Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 182121

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
421 PM EST Sat Nov 18 2017

A cold front crosses our region tonight with breezy to Windy
conditions expected on Sunday.  Cool high pressure will be in
control for much of the week. Another cold front arrives next


As of 415 PM EST Saturday:  Continuing to watch a line of frontal
convection working it`s way through the OH valley at this time.
Closer to home, gusty sw winds prevail amidst increasing cloudiness
from the west.  Starting to see increasing echo coverage across
portions of the Upper French Broad valley, which correlates well
the previous fcst pop trends therefore no changes were made on
that front.  Did tweak hourly t/td and sky to align with recent
trends, with no other changes made.

Previous Discussion:  Increasingly moist/strengthening SW flow will
continue into the evening hours in advance of a strong cold front
that will march across the TN Valley over the next few hours. Still
wouldn`t rule out some showers developing in response to mechanical
lift near the intersection of GA/NC/SC late this afternoon into
the early evening, and small pops will continue to be featured
there through that time.  Pops then ramp up steeply across western
areas by late evening, as strongly forced frontal band should be
knocking on the back door of the CWA by 10 pm-ish. If anything,
short term guidance has backed off a bit on the already-low sbCAPE
values forecast within the frontal convergence zone as the band
begins its trek across the southern Appalachians. However, shear
values will be more than adequate for a marginal high shear/low
CAPE severe threat, with perhaps a damaging wind gust or two,
and/or even a brief QLCS spin-up not out of the question. This
threat should mainly be confined to areas roughly west of I-26
this evening, as veering flow across the frontal zone will result
in weakening convergence as well as shear parameters, while the
sbCAPE is also forecast to diminish even further with time as the
front crosses the CWA.

While winds will be howling (gusts as high as 60 mph) this evening
on the exposed peaks and ridge tops of the southern Appalachians,
winds elsewhere should be quite tame until fropa occurs btw 03-09Z.
Strong winds should then descend into the lower elevations within
the cold advection regime. While advisory criteria gusts will likely
be hard to come by in the mtn valleys even then, guidance tends to
under-do the winds for the first 1-2 hours post-fropa during strong
cold-advection surges, so would expect at least occasional advisory
criteria gusts in the lower elevations during that time. Meanwhile,
the high peaks and ridge tops should continue to howl, with gusts
as high as 60 mph likely occurring through tomorrow morning.
That being the case, see no reason to deviate from the current
wind advisory.

Snow levels drop quickly across the high terrain from late evening
on, while a moist NW flow  quickly develops into the southern
Appalachians. Expect scattered showers, with high elevations snow
showers to develop late tonight/early Sunday. However, ingredients
are such that accumulating snowfall will be difficult to come by,
with moisture depth and residence time of moisture coincident with
cold air being the primary limiting factors. As such, any accums
should be limited to elevations above 4000` near the TN border,
if not 5000`, and even there less than an inch is expected.

Clearing quickly occurs Sunday morning, with most locations
(the TN border area being the primary exception) likely seeing
clear/mostly clear skies by sunrise. Max temps will be 10-15
degrees below normal in most areas.


As of 130 PM Saturday: Not much going over the short range period.
The models are in good agreement with the synoptic pattern which
features a 1026 mb sfc high traversing the area west to east while
subs remains strong thru most of the period. This high will center
off the the Atl coast early Tues...however strong llvl ridging will
remain over the FA Tue morning into the early afternoon. Thus...very
little moisture flux will be available before the ridge breaks down
in the afternoon as a low pressure trof advances from the west.
Soundings show little precip potential across most areas except
within an isent lift area aligned over the srn BL and srn Balsams
later Tue. Not expecting much precip with this scenario as deep
layered lift remains low-end as a broad area of vort energy
producing mainly upper and mid level clouds crosses a slowly
moistening BL.

Max temps will be held about 5-7 degrees below normal both days as a
llvl thermal airmass is slow to modify. Mins will also be about 10
degrees below normal with freezing conds supporting a wintry mix
over the sw/ly upslope zones early Tue...however advecting llvl
moisture will likely be too late for any sensible wx to materialize.


As of 215 PM EST Saturday: The medium range period around the
Thanksgiving holiday remains uncertain across the southeast due to
the continued run-to-run inconsistencies among the models. Nearly
all solutions have a broad upstream trough at the start of the
period over, or just east of, the MS river valley Tuesday night.
Differences then develop quickly through Wednesday. In a continued
role reversal from previous day solutions, the 12Z GFS now features
a strong cutoff low developing over the Deep South. The 00Z ECMWF
solution featured a more progressive open trough, however, the 12Z
data has trended back toward the deeper southern tier GFS low for
Wedneday night through Thursday. Fortunately, even the deeper,
slower solutions keep the abundant moisture shunted mainly toward
the coastline Thursday as the northern Gulf of Mexico surface low
redevelops over the southeast coastal waters. Will keep mainly
slight chances for light rain going across the southeast parts of
the forecast area for Thanksgiving day.

Deep layer northwest flow should set up behind the departing system
on Friday with lee troughing developing in surface high pressure
over the southeast. A strong cold front will then approach from the
northwest late next Saturday but with dry conditions likely
persisting through late day. Temperatures will gradually trend back
up toward climatology just ahead of the approaching front.


At KCLT and Elsewhere: MVFR stratocu is expected to continue
lifting/scattering through mid-afternoon, and there appears to be
little chance of restrictions at most terminals from that point
through early evening.  Cigs are then expected to lower to MVFR at
most terminals mid-to-late evening. SW winds will remain gusty at
most terminals into the evening hours, as a rather strong cold front
approaches the area from the TN Valley. A quick-moving/weakening
band of showers is expected to accompany the front across the
terminal forecast area from late evening into the pre-dawn hours.
Shower chances will decrease from west to east, so only PROB30s are
advertised at KCLT, while tempos are forecast elsewhere. As the
showers/frontal zone passes over the terminals, skies will clear
while winds will quickly shift to the W/NW and remain gusty. Gusts
may actually be quite strong in the 1-2 hours after fropa. Winds
will gradually diminish at most terminals (the primary exception
being KAVL) through the morning hours, but should remain at around
10 kts through the end of the period.

Outlook: Expect VFR from Sunday morning through at least the middle
of next week, under the influence of cold continental high pressure.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High  87%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  94%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     Low   58%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  94%     High 100%     High 100%     High  83%

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:


GA...Wind Advisory until noon EST Sunday for GAZ010-017.
NC...Wind Advisory until noon EST Sunday for NCZ033-048>053-058-059-


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