Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
642 PM EDT Sat Jul 24 2021

.SYNOPSIS...
Air quality should improve somewhat over the weekend while afternoon
showers and thunderstorms increase in coverage and intensity through
Monday.  A backdoor cold front builds into the forecast area early
next week, bringing more active weather at least through Tuesday.
Late week, additional rainfall is possible, though significant
uncertainty remains at that point.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 630 PM: Scattered showers and storms continue to track off the
higher terrain and slowly migrate across the foothills and piedmont.
The isolated coverage in areas south of I-85 has mostly come to an
end, but the storms along and north of I-85 still have a chance to
track towards the southeastern zones before all is said and done.
Most of the clouds coverage consists of cu or convective debris from
the storms that have developed. Expect this trend to continue
through sunset before the activity slowly starts to wane off.

Meso analysis shows better than expected effective shear across the
area, with moderate to very unstable CAPE values. DCAPE values are
generally modest but could increase to 800 J/kg ore higher with
heating. Therefore, a few severe storms will be possible. Storm
motion looks to be faster than previous days, but PW values remain
high. Isolated areas of excessive rainfall and flooding will be
possible, especially locations where multiple storms traverse.

Showers will linger into the early overnight but should diminish as
the low level flow turns more southwesterly. That said, can't rule
out an isolated shower in any location overnight. Expect mountain
valley fog and stratus once again, but confidence lower for
locations outside of the mountains. Lows should be near or slightly
above normal.

Northwest flow remains over the area Sunday with moderate
instability developing. Again, no strong trigger mechanism, but
differential heating and a lee trough should be enough for scattered
diurnal convection. An isolated strong to severe storm or area of
heavy rainfall will also be possible. Highs will be near normal.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 150 PM EDT Saturday: Broad ridging over the western two-thirds
of the country will entrench the eastern CONUS in a subdued weather
pattern at the start of the short term forecast period on Monday.
A deep shortwave digs into the Mid-Atlantic throughout the day,
but guidance still keeps the most potent QG forcing associated
with this trough well north of our forecast area.  Instead,
our active weather will be driven primarily by low-level forcing
associated with a cold front impinging on the region from the north
by Monday evening.  With the Bermuda High enhancing southerly
flow throughout the weekend and Monday, the front will find a
moisture-rich environment in the Carolinas - models agree that
PWATs across the foothills and Piedmont will have risen into the
1.9-2.3" range, and will be pushing at least 1.75" in the mountains.

Prefrontal precipitation should begin by Sunday night or early
Monday morning for the NC mountains, and over the course of the
day, this activity will gradually spread south and east over
the CWA, eventually encompassing the entire forecast area.
Coverage and intensity of showers should increase throughout
the day, before convective potential begins to rise late morning
and early afternoon.  The ECMWF, CMC, and NAM all suggest SBCAPE
values of 1500+ J/kg, with the outlier GFS carrying a more modest
500-1000 J/kg.  Given that we'll be overcast much/all of the day,
and that rain will likely be underway for most areas by midmorning,
the current thinking is that those higher instability values are
overdone.  So, expect this front to feature widespread showers
with some embedded non-severe thunderstorms.  The big concern will
be excessive rainfall: with PWATs at/near 2", a deep warm cloud
layer in excess of 12 kft, and weak MBE vectors less than 5kts
all afternoon, backbuilding should be a concern for some cells.
Furthermore, with the front likely stalling somewhere near our
forecast area, some spots are likely to see long durations of
heavy rainfall Monday night into Tuesday.

Temps are still on track to be above-normal both Monday and Tuesday,
though guidance continues to trend toward lower highs in light of
widespread clouds, continued rainfall persisting through the
forecast period, and weakened southerly flow/WAA as the front stalls.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 200 PM EDT Saturday: The details of the medium-range forecast
remain in question.  Global models continue to suggest that a strong
Bermuda High will limit the cold front's ability to fully exit our
region on by Tuesday night, so even into Wednesday we'll likely
continue to see frontally-forced rainfall as the boundary stalls
over our CWA or just to the south.  The GFS, ECMWF, and CMC all keep
us locked in a wet pattern through Wednesday, before the weakening
boundary finally sags out of our area, carrying low-level forcing
with it and allowing weak high pressure to filter into the area.
With broad high pressure persisting over the Atlantic, however,
the prevailing surface flow will still be primarily southerly,
albeit with an increasing westerly component.  This slight shift
in overall wind direction should limit PWATs - comparatively,
at least, as they still keep near 1" into Thursday and Friday.

By Friday, we may see the arrival of a shortwave as the upper
pattern slightly amplifies - which would result in another wave
of precipitation late in the week.  Once this passes, we may be
in store for some dry weather next weekend.

In any case, temperatures will generally be near- or slightly
above-normal throughout the period.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Convection continues to spread across the
CFWA and staying out with a mix of VCSH/VCTS and TEMPO TSRA based on
current coverage and guidance showing TSRA will be over or near all
TAF sites sometime this evening. Showers may linger into the early
overnight, so have VCSH for those. Guidance agrees on mountain
valley fog/stratus again by morning but disagree on restrictions
outside of the mountains. Have restrictions at KAVL and KHKY where
chances are highest, but left out elsewhere given the low
confidence. Convection will develop again Sunday afternoon, but have
PROB30 at all terminals with the best chances developing after 18Z.
S to SW wind this evening becomes light S or light and variable
overnight. Light SW wind for Sunday.

Outlook: Diurnal thunderstorm chances will steadily increase through
early next week, with some drying expected during mid-week. Early
morning fog or low stratus will be possible each day, primarily in
the mountain valleys.

Confidence Table...

            22-04Z        04-10Z        10-16Z        16-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  97%
KAVL       High 100%     High  85%     Med   60%     High 100%
KHKY       High 100%     High  95%     Med   67%     High 100%
KGMU       High 100%     High  85%     High  82%     High  97%
KAND       High 100%     High 100%     High 100%     High 100%

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...MPR
NEAR TERM...CAC/RWH
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...CAC/RWH