Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 101131

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
631 AM EST Sat Dec 10 2016

Cold high pressure will remain in place through the weekend. A cold
front will approach the Western Carolinas late Sunday and move
through the area on Monday, increasing rain chances through mid-
week. Cold air will approach the area again by the end of the next
work week.


As of 625 AM, A quasi-zonal upper flow pattern will dominate through
the end of the near term period. Other than some cirrus that may
spill into the area late this afternoon through tonight on the
southern side of the upper jet axis to our north, skies are expected
to remain largely cloud-free through the period. The center of broad
surface high pressure will begin oozing east of the forecast area by
this evening, supporting establishment of weak return flow. However,
thickness values are forecast to be quite similar to what was
observed on Friday, and the return flow should not be sufficient to
warm the air mass significantly over what was observed yesterday.
The only exception will be over the mtn valleys, which in the
absence of cold advection will see max temps of 5-10 degrees above
Friday`s highs. Tonight`s mins should be warmer than this morning`s,
but still 5-10 degrees below climo.


As of 2 AM Saturday: On Sunday, the center of a large low pressure
system is forecast to track from the Great Plains across the Mid
West. The warm front associated with the low is expected to remain
north of the Ohio River Valley as a cold front sweeps east across
the mid Mississippi River Valley. This pattern will support
strengthening llvl winds across the southern Appalachian region. By
21z, short range guidance indicates that H85 winds from the SSW are
forecast to range from 30-40 kts. Late Sunday afternoon and evening,
moisture from the Atlantic is expected to increase across NE GA and
the western Carolinas. The strengthening llvl flow will likely yield
weak isentropic lift, nicely defined around 290K. Based on the
timing of moisture, upslope flow, and isentropic lift I will time
PoPs to increase after 21z with SCHC covering most of the CWA.
Precip will fall as rain.

Sunday night through Monday, the cold front will slowly slide across
KY and TN, with a large band of moisture spreading across the
southern Appalachian region. H85 winds ahead of the front are
forecast to peak around 50 kts at daybreak on Monday. I will
indicate gusts around 50 kts across the ridges early Monday morning.
Coverage of rainfall should increase through the overnight hours,
largely supported by a region of H3 jet divergence and passing H5
vort max. I will indicate a range of PoPs Mon morning from cate
across the TN border counties to CHC east of I-85. Morning mtn.
temperatures may support a few periods of FZRA, but temps are
marginal for any accums exceeding a trace. In fact, temps may begin
to warm across the high terrain before sunrise Monday. Using a blend
of temp guidance, highs on Monday are forecast to range from the low
to mid 50s across the mtns to the low 60s across the Lakelands.

Monday night, the GFS continues to indicate that the cold front will
push south of the forecast area by 12z Tues. However, the ECMWF
continues to indicate that the front will stall along the southern
tier, then may begin to lift north as a warm front as a wave
develops over MS. I will continue to use a blend of both solutions,
holding on to clouds and CHC to SCHC PoPs. Lows in the upper 30s to
mid 40s should support only a chilly light rain.


As of 230 AM EST Saturday: The trend of medium range
uncertainty/guidance disagreement continues with this forecast
package. The extended period picks up Tuesday morning with a slight
discrepancy between the global models, with the ECMWF carrying a
more potent H5 shortwave and therefore a slightly more progressive
surface frontal passage. This leaves the ECMWF solution drier
(though generally in better agreement with the wet GFS), while the
GFS struggles to clear the front out of our region and maintains
upglide-induced precip through the day on Tuesday across most of the
area. Because of the continued model inconsistencies, little change
was made to the previous GFS-weighted forecast, and chance pops
(with light QPF) were maintained for the entire area through
Wednesday, tapering off by daybreak Thursday as higher elevations
see a change to light snow or a rain/snow mix. A transient surface
high will build into the area briefly during the day on Thursday,
providing something of a lull in pops for the Foothills and Piedmont
by the afternoon.

Friday...the forecast gets more interesting. Though global model
solutions are beginning to resemble one another, there are still
discrepancies in the details that could have a very large impact on
sensible weather for much of the forecast area. Cold high pressure
will settle somewhere over the Appalachians...possibly leading to a
CAD scenario of some type, but the location, speed, and intensity of
this high are still quite uncertain. The ECMWF, with the stronger
and slower "Classic CAD" parent high, has a more robust-looking
damming event set up over the southern Appalachians Friday and into
Saturday. The GFS maintains a weaker, more progressive high (more of
an "in-situ" damming type), and keeps the wedge of cold air somewhat
weaker and erodes it faster. As this possible CAD event takes
shape, a surface wave will lift out of the Gulf of Mexico. The
strength and track of this wave remain muddled in guidance once
again, though both major models do provide some decent moisture over-
running a wedge of cold air Friday night and into Saturday morning.
This could present some p-type issues as light precip spreads across
much of the area, though considering the lead time and inconsistent
model performance up to this point, much could change in coming runs
and forecast iterations. Confidence in the forecast at the end of
the medium range remains low, and it still remains to be seen
whether or not any of the aforementioned p-type issues materialize
at all.


At KCLT and elsewhere: VFR conditions will continue through the
terminal forecast period as cold and dry surface high pressure
continues to dominate. Light NW (KAVL) to NE (most mtns/fhills
terminals) will continue through much of the morning, but are
expected to gradually back around, likely becoming SW at most
terminals by mid-afternoon, as the center of high pressure pushes
east of the area. Otherwise, SCT mid and high level clouds may begin
increasing across the area this evening.

Outlook: Moisture will return ahead of an approaching cold front
late Sunday through Monday with mainly a cold rain and associated
restrictions expected. Conditions remain unsettled through mid week
as the passing front stalls just to the south of the area.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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:




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