Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 122107

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
507 PM EDT Thu Oct 12 2017

A wedge of cool high pressure will set up through tomorrow,
providing a brief break before warm, moist conditions return over
the weekend. A more robust cold front will approach the Carolinas,
bringing a chance of showers to the area Monday. Considerably cooler
and drier air will spread over the region in the front`s wake and
linger through the middle of next week.


As of 500 PM EDT: The latest LAPS sbCAPE values indicate some 1500
to 2000 J/kg values just southeast of CLT, and this plume of
instability along the frontal zone retreating southward through the
piedmont will be the focus of any scattered convection into early

Otherwise, hybrid cold air damming will continue to gradually evolve
across the forecast area, as parent surface high pressure over
northern New England moves into an increasingly favorable position.
Very moist low level conditions in the vicinity of the low level
ridge will support widespread low clouds, with areas of fog and
drizzle and/or -RA overnight into at least the first half of Friday.
Pops will generally be advertised in the chance range overnight,
with an area of likely pops advertised across eastern escarpment
counties in response to an E/SE upslope flow.

On Friday, the parent high is forecast to become centered near Nova
Scotia, thus the CAD will have some difficulty locking in. However,
extensive low cloud cover is expected to persist through the day,
and this alone should be sufficient to bring max temps crashing down
to right around normal levels. Precip chances should gradually
diminish through the day.


As of 215 PM EDT Thursday: Cold air damming will continue Friday
night, as guidance in good agreement on low stratus filling back in
(into areas that may have scattered earlier in the AFTN). Forecast
soundings lack of low-level flow and thin saturated layer. So only
spotty sprinkles/drizzle possible mainly in upslope areas. Lows will
be well above normal.

On Saturday, sfc high pressure drifts well off the New England
coast, leaving a weak wedge trailing back to the Carolinas. There
should be little to no precip under the wedge, so guidance shows
erosion with some scattering of stratus by late AFTN. Temps rebound
to mid 70s to possibly a few lower 80s in the southern piedmont. A
piece of the sfc high will settle over the CWFA Saturday night,
while skies should clear out for the most part. So guidance is
hinting at areas of fog, as dewpts remain elevated and good
radiational cooling occurs. I opted to add patchy fog wording for
early Sunday morning. Temps will continue to be above normal.

On Sunday, a cold front will approach from the west, as a deep
trough tracks across the Midwest. The 12z GFS has come in a bit
faster with the timing of the front, bringing the sfc boundary into
the NC mountains by early evening. While the 00z ECMWF has the front
arriving about 12 hours later. I will keep some CHC PoPs coming into
the western zones before 00z Monday. Otherwise, Sunday should be dry
with fog/stratus burns off by late morning. Temps will be back into
the lower to mid 80s east of the mountains.


As of 230 PM EDT Thursday: the medium range starts at 00z Monday,
with uncertainty on exact timing of cold front`s arrival. At time of
this writing, still awaiting the 12z ECMWF. But the 12z GFS has the
front thru the area by 12z Monday. If the front ends up being that
fast, most of the shower activity associated with the front will
likely struggle to cross the mountains. This also means prob of
thunder and severe threat should be low. Will time peak PoPs early
Monday, and mainly just shower mention. Temps were also lowered a
tad for Monday due to the faster timing.

Behind the front, cool and dry high pressure will quickly build in
and bring back more typical mid-October temps to the region for
Tuesday thru the end of the medium range. Highs will be mainly in
the 60s to lower 70s and lows in the 40s to lower 50s. Although a
few upper 30s will be possible in the highest elevations.


At KCLT and Elsewhere: Much of the terminal forecast area will see
SCT to BKN stratocu in the low VFR range through late afternoon,
with tempos for MVFR warranted at KHKY into the evening. Isolated
convection has been developing within a modestly unstable air mass
over the southern NC Piedmont over the past couple of hours just
southeast of KCLT. This instability does extend into the KCLT area,
and cannot completely rule out a SHRA there this afternoon, but the
probability is around 20%, so nothing worse than VCSH is needed at
the airfield. Convective trends will need to be closely monitored
through early evening with the surface boundary close by.

A cool/moist wedge of high pressure is expected to become gradually
established across the terminal forecast area tonight. Conditions
will deteriorate beginning this evening, with at least IFR cigs
expected by midnight at most terminals. Diminishing visby is
expected to lag the lowering cigs by a couple of hours, but with NE
winds remaining in the 5-10kts range overnight, most terminals
should see visby no worse than 2-3SM by daybreak. KAVL may be an
exception, with LIFR visby (or worse) likely there during the
pre-dawn hours. Otherwise, most terminals could see periods of
-SHRA or -DZ, and this is primarily covered with a prob30.
Conditions should be slow to improve on Friday, with most terminals
likely only seeing IFR/low MVFR conditions by the end of the period.

Outlook: Restrictions/possible showers will continue into Friday
afternoon with the damming in place. Some improvement will return
into the weekend, but conditions will remain moist with morning
restrictions continuing.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High  90%     Med   77%     High  91%     High  83%
KGSP       High 100%     High  86%     High  81%     Med   72%
KAVL       High  95%     Med   73%     Med   64%     Med   78%
KHKY       High  80%     Med   79%     Med   66%     Med   73%
KGMU       High 100%     Med   75%     Med   76%     Med   79%
KAND       High 100%     High  88%     High  81%     Med   79%

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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