Area Forecast Discussion
Issued by NWS Greer, SC

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952
FXUS62 KGSP 200734
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
334 AM EDT Sun May 20 2018

.SYNOPSIS...
A persistent southerly flow will maintain a tropical-like moist
atmosphere atop the region through at least Tuesday.  This will
result in a continued high chance of showers and storms each
afternoon and evening.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 315 AM EDT: Nearly all of the shower activity has ended across
the fcst area over the past few hours. Only a few pockets of light
showers remain over Caldwell and Davie Counties in NC. Conditions
should remain mostly quiet into the morning with lower cloud cover
already moving over portions of the Upstate and Piedmont and fog
settling in over the mtn valleys. increasing overnight with low
temperatures dipping into the 60s. Compared to the past couple of
mornings, the guidance is more bullish on fog across the lower
terrain this morning. I expect it will be mostly patchy to some
areas of fog, but doubt it will become widespread.

Otherwise, an upper trough will continue to lift north and deamplify
as its axis moves NE of the CWFA. At the sfc, the subtropical
Bermuda High will continue to maintain control of the synoptic
pattern over the Southeast CONUS. Although we don`t expect a
significant change in the moist airmass that has prevailed over our
area, the latest guidance depicts slightly drier air and less
convective coverage today compared to the last few days. Very weak
shear and SBCAPE in the 1500 to 2000 J/kg range will support pulse
convection with a minimal severe threat today. Highs will climb into
the low to mid 80s across the lower terrain and mid to upper 70s
across the higher terrain.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 230 AM EDT Sunday: The subtropical upper low will remain in
place over the Deep South as we begin the short term period, while
the surface high in the Atlantic will continue the low-level
warm/moist southerly feed. The upper low will weaken and work slowly
north through the period as the longwave pattern tries to absorb it,
but the upper flow over the Southeast will be so weak that the
weakness will sort of just sit across the area through the period.
Convection will be mainly diurnal in nature, but enhanced by the
upper low. CAPE values will be minimized by the weak lapse rates,
but generally between 1000-1500J/kg each day. Tuesday afternoon
looks a little curious as the surface pattern is resembling the
development of a bit of insitu damming, though not reflected in the
surface winds which remain out of the south, and guidance is
lowering max temps a couple of degrees accordingly. High PWs,
surface instability, convective precip rates combined with very
saturated soils will keep the flash flood threat in place.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 315 AM EDT Sunday: Upper high over the western Atlantic as
well as a building upper ridge up the Plains will be in place to
start the extended period. Broad upper trough over eastern Canada
will continue to try to absorb the weakening subtropical upper low
over the Southeast, but the two ridges will slowly begin to merge as
the Canadian upper trough strengthens and forces the Atlantic upper
high toward the Bahamas. A weak surface trough will be in place over
the Piedmont and will slide slowly south as we pass midweek,
swinging winds around to NW and eventually NE. Not much change in
temperatures as the airmass does not change despite the surface
trough. Convection will remain very diurnal in nature through still
enhanced by the Southeast upper weakness and like the short term
with PWs remaining high, lapse rates will be minimized. Severe
threat will remain low but isolated/localized flash flooding
concerns will remain through the period.

And then, as mentioned previously, things could get a tad more
complicated at the end of the period with global models wanting to
develop some sort of tropical system to impact the Southeast. Way
too far out for any details but naturally the concern would be the
continued increase in moisture as a result. Without much of a break
through the period, as mentioned in the WPC morning discussion, the
overall pattern is pointing to a heavier rainfall trend.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: For the overnight, as low-lvl flow veers to
the west, stratus is expected to develop bringing the potential for
IFR cigs and patchy MVFR fog as well. After daybreak, expect fog to
dissipate with a gradual improvement of cigs to VFR. Isolated
showers and thunderstorms will be possible again today, though
coverage is expected to be less than the trend over past few days.
Aside from varying wind direction with any convection or outflow
boundaries, expect winds to remain SLY to SWLY in the 5 to 10 kt
range thru the TAF period.

Outlook: The unsettled weather pattern will continue into early next
week with isolated to scattered SHRA/TSRA and possible flight
restrictions each day.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  82%     High  87%     High 100%     High 100%
KGSP       Med   72%     High  90%     High 100%     High 100%
KAVL       High  81%     High  91%     High 100%     High  96%
KHKY       Med   66%     High  86%     High 100%     High  96%
KGMU       High  81%     High  89%     High 100%     High 100%
KAND       Med   68%     High  81%     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 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...CSH
NEAR TERM...JPT
SHORT TERM...TDP
LONG TERM...TDP
AVIATION...JPT



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