Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210801

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
301 AM EST Sun Jan 21 2018

High pressure will move offshore today bringing warm southerly flow
to the region. A wet cold front will cross the region late Monday
into early Tuesday. In the front`s wake, dry high pressure will
overspread the region and persist through the work week.


A weak ulvl trof will cross south of the fcst area today with
little to no impact on the sensible wx. There will some increasing
cloudiness over the higher terrain as moist h85 flow is lifted in
weak topo forcing. No precip will occur with this moisture thru the
period. At the sfc...high pressure will broaden out over the region
and make for weak sfc flow and dry conds. Soundings show PWAT values
remaining steady arnd 0.6 inches thru tonight and nondescript flow
thru a deep layer. There will be Ci passing throuhout the day and
some lower clouds by this evening and overnight. With good
insol...expect max temps to reach a few cats or arnd 10 degrees abv
normal this afternoon. Overnight...steep sfc-based inversions will
set up as the llvl flow decouples and goes calm most locales. This
could allow for shallow ground fg arnd daybreak as crossover temps
are reached. Mins will be a little warmer with some increase in td/s
this afternoon...likely remaining arnd 8 degrees abv normal.


As of 230 AM EST Sunday: A deep closed upper low will be working its
way across the Plains as we move into the short term, with a
strongly occluded surface front sweeping toward the MS Valley. New
guidance has slowed down onset of precip just a tad, weakening the
isentropic upglide ahead of the system before upslope takes over
Monday across the SW mountains. The front itself will be fast-
moving, with a quick shot of QPF pushing through mainly overnight
Monday night, but even so storm totals are less than 1" even in the
favored upslope areas. Deep-layer shear with the storm is very
impressive, with very strong upper jet approaching 150kt and an area
of channeled vorticity wrapping around the upper low, leading to
widespread values of over 90kt 0-6km shear. However, instability
leaves something to be strongly desired, with SBCAPEs struggling to
get over 100J/kg with the nocturnal frontal passage. Have continued
mention of mainly slight chance thunder just due to the strongly
dynamic system as the LLJ and lapse rates increase.

With the very strong winds aloft, momentum transfer down to the
mountaintops will be a concern, especially as lee troughing should
hold in place during the day on Tuesday which will serve to keep the
mountain pressure gradient stronger. Current forecast winds/gusts
are generally below advisory criteria, but the trend is up, so in
coordination with RNK, will highlight the northern mountains for
winds in the HWO.

The system lifts out pretty rapidly Tuesday, and dryslotting moving
in just after daybreak will serve to limit any concerns with
snowfall, though temperatures will drop in the CAA across the
mountains to near freezing Tuesday morning. Have kept mention of
light snow across the higher elevations of the mountains, but with
moisture decreasing, only minor accumulations expected at this time.
For Tuesday night, really only the GFS shows some brief NW flow snow
showers as temperatures drop into the 20s by the end of the period.


As of 245 AM EST Sunday: Pretty quiet for most of the extended in
the wake of the short term front. Cool high pressure centered to the
west will slowly slide east and will dominate the area; despite
nearly full sun, temperatures should be near seasonal for most of
the period. A sharp shortwave passes to our north Wednesday night,
and the surface high crosses the Appalachians behind this wave while
ridging aloft builds in toward Friday. Increasing amplitude of the
wavetrain expected at the end of the week as a strong trough digs
into the Rockies and a shortwave lifts into the Plains. The surface
high moves offshore bringing a moist SE fetch to the area, and lift
ahead of the deep trough will allow precipitation to lift north up
the MS Valley under the diffluent flow aloft. Pops increase
significantly Friday night as isentropic upglide increases, and the
forecast does (for now) have some overlap of cooler surface temps
and the incoming moisture to allow for some brief rain/snow mix
Saturday morning. QPF increases more significantly during the day on
Saturday and into Saturday night and should continue beyond the end
of the period.


At KCLT and Elsewhere: Deeply dry conds will prevail across all TAF
sites thru the period. Very good radiational cooling overnight will
develop steep llvl LRs and increase the potential for BR/FG. Current
Tdd/s are running a little low and with the recent snow melt and
moist flux...the sfc layer shud support ground fog arnd daybreak.
Most guidance is rather optimistic with went with high-end
MVFR all sites. Winds will remain weak and generally s/ly to sw/ly
thru the period as a sfc high broadens over the region.

Outlook: Potential for MVFR/IFR conds Monday evening into Tuesday
with an approaching cold front. Dry high pressure will dominate thru
the remainder of the week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
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  80%
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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