Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171946
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
246 PM EST Sat Feb 17 2018

.SYNOPSIS...
Dry high pressure will bring a break from rain on Sunday.
Temperatures will climb through the middle of next week as another
cold front arrives from Tennessee by Thursday. This front will stall
and keep rain chances in the forecast through the end of the week.

&&

.NEAR TERM /THROUGH SUNDAY/...
Hybrid cold air damming continues from the Blue Ridge south and east
this afternoon, with mainly -RA and drizzle continuing to expand in
response to isentropic lift/upslope flow atop the shallow cool air.
The bulk of this activity should begin shifting more toward the
central NC and Virginia Piedmont toward early evening, as the deeper
isentropic lift shifts northeast and flow above the cool pool
continues to veer. Meanwhile, widespread/heavier precipitation
continues across East Tenn in association with weak surface wave
riding northeast along the baroclinic zone to our west. While it`s
been taking it`s time, this area of rain should move into the NC
mtns by early evening, supporting categorical pops, esp along the TN
border. Guidance continues to suggest diminishing precip potential
this evening across areas from the Blue Ridge south and east, in
response to veering pre-frontal flow in the H9-H8 layer, and
stronger QG forcing passing to our north. Nevertheless, small pops
are still warranted this evening across the Piedmont.

Although temps may initially warm this evening in response to
weakening of the hybrid CAD, they should undergo a slight tumble
late tonight, as cold front sweeps through the area. Min temps are
nevertheless expected to average 5-10 degrees above climo. Initially
weak downslope flow and slow dry air advection and eroding wedge air
mass may yield some fog, perhaps patchy dense fog concerns across
the area from late evening or so through sunrise. Otherwise, pops
taper off after midnight across all but the far western NC mtns,
where some upslope flow and lingering shallow moisture could wring
out some sprinkles (or high elevation flurries).

Insolation and downslope flow should erode any lingering low clouds
and/or fog Sunday morning, giving way to a rare sunny/mostly sunny
day with not-quite seasonable temps (maxes forecast at around 5
degrees above climo) across the forecast area. However, clouds will
once again begin increasing from the southwest by the end of the
day.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 145 PM Sat: Sunday night, high pressure will remain
situated off the East Coast as we remain under a low-amplitude
upper ridge. The frontal boundary along the southern fringe of
the sfc high will pivot north as a warm front, in conjunction
with cyclogenesis in the lee of the Rockies. The warm front will
bring a modest chance of rain to the entire forecast area early
Monday, being most likely in the southerly upslope areas of the
NC mountains. Min temps will be kept close to 15 degrees above
normal by the cloud cover.

PoPs will diminish somewhat as the front exits the area to the
north Monday, with only very weak WAA continuing thereafter. It
still is questionable whether actual CAD will be able to develop
before this occurs. Raw model max temps, especially the NAM/GFS,
seem to reflect the presence of a wedge. With limited precip this is
not a "lock," but abundant cloud cover still is expected. The max
temp fcst has been nudged in the direction of the cooler guidance,
though values are still 2-3 categories above normal. Late Monday
upper heights do rise appreciably, and the sfc high will have
settled into a position well offshore, so it looks unlikely any
wedge would last beyond that time.

Monday night and Tuesday morning we effectively will be in
the warm sector of the frontal system extending from Texas to
southeastern Ontario. Around here, the primary source of forcing
will be upsloping near the Blue Ridge, keeping PoPs too small to
mention over the lower Piedmont. However, guidance indicates an
uptick in moisture late in the day Tuesday which will lead to an
expansion of precip chances at that time. Skies are expected to
remain mostly cloudy; increased thicknesses warrant max temps in
the upper 60s to mid 70s, coming within a couple degrees of record
highs at GSP and CLT.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 2:30 PM EST Saturday: During this part of the forecast cycle
we continue to follow the strong H5 high pressure system along the
southeast coast. It continues to place our forecast area (FA) in a
broad southwest flow aloft. We do note some semblance of the upper
level high repositioning to the south late in the forecast cycle,
with upper level heights gradually falling.

The models look to be in very good agreement Tuesday night and
Wednesday with a glob of Gulf of Mexico moisture being funneled
north into our forecast area. Southwest H85 winds have about a 30 to
35 knot southwest component. Meanwhile we note a very nice signal of
precipitable water values trending up toward 1.50 inches Wednesday.
We will ramp POPS up into the the likely category (or more) later
Tuesday night and Wednesday. This would be in our west facing
upslope areas in northeast Georgia, mountains of upstate South
Carolina and the escarpment of western North Carolina. Meanwhile,
with ripples of energy embedded in a moist environment, we have
chance POPS at other locations, with the lowest POPS east of I77.

We continue to follow a sluggish moving cold front Wednesday night
through Friday. The GFS has backed off on its rapid movement through
our area, while the European has kept pace with earlier forecast
runs. As a result, we shifted higher likely POPS back to the NC
mountains, with lower chance values east. This is tied into a
favorable jet structure and deeper moisture, and local convergence.
As far as thunder possibilities, we note surface based CAPES from
the GFS and European models of 500 to 1000 J/KG both Wednesday and
Thursday afternoon. We have kept a mention in those areas, which is
mainly along and south of I85.

How this all unfolds at the end of the week and into the weekend is
still questionable. The best solution is for the approaching cold
front to lose steam, while its eastern flank flops south into the
northern parts of our FA Thursday night or Friday, with the western
end struggling to make much headway. Thereafter, a push back to the
north is expected. We have tried to capture the essence of the POPS
with high chance values north FA at the end of the work week, and
into the start of the new weekend and lower elsewhere.

Temperatures will remain above normal until late in the work week.
At that point, wherever the front actually stalls, we could have a
very large temperature difference from north to south. We have
started that trend, but did not bite on any of the outlier values.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: IFR cigs have been filling in rather quickly
in response to -RA and DZ across the mountains and foothills early
this afternoon. Once these settle in, they are expected to continue
through at least the afternoon. Meanwhile, IFR/MVFR visby can be
expected to accompany the periods of precip. Meanwhile, KCLT
stubbornly remains  at the high end of MVFR, but as light precip
continues to overspread the NC Piedmont from the southwest, KCLT
should see cigs lower to IFR no later than 21Z. While the precip is
expected to taper off this evening, lingering low level moisture
within cool wedge will continue to pose the potential for BR, while
cigs should remain IFR, if not lower to LIFR. A cold front will push
across the area this evening into the overnight. Winds will shift to
light NW at most terminals (except for the usual up-valley gustiness
at KAVL), with some improvement in flt conditions forecast. However,
the introduction of drier air behind the front will be somewhat
slow, and winds will not be strong enough to strongly mix the
atmosphere. Therefore, restrictions in BR and perhaps low stratus
will remain possible, especially across the foothills and Piedmont
through at least sunrise Sunday. (Confidence in the forecast is
lower than normal from about 06Z-12Z or so Sunday.)

Outlook: VFR conditions expected for the bulk of the day Sunday.
Moisture returns early next week, and the weather will remain
periodically unsettled, with occasional restrictions likely through
much of next week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       Med   70%     Low   56%     Med   68%     High  96%
KGSP       Med   77%     Low   59%     High  82%     High 100%
KAVL       Med   76%     Med   73%     Med   71%     High 100%
KHKY       Med   77%     Low   57%     Med   73%     High 100%
KGMU       Med   72%     Med   76%     High 100%     High 100%
KAND       High  84%     High  83%     High  85%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...Carroll
NEAR TERM...JDL
SHORT TERM...Wimberley
LONG TERM...TS
AVIATION...JDL



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