291 
FXUS62 KGSP 211751
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
151 PM EDT Sat Aug 21 2021

.SYNOPSIS...
An upper level low over the Mid-Atlantic states will gradually lift 
northeast into New England this weekend into early next week while a 
ridge of high pressure in the southern United States gradually 
builds into the southeastern United States. This upper-level pattern 
will lead to only isolated shower and thunderstorm activity this 
weekend into early next week across northeast Georgia and the 
western Carolinas. Meanwhile Hurricane Henri will remain out in the 
western Atlantic tracking north towards New England throughout the 
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1030 am EDT: A few showers have developed in the Lakelands 
area of the Upstate had have updated pops accordingly. Otherwise, 
grids are in pretty good shape this morning for a seasonally hot and 
muggy day across the area with typical summertime pops mainly across 
the mountains in the NW flow.

Otherwise, an H5 closed low center over the central Appalachians 
will continue to spin north of the region, with various spokes of 
vorticity moving through the southern Appalachians and crossing the 
forecast area today. Forcing from these vorticity lobes may be 
better in the northern tier today, but moisture and instability will 
be better in the southern half, and that is where the better chance 
PoPs will be featured during peak heating as sbCAPE values climb to 
2500+ J/kg. Initiation should get started near the mountains again, 
especially the southern escarpment area, but with a secondary focus 
along a weak lee surface trough. Maximum temperatures should be 
comparable to yesterday.

Tropical System Henri will remain well east of the Carolinas today 
through tonight, with no impact to the region expected. Otherwise, 
any diurnal convection will die out quickly this evening, with 
another night of patchy fog along with some dense fog in the 
mountain valleys. Mins remain above climo.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 100 AM EDT Saturday: Dry air advection from NW-ly flow
aloft and weak high pressure over the southern Appalachians will set 
the stage for very limited convective coverage on Sunday. A closed 
upper low over the mid-Atlantic will continue to lift northeast, 
while pulling TS Henri further west and potentially stalling over 
New England and portions of the Northeast. In our neck of the woods, 
subsidence aloft and rising heights will filter into the area from a 
deepening upper ridge over the Southern Plains and Lower Mississippi 
Valley, which allows for mostly dry conditions across the CFWA 
outside of isolated ridgetop convection and a pop up storm or two 
east of the mountains for Sunday. With ~590 dm thicknesses 
continuing to set up shop over the region, expect for the very warm 
and humid airmass to persist through the short term period. With 
little change in the overall synoptic pattern and mesoscale 
processes during the early part of the upcoming week, expect a very 
similar forecast for both Sunday and Monday. The main difference 
between both days will be temperatures. With the upper ridge 
extending further east into the southeastern CONUS on Monday, 
temperatures should uptick a few degrees from Sunday to Monday. 
Either way, temperature values will run above normal for both days.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 230 AM EDT Saturday: Summertime Bermuda High pattern will
begin to set up shop during the medium range. The Bermuda high will
gradually bring a return flow back into the CFWA by the middle
part of the upcoming week. Two areas of upper ridges will leave
the CFWA hot and humid with high heights continuing to linger over
the area. Diurnal convection will make a return on Tuesday due
to the Atlantic fetch from the Bermuda High. PoPs will still run 
below climo due to the stagnant subsidence aloft. By Wednesday and 
Thursday, model guidance shows the upper high to the west 
retrograding as the subtropical ridge over the western Atlantic 
pushes towards the southeastern CONUS. An open tropical wave will 
ride the southwestern peripheral of the subtropical high as the wave 
pushes through the Bahamas. The track of the tropical wave will be 
influenced by the western Atlantic subtropical high. Deterministic 
models have pushed this into the western Carolinas and northeast 
Georgia over the last couple of model runs by Wednesday and 
Thursday. If this trend continues, expect PoPs and PWAT values to 
surge as the atmospheric column becomes saturated from the 
relatively deep tropical moisture. The GFS and ECMWF also introduce 
the potential for the aforementioned tropical wave to stall over the 
CFWA and develop a closed low. There are a lot more questions than 
answers at this time, but the quiet pattern that is forecasted 
during the short-term could come to an abrupt end during the 
extended period if this scenario ends up playing out as it is 
currently being displayed. Temperatures will start out above normal 
for Tuesday, but values will likely decrease to near-normal readings 
through the rest of the forecast period due to the uptick in 
precipitation chances and cloud cover.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: VFR this afternoon, with some pockets of MVFR 
Cu as convection is expected to begin firing this afternoon, but at 
this time not widespread enough to introduce TEMPO; have left VCSH 
in TAFs at this time. Expect typical mountain valley fog again 
tonight with MVFR to IFR conditions expected again at KAVL. Slightly 
better chances for afternoon convection Sunday but too early to 
introduce PROB30 at this time. Winds are generally lgt/vrb in the 
weak pressure pattern but by tomorrow should be favoring the S/SW.

Outlook: A typical, diurnal summertime convective pattern is 
expected to prevail the rest of the weekend through the middle of 
next week, with scattered coverage at best. Mountain valley fog will 
be possible each morning, as well as at sites that receive heavy 
rainfall the previous afternoon.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z 
KCLT       High 100%     High  99%     High 100%     High 100%     
KGSP       High 100%     High 100%     High  80%     High  92%     
KAVL       High  93%     High 100%     Low   51%     Med   79%     
KHKY       High 100%     High  95%     Med   79%     High  91%     
KGMU       High 100%     High 100%     High  81%     High  89%     
KAND       High 100%     High 100%     High  99%     High  82%     

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...HG
NEAR TERM...HG/TDP
SHORT TERM...CAC
LONG TERM...CAC
AVIATION...TDP