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Area Forecast Discussion (AFD)

Issued by Greenville-Spartanburg, SC (GSP)

                            
000
FXUS62 KGSP 191108
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
708 AM EDT Mon Aug 19 2019

.SYNOPSIS...
A trough of low pressure and elevated moisture and instability will 
persist over the western Carolinas and northeast Georgia today 
through Wednesday. A cold front will approach the region from the 
northwest on Thursday and then stall in the vicinity on Friday, with 
abundant moisture remaining in place. The front will gradually 
settle just south of the area through the weekend as high pressure 
builds over to the north. Cooler temperatures will develop but 
moisture will linger.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 700 AM: No big changes to the forecast with this update. 
Tweaked fog mention in the mountain valleys. But sky and temps and 
pops look on track.

As of 250 AM: There will be a couple of vort maxes that will meander 
over the Carolinas and GA today between two ridges of high pressure 
at 500 mb---one over the southern plains and the other over the 
Atlantic. They will not provide much upper forcing due to their slow 
motions. However, guidance is in good agreement on a pocket of 
slightly cooler temps within this weakness aloft, which should 
result in strong destabilization this aftn across the region. With 
sbCAPE in the 2000-3000 J/kg range today and plenty of mid-lvl dry 
air along with 0-3 km shear less than 10 kts, the atmosphere will be 
supportive of microbursts. So I expect scattered storms to develop, 
starting in the high terrain due to typical mountain-top 
differential heating, then expand east with help from lingering 
outflow boundaries. PoPs will feature mainly mid to high-end chc 
this aftn into early evening. Temps will be similar to yesterday's 
readings, which are a couple categories above normal. Dewpts in the 
upper 60s to lower 70s will result in heat index values in the 95-
100 deg range, with a few spots slightly over 100 across the 
piedmont.

Convection should steadily wane mid to late evening, but a few 
isolated showers could linger into the overnight. A weak low over 
southern GA may start to drift slowly north overnight, bringing a 
very light moist upslope SELY flow atop the FA. This flow may also 
help some stratus to develop and expand across much of the piedmont 
to the escarpment. The clouds will hold min temps slightly above 
normal.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 2 am EDT Monday: A weak 500 mb trough axis will persist atop 
the forecast area on Tuesday between offshore Bermuda high pressure 
and stronger higher pressure centered over the southern plains. The 
best plume of above climo precipitable water will stretch across 
piedmont areas. Moisture filling in at mid-levels, slightly weaker 
lapse rates, and more clouds keeping temperatures in check should 
reduce the severe weather potential a touch below Monday values. 

The weakness in the height field will persist on Wednesday. Moisture 
will continue to increase in profiles, but lapse rates will start to 
steepen back up a bit to permit better afternoon instability and a 
modest severe thunderstorm/heavy rain threat. Confidence, however, 
is low on this given the uncertain forcing.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 230 am EDT Monday: More prominent height falls will develop 
from the northwest on Thursday. Deep moisture and better forcing 
should arrive during peak heating Thursday afternoon and evening as 
a surface cold front begins to approach from the northwest. 
Anticipate an uptick in PoP, with high end likely thunderstorm 
coverage in the northwest half by late Thursday, and solid scattered 
coverage southeast. Wind shear will be limited despite the falling 
heights, and moistening profiles will make the instability, and 
associated severe thunderstorm threat, a bit uncertain.

The frontal boundary will slow or stall across the region on Friday, 
with a very moist axis of PWAT values at or above two inches draped 
over the region. Showers and thunderstorms will become increasingly 
widespread given the moisture, forcing, and instability. With high 
pressure building to the north of the area into the weekend, cold 
air damming conditions may set up across the western Carolinas east 
of the Blue Ridge mountains by Saturday as the boundary slowly 
settles southward. Moist cold air damming could persist into Sunday 
as the boundary moves just south of the area. Anticipate smaller 
diurnal ranges on temperatures over the weekend with the cold air 
damming in place.

&&

.AVIATION /11Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: All sites starting out VFR this morning, with 
VFR-level cu developing by midday. It should be come seasonably 
unstable again this aftn. Though a weak upper low will begin to take 
shape over the region, there is little forcing aside from the 
diurnal heating, so PROB30s are maintained at all sites for aftn 
TSRA and associated vsby restrictions. Tonight, a weak low will get 
a little stronger over central GA, producing a light SELY low-level 
flow over the region. This flow may support some stratus development 
over the piedmont toward daybreak Tuesday. Some of the guidance hits 
vsby restrictions harder than I would expect, given forecast 
soundings show potential for plenty of mid and high clouds lingering 
thru much of the night.

Outlook: Moisture and instability will gradually increase through
the week, resulting in a steady uptick of afternoon/evening
convective coverage each day, especially mid to late week as a
cold front approaches from the northwest. Morning fog and low
stratus will be possible each day across the mountain valleys.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z 
KCLT       High 100%     High 100%     High 100%     High 100%     
KGSP       High 100%     High 100%     High 100%     High  83%     
KAVL       Low   55%     High 100%     High 100%     High 100%     
KHKY       High 100%     High 100%     High 100%     High  83%     
KGMU       High 100%     High 100%     High 100%     High  83%     
KAND       High 100%     High 100%     High 100%     High  83%     

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
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...ARK/Wimberley
SHORT TERM...HG
LONG TERM...HG
AVIATION...ARK