Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180532

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1232 AM EST Sun Feb 18 2018

Dry high pressure will bring a break from rain today.
Temperatures will climb through the middle of the week as
another cold front arrives from Tennessee by Thursday. This front
stalls out and lingers just north of our area next weekend. Chances
for rain remain in the forecast each day.


As of 1230 AM: Precip continues to move out of the area with dry
conditions expected for much of the rest of the night. Gusty winds
will continue across the mountains. Expect the current patchy dense
fog to dissipate early this morning but return across much of the
area by daybreak.

Otherwise, 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


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.


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.


At KCLT and elsewhere: IFR outside of the mountains will briefly
scatter out early this morning before fog returns before daybreak.
Expect MVFR tempo LIFR. KAVL will see low VFR with some MVFR through
daybreak. Expect conditions to improve quickly after daybreak with
mainly cirrus until evening when low VFR then MVFR moves in from the
SW. Precip will be possible overnight. Light NE to E winds continue
through the period. KAVL the exception where gusty NNW wind becomes
light after daybreak. Wind turns SSE for the afternoon and evening.

Outlook: The weather will remain periodically unsettled, with
occasional restrictions likely through much of next week.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High  93%     High  94%
KGSP       High 100%     High  94%     High 100%     High 100%
KAVL       High 100%     High  81%     Med   71%     Med   69%
KHKY       High 100%     High 100%     Med   64%     High 100%
KGMU       High 100%     High  94%     High 100%     High  81%
KAND       High 100%     Med   69%     Med   64%     High  94%

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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