Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 071947

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
247 PM EST Wed Dec 7 2016

A reinforcing cold front will move through the area overnight.
In its wake, cold high pressure will gradually spread across the
region, bringing the coldest temperatures so far this season.
Another cold front will approach the Western Carolinas late Sunday
and move through the area early Monday with drier high pressure
in its wake.


As of 130 PM EST Wednesday: Visible satellite imagery shows that low
clouds continue to scatter and thin out across most of the Upstate
and CLT metro area, but remain trapped under the inversion in the
Savannah River basin. The general idea is that clouds might develop
and expand more as the afternoon progresses, but the model guidance
appears to be overdone with the extent of the low level moisture and
thus the development of patchy light precip. The fcst will remain
dry. Temps are generally in line with expectations.

The upper pattern is expected to slowly evolve over the next 24
hours as a large closed low moves across eastern Canada bringing the
axis of the long wave trof in from the west. An old weak boundary
strung out near the mtns will be re-invigorated to some extent as a
large cold continental high drops down into the nrn Plains and
increases the pressure and temperature gradient. Meanwhile, a weak
sheared short wave and weak upper divergence will move over the top
of this feature. The models suggest that some light precip might
develop along the boundary after midnight, but particularly around
daybreak, mainly across the mtns. The previous fcst went along with
that idea so little change is necessary in that regard. The timing
is such that if any precip falls, there could be a brief window of
wintry p-types mainly at high elevations. Fcst soundings suggest
this could be a mix of valley rain, high elevation snow, and
mid-slope freezing rain. Either way, it will not amount to much. In
the first place, the chance is only about 20 pct. Secondly, amts
will be no more than a few hundredths. Temps will remain above
freezing in most places away from the TN border. The front will push
southeast on Thursday with drier air spreading across the region as
the high builds in. Downslope east of the Blue Ridge and sunny sky
should afford us one more day with near normal temps.


At 2 PM Wednesday: On Thursday night a positively tilted low
amplitude upper trough will extend from Quebec to Mexico. This
feature slowly progresses to the Eastern Seaboard and the Gulf of
Mexico by Friday evening. Channeled vorticity rounding the base of
the trough will pass north of our area on Friday. The upper pattern
deamplifies such that by Saturday nearly zonal flow extends across
the CONUS.

At the surface, on Thursday night dry air will move into our area in
the wake of a departing cold front. Although model guidance does
show a shallow moist layer near the TN border at around 875 MB
overnight, confidence is low that any snowfall would result. A tight
pressure gradient behind the front will allow winds to only slowly
diminish through Friday. A surface high will cross the Southern
Appalachians late on Saturday. Temperatures will run well below
normal in the cooler air mass behind the front.


As of 245 PM EST Wednesday:  The medium range fcst period kicks
off on Saturday evening amidst zonal flow aloft across much of
the eastern CONUS, while the surface pattern is to be highlighted
by a broad/cold high pressure centered over the southern/central
Appalachians.  Pattern evolution through the period will feature
amplification of the synoptic pattern by way of a digging H5
shortwave into the central plains states, which will drive a
surface cold front into/through the MS river valley into Sunday.
Guidance is rather discontinuous with regards to timing of the
frontal passage through northeast GA and the western Carolinas
with ECMWF being slower and warmer than the GFS.  Nevertheless,
wintry precip is possible in the form of high elevation snowfall
and/or sheltered valley icing into Monday morning, with perhaps
better chances for mtn snowfall being on Monday night if the ECMWF
verifies.  Cool/dry high pressure to prevail behind this system,
into Wednesday/Thursday before another cold front pushes into the
region. As with the aforementioned system, the op guidance is still
out of phase thus timing is yet to be determined, however there
seems to be some convergence in that another round of wintry precip
is possible late in the week as an arctic airmass surges southward.


At KCLT and elsewhere: VFR for the remainder of the daytime hours. A
stratocu deck will continue to develop and expand underneath a
capping inversion. The cloud base should be generally in the 040 to
060 range, so have opted for 050 at KCLT. Went prevailing BKN to
start but think we will see some SCT holes in the ceiling through
late afternoon. Wind should be light, favoring a switch to SE.
Another round of moisture ahead of the next front will follow
quickly, with low clouds expanding and thickening more this evening.
The guidance is on board with cigs eventually dropping to MVFR along
with the possibility of some light fog, but this was pushed back
until after 06Z outside the mtns. The passage of a front and a weak
upper wave may help to develop some light precip, mainly over the
mtns and foothills, around daybreak. This could produce some wintry
precip at KAVL, but the chance is too low to mention above a VCSH.
As the front passes, low clouds will get moved to the east and wind
will veer to the NW and strengthen.

Outlook: The front will push through the area by Thursday afternoon,
with drier and much colder conditions into the weekend. Another
front will approach the area Sunday afternoon bringing a chance of
precipitation and restrictions.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High  84%     High  90%     High  90%
KGSP       High  91%     High  84%     Med   77%     High 100%
KAVL       High  87%     High  92%     High  80%     High 100%
KHKY       High 100%     Med   75%     Med   67%     High 100%
KGMU       High  94%     High  86%     High  83%     High 100%
KAND       High 100%     High  85%     High  84%     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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