another weak mid-level impulse
forces some better upslope flow to result in more showers. Similar
to yesterday, the extensive cloud cover may delay destabilization
until the early afternoon, at which time convection will
develop quickly and will probably end up numerous in the mid/late
afternoon. Model guidance shows some vorticity, emanating from the
circulation associated with Fred, moving northward across the region
late in the day, which should help do the trick. Won't rule out a
few strong thunderstorms, but the updraft strength looks poor on
the HREF and profiles are wet with poor lapse rates, so by far the
bigger concern is localized flash flooding. Will not be surprised
if a few locations need a Flash Flood Warning, but confidence in
where the bigger threat might be located precludes issuing a Watch
at this time due to the convective nature of the threat. Temps
will stay on the cool side of normal. As we work into tonight,
the threat for heavy rain will steadily ramp upward mainly over
the upslope areas, particularly southwest NC and the headwaters
of the French Broad River. All ingredients should be in place,
with PW above 2 inches and deep warm cloud depth, we just might
have to wait for something to trigger and focus. Precip chances
increase as a result, mainly after midnight over the southwestern
zones. Temps will stay mild.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 325 AM EDT Monday...The remnants of TC Fred will be tracking
north, generally along the AL/GA border to begin the period. This
system will interact with an existing wedge bndry and push it north
as a warm front thru Wed. Confidence in high rainfall across
the srn escarpment and NE GA has increased, yet the 00z GFS remains
a low QPF outlier as it has the past several runs. The GFS is
indicating convection along the SC coast, which would rob moisture
transport toward the FA, while the other op models generally agree
with high precip rates developing aided by mech lift over the wrn
zones beginning by mid day Tue. This idea is more accepted by the
WPC as well and a Day 2 Moderate risk of excessive rainfall has been
issued for the srn BR escarpment, nw/rn Upstate SC, and NE GA north
of I-85.

Currently thinking about 5-7 inches of rainfall over these regions
with up to 10 inches possible in the favored Lake Toxaway area. This
amt of rain would certainly create flooding issues with small
streams and main-stem rivers like the French Broad by Tue
afternoon/evening. Additionally, with highly moist antecedent conds,
there exists a landslide threat across the srn BR escarpment ridges.
With no sigfnt changes to the QPF forecast anticipated, a Flash
Flood Watch will likely be issued by the day shift today as the
overall threat moves into the 2nd period. This system wont bring
real strong winds, however, low to moderate gusts will develop and
with saturated soils, weak/dead trees could be blown down.

The fcst remains somewhat uncertain going into Wed as the NAM and
ECMWF both linger a high precip amts as the sfc wave elongates
across the Apps, while the GFS takes the better precip well north of
the FA. In any case and considering the degree of soil saturation,
there will likely be additional hydro issues on Wed and the WPC SLT
risk of excessive rainfall looks good across the higher terrain. Max
temps will drop abt 10 degrees below normal Tue and a couple degrees
below normal Wed. Mins will remain abv normal thru the period.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 345 AM EDT Monday...The ext range remains unsettled as an
upper ridge briefly develops overhead Thu, before a couple h5 trofs
and cold fronts work into the area into the weekend. Soundings show
decent sbCAPE and an uptick in deep layered shear ahead of the first
front Fri, which is making the case for a semi-organized squall line
ahead of the sfc bndry progged to cross the NC mtns arnd max
heating. Another upper wave looks to support a sfc front on Sat, but
confidence in the strength and organization of this system is not
great considering the higher h5 heights and weaker trof seen on the
ECMWF. Still, looks like a return to more normal late summer conds
with tstms developing each afternoon thru the weekend. Max temps
will remain at or a bit abv normal each day with mins holding abv
normal as well.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Now that low level moisture is abundant
across the region, with a S/SE low level flow, low clouds and
showery precip could develop at any location at any time through
the period. Away from the mtns, the upslope flow will mainly help
to keep a low cloud ceiling in place, probably dropping down to
IFR/LIFR in the pre-dawn hours, then struggling to scatter/lift
during the middle part of the day. Over the mtns and near the Blue
Ridge Escarpment, showers and a few thunderstorms will continue
off and on. The low clouds should eventually scatter enough for
terminals to go VFR, which the guidance indicates will happen around
18Z. Timing of shower/tstm development is problematic. Chances are
good that all terminals will get something during the period, but
for this issuance we will leave it at a PROB30 for the afternoon
and early evening thanks to daytime heating. We will attempt to
narrow it down in the 12Z TAFs. Winds should be SE to S through
the period, stronger near thunderstorms.

Outlook: Coverage of mainly afternoon/evening showers and
thunderstorms is expected to remain high through mid-week, with
Tue looking like the most active day. A gradual downward trend
in diurnal convection is expected during the latter half of the
week. Widespread low stratus and areas of fog are expected each
morning through at least mid-week.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       Med   79%     Med   76%     High 100%     High 100%
KGSP       High  95%     High  81%     High 100%     High 100%
KAVL       High  81%     High  80%     Med   60%     Med   68%
KHKY       High  87%     High  83%     Med   60%     High  91%
KGMU       High  93%     High  83%     High 100%     Med   76%
KAND       Med   78%     Med   78%     High  99%     Med   74%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

SYNOPSIS...DEO/SBK
NEAR TERM...PM
SHORT TERM...SBK
LONG TERM...SBK
AVIATION...PM

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
1152 PM EDT Sun Aug 15 2021

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-170400-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Avery-Alexander-
Iredell-Davie-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-Catawba-
Rowan-Graham-Northern Jackson-Macon-Southern Jackson-Transylvania-
Henderson-Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Union NC-
Caldwell Mountains-Greater Caldwell-Burke Mountains-Greater Burke-
McDowell Mountains-Eastern McDowell-Rutherford Mountains-
Greater Rutherford-Polk Mountains-Eastern Polk-Oconee Mountains-
Pickens Mountains-Greenville Mountains-Greater Oconee-
Greater Pickens-Greater Greenville-Spartanburg-Cherokee-York-
Anderson-Abbeville-Laurens-Union SC-Chester-Greenwood-
1152 PM EDT Sun Aug 15 2021

This Hazardous Weather Outlook is for northeast Georgia, piedmont
North Carolina, western North Carolina and upstate South Carolina.

.DAY ONE...Tonight.

Widely scattered thunderstorms are expected through daybreak on
Monday. Cloud-to-ground lightning and torrential downpours will be
possible with any storm. Some localized flooding of poor drainage
areas is possible, particularly in areas that have had heavy rain in
the last day or two.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Deep tropical moisture associated with the remnants of tropical
storm Fred will interact with an old front and the mountains to
produce periodic heavy rainfall through the middle part of the week.
The flood threat will steadily increase on Monday as showers and
thunderstorms wet the soil. At this time, the greatest potential for
excessive rain appears to be Tuesday and Tuesday night as a remnant
circulation from Fred moves northward to the west of the region.
This could result in a flash flooding threat, especially in areas
that receive multiple rounds of heavy rainfall.

.SPOTTER INFORMATION STATEMENT...

None.