Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 150912
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
412 AM EST Sun Jan 15 2017

.SYNOPSIS...
A surface front will lift north across our area, allowing well-above
normal high temperatures to return for the end of the weekend.
Another cool high pressure over New England on Monday will force the
front south into Georgia again. The front retreats north again by
mid-week as a cold front approaches from the west. The cold front
may stall near our region late next week allowing rounds of showers
to persist through the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
400 AM EST Update...no need for any sigfnt changes. The atmos has
become rather steady state enabling slow changes to temps/tds. Dense fg
still persisting across NE GA and the wrn Upstate....with little
expansion east. Pops are in good shape across the NC mtns as precip
has increased in coverage with passing ulvl s/w energy.

Dense fog is affecting the wrn Upstate and NE GA as a weak p/grad
and a very moist BL have made for stagnant conds. The dense fog
could work it/s way east through the morning...but should remain in
the SC zones. The DFA may need to be expanded.

A strong subtrop high remains dominant over the SE region and a
building ridge axis will move into GA through the period. Good ulvl
confluence over srn New England will support a 1030 mb sfc high over
ern PA by the end of the period. This high will ridge down into
the CWFA and combine with a moist sta bndry bringing a potential for
-shra mainly across the NC zones overnight. Meanwhile...crossing s/w
energy will maintain showery activity across the NC/TN border and
the central/nrn mtns and fthills this morning. These showers will
dissipate as the s/w pushes east.

Decent afternoon heating will produce low end instability across the
srn NC mtns...where a stray/brief thunderstorm or two will not be
out of the question. Too uncertain for an HWO mention. After the fg
and stcu wane...breaks in the cloud cover and a w/ly compressional
h92 flow will add to a warm sw/ly sfc flow and allow for max temps
well above normal across NE GA...the SC Upstate...and the srn part
of NC. There will be a t/grad to the NE....but temps near 10 degrees
above normal will be possible arnd Statesville/Mocksville. With
another moist wedge building in overnight and dry air aloft
associated with increasing subs...the potential will be good for
dense fg and low stcu through daybreak Mon.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 310 AM Sunday: A split flow regime will be in place across the
Conus at the start of the short term, with a deep trough across the
Baja/southwest U.S. and a ridge across the Southeast. A rather
potent short wave trough will be ejecting across the southern Great
Plains Monday morning, and this looks to be the next potential
significant weather-maker for the East. In the interim, transient
surface high pressure centered over the Mid-Atlantic at the start of
the period will support another intrusion of cool air into our
forecast area on Monday, with considerable uncertainty as to how far
southwest the front will push before stalling in the face of
increasing SW flow. While isentropic lift will increase above the
boundary on Monday, forcing will remain weak, and it`s doubtful that
we will see a great deal of -RA or even drizzle to diabatically
enhance the cold pool. In other words, it appears that this scenario
will be rather similar to Saturday`s "wedge" event, in which temps
ended up being quite a bit warmer than forecast. That being said,
there is enough of a confluent flow aloft to hold the surface high
more or less in place for about 18 hours, so this event may linger a
bit longer. Forecast maxes will range from the mid-to-upper 60s
across the upper Savannah River Valley (and these may well prove too
cool) to around 50 across the northwest NC Piedmont.

Mon night into Tue, S/SW flow continues to accelerate above the
surface as the short wave/attendant cyclone approaches the Great
Lakes. For our area, this will mean increasing mechanical lift
across SW NC and northeast GA, and chance pops for SHRA will be
maintained in these areas throughout this time. Temps should once
again increase to above-climo levels in all areas by the end of
Tuesday, as the surface high supporting the wedge early in the
period will be hundreds of miles off the East Coast by 12Z Wed.

As is to be expected, the deterministic models are struggling in
handling the interaction/potential phasing of the northern and
southern streams across the eastern Conus late in the period,
resulting in significant timing differences regarding the incursion
into our area of the frontal zone associated with the Great Lakes
cyclone. For example, the NAM is roughly 12 hours faster with the
front than the GFS, with the ECMWF somewhere in between. We will
simply advertise pops gradually ramping up Tue night into early Wed,
ranging from low chance across the eastern zones, to likely across
the mtns by daybreak.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 230 AM EST Sunday: Guidance is in agreement on the general
idea that the next week will feature a wet pattern, though the
timing and location details regarding any precip are still on the
messy side. The forecast picks up Wednesday morning at 12Z with a
frontal zone and surface wave in the process of passing through the
region. Fine details as far as timing and spatial QPF distribution
are fuzzy at the moment...but with plentiful moisture and a surface
wedge of cool air in place over the spine of the southern
Appalachians, major global models agree reasonably well on at least
chance pops across the entire CWA during the day on Wednesday.

By Thursday morning, the front will most likely have made it through
the area, though the timing and track of the next upper wave/surface
reflection is uncertain as well. This uncertainty is bred from
differences in the handling of an upper level shortwave crossing the
Appalachians...with a more southerly wave implying a wet Thursday,
while the solution carrying the more northerly track is very close
to completely dry. A multi-model blend compromise was made in order
to reflect chance pops in the grids, and this remains a low-
confidence portion of the forecast.

By Friday, the beginnings of an upper level omega block begin to
shape up over the central CONUS. The southern stream remains active
as an  upper wave or two is allowed to slip under the blocking
pattern Friday into Saturday, and considering the combination of so-
so support aloft with a wedge and surface moisture in place over our
area, at least some isentropic lift should be able to produce precip
both days. The end of the extended period also marks the approach of
a more substantial cold front into the southeast, which should keep
rain chances elevated through next weekend. The entire medium range
is still marked by anomalously high upper heights, so max and min
temperatures will remain generally 10-15 degrees above average,
despite the likelihood of precipitation and the potential for CAD.

&&

.AVIATION /09Z SUNDAY THROUGH THURSDAY/...
At KCLT and Elsewhere: Lowering cigs and vsbys at all TAF sites this
morning. KAND and KAVL are already socked in at VLIFR vsby and shud
remain so thru daybreak outside of TEMPO improvements. With moist
bndry stalled across wrn NC...expect KGSP, KGMU, and KCLT to drop
into or remain in VLIFR cigs with vsbys likely dropping into the IFR
range this morning. A passing h5 s/w could instigate -shra today...so
Prob30s are included at KAVL, KHKY late this morning...and this
evening at KCLT. The hires models and a couple op models are
uncertain on the qpf response...so this scenario could change by the
next TAF set or earlier. Otherwise...expect gradual improvement
across all sites...except KAVL and KHKY...by the afternoon then
deteriorating conds return arnd midnight as a moist wedge of high
pressure builds in throughout the overnight period.

Outlook: Cool high pressure ridges down the East Coast Sunday
evening, bringing yet another round of precipitation and
restrictions to start the work week.

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z
KCLT       Med   78%     High 100%     High  93%     Med   66%
KGSP       Med   60%     High 100%     High 100%     Low   54%
KAVL       Low   50%     Med   66%     Low   57%     Med   75%
KHKY       High  93%     High 100%     Med   64%     High  80%
KGMU       Low   45%     High 100%     High 100%     Low   48%
KAND       Med   61%     High  92%     High 100%     Med   77%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Dense Fog Advisory until 9 AM EST this morning for GAZ010-017-
     018-026-028-029.
NC...None.
SC...Dense Fog Advisory until 9 AM EST this morning for SCZ001>007-
     010>012-019.

&&

$$
SYNOPSIS...LEV
NEAR TERM...SBK
SHORT TERM...JDL
LONG TERM...LEV
AVIATION...SBK



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