Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201751
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
151 PM 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 145 PM EDT: Convection firing across the ridgetops as
expected by the early warmup and destabilization across the
mtns. The convection should gradually expand eastward on a diet of
sbCAPE on the order of 3000 J/kg just east of the Blue Ridge this
afternoon. However, the mesoanalysis bears out the suppressing
effects of some thicker high clouds over the Piedmont and Upstate
in a corridor between the mtns and the convection firing over the
SC Midlands. Still think there is time for the high clouds to thin
out enough and for convection to eventually develop in the I-85
corridor, as indicated by the 12Z HREF.

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. 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 /18Z SUNDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Low clouds with bases generally in the
range of 020-025 continue to scatter out across the region
early this afternoon. Cannot rule out a brief MVFR ceiling away
from ongoing convection, but this detail will not be included
for brevity. Instead, prefer to employ a 3-4 hr TEMPO to handle
thunderstorm restrictions later this afternoon at all sites other
than KAND. The showers and storms should diminish fairly early
this evening with only convective debris beyond 23Z-01Z. Locations
that see rain this afternoon/evening will have a good chance at
a fog restriction in the pre-dawn hours Monday if recent past is
any indication. All sites get a short window of MVFR visibility
around daybreak to account for this. Otherwise, Monday looks to
be nearly a copy of today. Wind should remain light S/SW apart
from thunderstorm outflows today, then light/variable overnight,
and SW again Monday.

Outlook: The unsettled weather pattern will continue into the first
half of the week with flight restrictions possible each day around
isolated/scattered showers and storms.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  99%     High  91%
KGSP       High 100%     High 100%     High  94%     High  85%
KAVL       High 100%     High 100%     High  93%     High  98%
KHKY       High 100%     High 100%     High  92%     High  89%
KGMU       High 100%     High 100%     High  89%     High  81%
KAND       High  99%     High  99%     High  80%     High  81%

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/PM
SHORT TERM...TDP
LONG TERM...TDP
AVIATION...PM



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