Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 081805

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
105 PM EST Thu Dec 8 2016

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


As of 1245 PM EST: Post-frontal NW winds are ushering in the leading
edge of the coldest air of the season across our forecast area. The
broad trough axis remains west of the mountain chain this afternoon
and IR imagery shows clouds thickening overhead east of the trough
as the upper jet axis sharpens up along the Appalachians. The upper
jet axis will gradualy pass off to the southeast overnight as
mid/high level clouds diminish. Shallow moisture at low levels will
intermittently bank up against the NC mountains in NW flow to
produce some shallow stratocumulus in the upslope preferred areas.
Anticipate moisture remaining too shallow for any low end snow PoPs.

The main issue will then be brisk to windy conditions in the high
terrain as the NW flow 850 mb jet ramps up to 35 kt 06z to 12z. This
will combine with very chilly min temps to push northern mountain
and high peak wind chill values down to around minus 5 C. A wind
chill advisory could be needed.

The shallow upslope moisture and pressure gradient in the NW flow
will gradually relax through the day on Friday. Some weak lee
troughing will likely develop during the day as downsloping
continues. Temperatures will likely not recover above freezing in
many NC mountain locations and lower to middle 40s maxes are likely
east of the mountains Friday afternoon.


As of 230 AM EST Thursday...Quasi-zonal flow will persist in the
upper levels across the entire CONUS during the short term. At the
surface, a cold high pressure system will gradually shift east from
the Mid-MS Valley to the Mid-Atlantic Coast by 12Z Sunday. Skies
will be mostly clear thru the period, with temps well below normal.
Highs will range from the 20s to mid 30s in the mountains Friday, and
mid 30s to lower 40s Saturday. The Piedmont will see temps in the
40s both days. As for lows, temps will be in the teens in the
mountains Friday night, and 20s Saturday night. In the Piedmont,
lows will be in the 20s both nights.


As of 250 AM EST Thursday: The first part of the ext range features
more zonal flow than the previous package as the models have slowed
the development of a midwestern h5 s/w. This has the effect of
keeping the deeper moisture west of the NC mtns...esp during the day
Sun while a weak sfc low develops through the OH Valley. Pops were
adj down a bit to lower and mid range chances overnight as the best
moisture flux may not reach the mtn spine zones till 12z. Enuf cold
air will be in place for -snsh or a ra/sn mix across the nrn mtns
with little accums before sfc temps warm above freezing. The GFS
model soundings dropped across Avery Co show little moisture
available while temps are favorable for wintry precip...while the
ECMWF soundings show very dry low levels. There is a strong warm
nose however...and if moisture levels pan out earlier than
fcst/d...areas of -fzra will be possible before daybreak.

There is a general consensus with the eastward progression of the
cold front thru the day Mon...however the guidance has wholly
varying ideas with the amount of available moisture. For now will
expect a low to mid chance pop event over the cwfa with lingering
weak nw flow -snsh overnight. Not sure what to think about Wed with
the models widely differing in their synoptic/hemispheric solns. The
GFS is much more amplified than the ECMWF and quickly induces
cyclogenesis up toward the glakes region while pushing a strong moist
cold front across the area thru 12z...and very cold arctic high
pushing in behind that. The ECMWF on the other hand...has more zonal
flow and a weaker sfc high...while a stationary front remains to our
southeast. Thus...with high uncertainty will keep pops limited to
slight or low-end chances thru the last period....mainly nw flow
-snsh for now. As far as temps...the period begins a couple cats
below normal Sun...rebounds to arnd normal Mon-Wed...then the colder
arctic air pushing in Wed night.


At KCLT and elsewhere: High clouds will remain expansive across the
terminal forecast area as an upper jet moves over the mountains. The
clouds will diminish from west to east overnight as the jet axis
slips east. A few stratocumulus could develop in upslope near the TN
border, but these will likely not reach KAVL. Anticipate low ends
gusts across the region this afternoon with mixing and cold
advection, with the highest gusts into the mid 20s at KAVL. KAVL
gusts could well linger through the end of the period of Friday as
well as cold advection continues. Otherwise, VFR conditions are
expected throughout.

Outlook: Drier and much colder conditions will continue through most
of the weekend. Another front will approach the area Sunday
afternoon, with uncertain chances of precipitation and restrictions.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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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