Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 222052

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
452 PM EDT Tue May 22 2018

A moist air mass will remain in control of our weather through
Wednesday, with good chances for showers and thunderstorms in the
afternoon and evening. A weak backdoor cold front will move into the
forecast area from the north on Thursday and should allow for some
brief drying for Thursday and Friday. Another round of deep tropical
moisture is expected to lift across the area over the weekend and
into early next week.


As of 440 PM: Outflow boundaries working northward across the region
today are trigging numerous showers and embedded thunderstorms in
the very moist profiles. Forcing generally remains quite weak this
evening, and diminishing coverage is expected following peak heating.

Overnight, the shortwave trough now over the middle Ohio Valley
will move toward the East Coast. Low-level winds will shift behind
its axis. The wind shift is expected to occur between midnight and
dawn roughly over the northwest half of the area. This will provide
a slight enhancement to lift and warrants keeping at least a small
PoP throughout the night, with the highest values near the trough
axis. Min temps overnight should again reflect the muggy dewpoints,
and bottom out 7-10 degrees above normal.

The trough/boundary will remain nearly stationary over the southern
part of the CWFA through the remainder of the near term period. This
will enhance tomorrow`s round of diurnal convection over the
southeastern half of the area, though chance-range PoPs are forecast
elsewhere. The terrain along the southern Escarpment looks likely to
see precip develop by late morning; these areas in particular have
particularly saturated soils from the rainfall of the past week,
so we will be especially mindful of flash flood threat with any
cells that develop there. PWAT values will remain exceptionally
high south of the boundary, with SREF mean values above 1.6",
2 standard deviations above climo. Deep layer shear will be very
weak and storms are likely to be virtually stationary, so locally
heavy rain cannot be ruled out anywhere in this environment. On
the other hand, over the central/nrn mountains and I-40 corridor,
drier air will begin to fill in on the northwesterly flow occurring
there. While this may limit rainfall rates, it will also make
cells in that area more likely to produce strong wind gusts. Max
temps will be a few degrees above normal.


As of 220 pm Tuesday: Weak frontal boundary will be sagging slowly
south or stalling across the area, while becoming increasingly
diffuse during the short term. While slightly drier/lower theta-e
air will have modest success trickling into the area on Thursday
(mainly across the northern zones), this won`t last as the air mass
is forecast to modify back to soupy/unstable across the entire area
by the end of the period. Thu and Fri should therefore see more
diurnal convective activity, with pops Thu ranging from 20-30%
across much of the northern half of the area, to 40-60% across
southern zones. Easterly low level flow north of the boundary
underneath NW flow aloft will result in very weak (i.e., < 5 kts)
steering flow. With the boundary providing a potential focus for
slow-moving training cells, a localized flash flooding threat will
exist across roughly the southern half of the area Thu afternoon.
Max temps will remain near normal, and min temps 5-10 degrees above
climo through the period.


As of 235 pm Tuesday: All eyes remain on the tropics in terms of the
medium range, with the two camps re: tropical cyclone impacts along
the Gulf Coast sticking to their respective guns: the eastern Gulf
solution (GFS) and the Miss Valley solution (ECMWF). After it had
begun to trend to the west, the GFS has doubled-down on a FL-GA-
Carolinas track with its weakening surface low (and actually depicts
secondary surface development across the central Gulf Coast) early
next week. Meanwhile, the ECMWF continues to landfall a tropical
cyclone near the mouth of the Miss River on Sunday, then stalls it
for 24-ish hours before slowly lifting it north. (For what it`s
worth, the latest Canadian is somewhere in between these two
solutions). However, as has been the case for several runs now, the
global models are in good agreement in depicting a large moisture
shield around the cyclone. Thus even if the more western ECMWF track
pans out, ample moisture will exist along with modest upslope flow
to support 50-70 pops from late Saturday through the end of the
period. While there will be a diurnal peak in convective activity,
the pattern is such that some degree of convective coverage will
exist outside the normal afternoon/evening peak. Heavy rain/at least
localized flash flooding will remain a daily concern for the
foreseeable future.


At KCLT and elsewhere: SHRA/TSRA are lining up this aftn along a
weak sfc boundary and will move thru the SC sites bringing brief
restrictions. Additional, less organized convection is expected
across the NC sites, likely developing on the strength of weak
outflows, warranting TEMPOs at KCLT/KAVL and VCTS at KHKY. The
activity should weaken by late aftn. A few minor SHRA may last into
evening. Overnight a weak cold front is expected to settle into the
area from the N. This will induce a wind shift to NW at KAVL a
little before dawn. Convergence near the front may permit
redevelopment of some low to mid-level cloud cover, though given the
veered flow across the Piedmont, it is not expected to be as
widespread or prolonged as what occurred Tue morning in southerly
flow. Nonetheless a few hours of MVFR are mentioned at all sites
close to daybreak. Fog is possible overnight mainly where rain falls
this aftn/evening, but also in mtn valleys. The front will remain
across the region on Wednesday and looks to aid in SHRA/TSRA
development beginning late morning, mainly south of KAVL/KHKY, but
worthy of PROB30 mention at all sites.

Outlook: The unsettled pattern will continue the rest of the week,
with flight restrictions possible each day under periodic showers
and thunderstorms. Morning stratus/fog are possible especially
following heavy rain the previous day.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High  98%     Med   68%     Med   75%
KGSP       High  98%     High  92%     High  85%     Med   75%
KAVL       High  92%     High  92%     Med   75%     Low   59%
KHKY       High 100%     High  94%     Med   77%     Med   73%
KGMU       High  85%     High  90%     High  96%     High 100%
KAND       High  87%     High  94%     High  96%     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:




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