Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 222352

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
752 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 745 PM: Very weak steering flow up the column is
permitting showers and thunderstorms to produce very heavy localized
rainfall - with rates up to 2 inches per hour in some places.
Coverage has largely been limited to the mountains, north of I-40,
and extreme southwest sections, with limited triggering over the
foothills and piedmont given the very weak outflows. The convection
allowing models still feature mainly diurnal convection, with
activity waning somewhat after midnight, and low clouds and patchy
fog forming toward daybreak.

Meanwhile, the shortwave trough over the middle Ohio Valley will
move toward the East Coast. Low-level winds will shift behind the
trough axis 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

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: The better convection arriving with moisture
from the southwest is generally staying to the west and southwest of
the TAF sites this evening. Very weak steering flow should keep
coverage down a bit through late evening before the showers
diminish, so will mention mainly VCSH, except for VCTS at KAND,
across the terminal area. The main question going forward is when
will lowering cloud restrictions develop in the moist boundary layer
overnight, and how low will they go. At least MVFR looks likely
overnight given the MOS and model RH profile presentation, but a
brief window of IFR certainly looks possible around daybreak. Bases
will recover only very slowly with heating on Wednesday given the
high surface dewpoints, and MVFR could linger well into the mid or
late morning hours. Will feature mainly PROB30 for late day TSRA.
Winds will be generally light southwesterly, except for mainly NW at
KAVL overnight.

Outlook: The unsettled pattern will continue the rest of the week,
with flight restrictions possible each day under diurnal showers and
thunderstorms. Tropical moisture may increase through the late
weekend. Morning stratus/fog are possible each day - especially
following heavy rain the previous day.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  99%     High  87%     High  83%     High 100%
KGSP       High  94%     High  85%     Med   68%     High  93%
KAVL       High  90%     Med   64%     High  88%     High 100%
KHKY       High  98%     High  83%     High  95%     High 100%
KGMU       High  94%     High  81%     Med   75%     High 100%
KAND       High  88%     Med   76%     Med   73%     High  88%

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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