268 
FXUS62 KGSP 211047
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
647 AM EDT Sat Aug 21 2021

.SYNOPSIS...
An upper level low or trough will persist over the eastern 
seaboard, as Tropical System Henri moves over the offshore waters. 
Weak surface high pressure will set up over the southern 
Appalachians, with a lee trough persisting in the Piedmont. The 
upper-level pattern will remain relatively unchanged through most of 
the period. Isolated showers and thunderstorms will remain possible 
this weekend into early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 630 am EDT: Nighttime microphysics enhanced satellite imagery 
shows plenty of lower clouds floating around the region near 
daybreak this morning, but any widespread fog issues are impacting 
mainly the mountian river valleys. An SPS has been issued there for 
patchy dense fog through 9 am. 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 /12Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Scattered MVFR-level bases are 
floating around the foothills and Piedmont this morning in NE flow, 
but with any ceilings rather transient in nature. The main exception 
is KAVL, with lower stratus and fog filling up the mountain valleys 
and likely to impact the airfield for a couple of hours past 
sunrise. Expect FEW to SCT VFR cumulus to develop quickly with 
heating throughout today, with PROB30 TSRA or VCSH/TS for the mid to 
late afternoon hours at all but KHKY and KCLT where profiles will be 
drier. Light winds will favor a northerly direction early in the 
period and then a southerly direction the second half of the TAF 
period, except mainly NW at KAVL. Any afternoon convection will 
diminish quickly this evening. Other than early morning valley fog, 
any restrictions through the period will be confined to the 
immediate vicinity of thunderstorms.

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...

            10-16Z        16-22Z        22-04Z        04-06Z 
KCLT       High  95%     High 100%     High 100%     High 100%     
KGSP       High 100%     High 100%     High 100%     High 100%     
KAVL       Low   40%     High 100%     High 100%     High 100%     
KHKY       High 100%     High 100%     High 100%     High 100%     
KGMU       High 100%     High 100%     High 100%     High 100%     
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...HG
NEAR TERM...HG
SHORT TERM...CAC
LONG TERM...CAC
AVIATION...HG