Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 091815
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
115 PM EST Fri Dec 9 2016
Cold high pressure will remain in place through the weekend. A cold
front will approach the Western Carolinas late Sunday and move
through the area on Monday, increasing rain chances for early next
.NEAR TERM /THROUGH SATURDAY/...
As of 1250 PM EST: Cloud tops colder than minus 10 C continue to
rapidly diminish in coverage this afternoon along the northern
upslope areas and no further flurries will be featured in the
A shortwave crossing the central Appalachians this afternoon will
move off the eastern seaboard this evening. Upper flow behind the
wave will become increasingly zonal tonight through tomorrow. At the
surface, strong and cold high pressure currently centered over the
confluence of the OH/MS rivers will build east to the Appalachians
tonight through Saturday. A very cold night is expected as low level
thicknesses bottom out overnight and winds go light as the pressure
gradient relaxes everywhere but the highest peaks. Expect mountain
mins well down in the teens, with teens to lower 20s east of the
mountains, but most locations well above record mins for the date.
Weak westerly downsloping flow will continue at 850 mb through
Saturday. A modest upward bump in max temps are expected.
A round of mid level moisture will pass north of the area in the
increasingly zonal flow, but with little impact on sky cover through
.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
At 250 AM Friday: Short range guidance indicates that the center of
a large portion of a Canadian air mass will slide over the CWA on
Saturday. A H5, a broad longwave trof axis will ripple east,
resulting in zonal flow by Saturday afternoon. This pattern should
provide dry and cool conditions across the forecast area. Using a
blend of guidance, high temperatures are forecast to range from the
low 40s within the mtn valleys to mid to upper 40s east of the mtns.
Afternoon RH values may dip into the upper 20s to low 30s.
Saturday night through Sunday, the pressure gradient across the
western Carolinas and northeast Georgia will begin to increase
between high pressure off the mid Atlantic coast and low pressure
over the Great Plains. However, winds should remain light early
Sunday morning under partly clear sky cover. Low temperatures are
forecast to range from the low 20s across the mtns to upper 20s
across the foothills and Piedmont. Develop return flow should
support high temperatures 2-3 degrees warmer than Saturday, with a
15 percent increase in sfc RH. Moisture should arrive late in the
day, supporting increase sky cover across the region and SCHC PoPs
for -RA across the east facing mtn slopes.
Sunday night, S-SW H85 winds are forecast to strengthen across the
southern Appalachians as a cold front approaches from the west. Winds
across the NC mtn ridges may strengthen to 25-35 mph with higher
gusts during the late night hours. Otherwise, the strong llvl flow
should support expanding upslope rain across the high terrain during
the evening hours. Later in the night, a band of pre-frontal rain
should sweep across the region. Forecast sounding across the mtns
indicate a very strong warm nose between 3-5 kft during the
overnight. Most areas should see a cold rain, however, areas across
the ridges of the northern NC mtns may see periods of -FZRA. Low
temperatures will favor the 30s across the region.
Monday through Monday night, short range guidance is mixed with the
placement and timing of the cold front. The GFS indicates that the
front will slip south of the region late Monday and Monday night,
resulting in dry air to build across the mtns. The ECMWF is slower
and stalls the front over the upper Savannah River Valley on Monday,
then lifting north as a warm front Monday night. I will use a blend
of both solutions, keeping SCHC POPs across the CWA with CHC POPs
over the SC and GA Piedmont, increasing during the late night hours.
Temps on Monday are forecast to rise a couple of degrees above
normal during the afternoon, remaining above freezing across the
mtns Monday night.
.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 230 AM EST Friday: Picking up at 12Z Tuesday morning, the
medium range begins at a point of substantial global model
disagreement on the handling of a surface frontal system and
associated precip draped over our region. The drier GFS allows the
front to make a clear passage by Tuesday morning, and is therefore
mainly dry during the day on Tuesday before another low and
associated surface front raise pops once again on Wednesday. The
ECMWF disagrees...with the first surface front getting hung up over
the southern Appalachians and little break in precip between the
first and second lows/fronts. Both models have remained stubborn and
displayed decent individual run-to-run consistency...so as WPC has
done, I have gone with a blend of the two solutions, which seems
reasonable both as far as the progressiveness of the overall pattern
and depth of the second low pressure system.
As far as sensible weather goes, Tuesday through early Thursday
morning feature almost constant high-end chance pops. QPF looks
generally light through the period, regardless of whether there is a
break in precip between surface fronts. Though weak CAA will likely
be ongoing, Tuesday and Wednesday should remain sufficiently warm
that all areas except the high terrain will see liquid precip, with
a brief period of mixed precip/freezing rain possible before
daybreak Wednesday in the mountains. Though more substantial CAA
will set up overnight Wednesday into Thursday morning, the deep
surface low and impressive longwave troughing on the ECMWF seem a
bit too amplified in the context of other guidance and previous
runs. P-type was therefore kept liquid in the majority of the
Piedmont/Foothills, with the mountains receiving light snow as
precip tapers off Thursday morning. The period will begin with
temperatures just above normal...dropping to average mid-week and
then well below average by Thursday as a cold surface high sets up
over the Great Plains.
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: VFR conditions will continue through the
terminal forecast period as cold and dry surface high pressure
builds eastward from the OH/MS rivers to the Appalachians. Early
gusts at KAVL will relax through early evening, with NW to N flow
less than 10 kt otherwise through late evening. Light winds toggle
NE overnight east of the mountains before switching to southerly
near the end of the period. Expect continued light NW to N flow in
the French Broad valley near KAVL through the end of the period.
Outlook: Dry and cool high pressure will move from the Appalachians
off the east coast late Saturday through Sunday. Moisture will
return ahead of an approaching cold front Sunday night through
Monday. A cold rain and associated restrictions are possible with
any minor onset FZRA confined to NC foothills areas Sunday night.
Conditions remain unsettled through mid week as the passing front
stalls just to the south of the area.
18-24Z 00-06Z 06-12Z 12-18Z
KCLT High 100% High 100% High 94% High 100%
KGSP High 100% High 100% High 94% High 100%
KAVL High 100% High 100% High 94% High 100%
KHKY High 100% High 100% High 94% High 100%
KGMU High 100% High 100% High 94% High 100%
KAND High 100% High 100% High 94% 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: