Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 131942

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
242 PM EST Fri Jan 13 2017

A slowly moving cold front will settle southward across the region
tonight through Saturday as cool high pressure ridges down from New
England. The resulting wedge front will oscillate northward and back
southward Sunday through Monday before returning solidly northward
on Tuesday. A cold front will approach from the west during the
middle of next week.


As of 2:30pm Friday:  Temperatures have warmed nicely this
afternoon, especially in Piedmont areas that have not seen much
cloud as yet, with high temperatures surpassing records for the
date at some stations.  Saturday will be cloudy and considerably
cooler as a back door cold front/wedge front with a cold pool and
dense low clouds behind it flows across the area from northeast to
southwest tonight and Saturday morning.  Some light, stratiform
precip is also nicely depicted by high-res models behind the front
through Saturday morning.  With clouds and cool-advection, Saturday
temps. will have a narrow range from the upper 40s for lows in the
mountains to upper 50s for highs in Piedmont areas.

Parent surface high that supports northeasterly cool flow behind
front Saturday morning gradually dissolves Saturday afternoon and
dense low clouds associated with cool pool behind the front may
begin to erode Saturday afternoon or evening, with Sunday seeing a
warm-up back to above normal temps.

The balance of Friday will continue with increasing clouds and light
winds, and a few isolated light showers, with denser low clouds
coming with the back door front early Saturday morning.  Some
showers aided by westerly 850mb flow along the Southern Appalachians
will continue this evening, being joined by some light showers
behind the back door front later.  Upper flow remains zonal, with a
large upper high over FL and the Gulf Coast.

Some marginal convective instability has developed over western
parts of the CWA with MUCAPE to 250 j/kg.  Current radar and
satellite do not show any convective activity so far, beyond some
low-top stratocu.  However, some of the widely scattered showers
could become stronger over the next few hours before diurnal cooling
rapidly eliminates this marginal instability.


As of 200 PM EST Friday: A flat upper level ridge will be in place
across the southern tier of the central and eastern CONUS the latter
half of the weekend as a deep, closed low lifts from western Mexico
across West Texas. Deeper moisture feeding east of this system will
spill over the northern periphery of the ridge and stretch out close
to western North Carolina Saturday night into Sunday before
retreating northward Sunday night. Meanwhile, at the surface, weak
cold air damming may still be in place Saturday evening. The wedge
boundary will begin a period of oscillation, with some northward
movement through Sunday and then a possible backdoor retreat into
the area once again Sunday night as surface high pressure slides
east from the Great Lakes to the East Coast. Forcing over the
oscillating wedge appears quite limited during the period, but any
chance of weak moist upglide will be best over western NC.

A highly amplified pattern will remain in place on Monday with the
reinforcing trough axis gathering over the plains states while a
ridge builds over the southeast. Transient northern tier upglide may
continue into Monday and this could help pull the wedge boundary
back southward across our region. Any QPF through the period will be
light. Temps should see a smaller diurnal range across the more
wedge-impacted northern tier and more warming and less precip threat
across the southern tier.


As of 240 PM Friday, confidence has taken a hit for the medium range
as models are in flux with the potential weather systems moving
through the area. The ECMWF keeps the upper ridge over the eastern
CONUS in place on Tuesday while the GFS has it weakening. This
allows an upper low to cross the Great Lakes Tuesday night with its
trailing trough to cross our area late Tuesday night and early
Wednesday. The ECMWF brings the trough through late Wednesday and
early Wednesday night. The ECMWF keeps the flow zonal for Thursday
with a dampening short wave crossing the area. The GFS shows flat
ridging with a weak short wave. Both models show a progressive upper
low crossing the area Friday, but the ECMWF shows it weakening while
the GFS is stronger.

At the surface, the damming high over the area weakens Tuesday as a
cold front approaches from the west. The ECMWF brings the front into
the area Wednesday while the GFS moves it through Tuesday night. The
ECMWF stalls the front just to our south through Friday with some
weak moisture return and precip over the front. The GFS is dry for
Wednesday and Thursday with moisture and precip returning late in
the day on Thursday and continuing into Friday in developing low
level southerly flow. Given all these differences, have gone with a
mix of guidance blend and national center guidance. This means
slowly increasing PoP Tuesday and Wednesday. PoP drops off a little
Thursday into Friday but remains in the chance range. There is also
little to no agreement on whether any thunderstorms can develop, let
alone severe storms.

Highs 5 to 10 degrees above normal Tuesday rise to 10 to 15 above
normal for Thursday and Friday. Lows Monday night 15 to 20 degrees
above normal rise to around 20 above normal for the rest of the


AT CLT: Winds are currently light and variable with a WSW bias.
Winds are expected to continuing veering, though remain light,
becoming ENE by around 3Z when a back door/wedge front passes
through, bringing low-clouds with it.  SCT to BKN Upper clouds down to
FL100 are current spreading eastwards towards the area on westerly
upper flow.  Low clouds and CIGs will commence with frontal passage
after 3Z, along with some chance for light rain.  BUFKIT soundings
show thick low clouds from near the surface to 9000 ft AGL Saturday
morning, which may begin to reduce in the afternoon as cold surface
air erodes.  There will be some reduction in visibility at times on
Saturday, but the main limitation is expected to be very low CIGs.

Elsewhere:  Upper clouds down to FL100 are currently spreading from
west to east, and should cover all areas in BKN to OVC tonight.
Meanwhile, a cold frontal boundary associated with cold-air-damming
will enter the region from the northeast after 3Z, spreading across
the area through Saturday morning.  Surface cold-air will be
associated with think low-level cloudiness from near the surface up
to 9000 ft, with some light rain possible behind the front.  CIGs
down to LIFR are anticipated in most areas.  Some reduction in
visibility is also anticipated, though low CIGs will have a greater
impact. Winds will remain light and variable, with more reliable ENE
winds for awhile behind the front Saturday morning.

Outlook: Cool high pressure ridges down the East Coast over the
weekend, bringing chances for precipitation and restrictions.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High  85%     High  98%     High  83%
KGSP       High 100%     High  96%     Med   66%     High  93%
KAVL       High 100%     High  80%     High  96%     High  96%
KHKY       High 100%     Med   78%     Med   67%     High  90%
KGMU       High 100%     High 100%     Med   76%     High  93%
KAND       High 100%     High 100%     High  80%     High  93%

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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