Area Forecast Discussion
Issued by NWS Greer, SC

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504
FXUS62 KGSP 212359
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
659 PM EST Sun Jan 21 2018

.SYNOPSIS...
High pressure will move farther offshore tonight and tomorrow.
A wet cold front will then cross the region later tomorrow and
into early Tuesday. In the front`s wake, dry high pressure will
overspread the region and persist through the work week.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 640 PM: Changes to the fcst at this update are mainly TAF
related. Temp trends are currently on track except some of the
mountain sites are cooling much more rapidly than anticipated,
apparently due to drainage. Cloud cover has been increased over
the mtns late tonight after low-level isentropic analysis and in
light of cirrus seen on satellite upstream of the upper ridge axis,
which will be overhead by then. Patchy fog is still anticipated due
to abundant sfc moisture overnight, though the guidance sources
showing areas of dense fog developing appear to have analyzed
an unrealistic amount of snow cover, and are thus being
discounted.

Synoptic pattern has a surface high just southeast of the area,
with a broad,flat upper high aloft. Conditions will remain
warm and mild through most of Monday in most areas, with clouds
and some showers increasing from the west Monday afternoon as
upstream system advances.  Good dynamics with this system should
squeeze-out some precipitation, but system is fairly quick moving
with storm-total precipitation around a quarter to a half inch of
rain. Low-level moisture advances Monday afternoon ahead of a major
front that reaches the NC/TN stateline area Monday night. Southerly
to southwesterly flow into the mountains is expected to result
in some light showers by Monday afternoon, with more widespread
rain Monday night into Tuesday morning.  850mb flow also increase
ahead of the front to 40kts Monday evening, which could give some
elevated wind and gusts over mountain peaks.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 200 PM EST Sunday:  The short term fcst period kicks off with
a bang Monday night as a deep closed upper low works through the
mid/upper MS river valley, while it`s associated surface trof/front
complex drives eastward across the OH/TN Valleys.  Out ahead,
persistent sly/sely H850 flow into the southern Appalachians will
warrant hefty pops along the s/se facing slopes of the Blue Ridge
given good moist upsloping potential.  As the entire system shifts
east through the night, the front will cross the region thus
warranting high end pops regionwide, with overall fcst QPF generally
in the half inch range.  That being said, the dynamics associated
with this front are quite strong given a potent low/mid level
jetmax, with wind profiles being veered overall.  As for
instability, a Bufkit profile analysis indicates better lapse rates
in the GFS vs the NAM in the mid/upper levels which in turn yields
some marginal elevated instability.  Likewise, although very
minimal, there is at least some positive sbcape which could promote
a deep enough mixed layer to mix into the aforementioned potent
~55kt llj.  All said, given the nocturnal nature yielding minimal
CAPE as well as veered profiles, would be suprised to see any deep
strong/severe convection.  Thus, for now the fcst will feature
increased pops in association with the fropa into Tuesday morning
with pops tapering into the afternoon amidst clearing and caa.  One
other note regarding Monday night, the sly llj will likely yield
gusty winds across the higher terrain, upwards of 45-50mph,
therefore a wind advisory cannot be ruled out.

Moving on, once the upper trof axis swings through the region into
Tuesday afternoon, profiles will be solidly veered nwly, which
combined with caa and ample llv moisture could lead to a brief round
of nwfs Tuesday night into Wednesday.  Therefore the fcst will
feature such by way of slight chance pops along the TN line, with
little accumulation fcst at this time.  In addition to the light
snowfall, winds will once again be elevated amidst the nwf pattern
across the high terrain on Tuesday night, thus another wind advisory
may be needed, especially over the northern mtns.  The remainder of
the short term looks quiet as high pressure continues to build into
the region from the west amidst sustained nwly flow yielding dry and
slightly cooler conditions.  Overall temperatures through the period
will initialize above normal Monday night, gradually cooling to at
or just below normal levels by periods end.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 1230 pm Sunday: A fairly amplified, but progressive pattern
will be in place at the start of the period, as upper trough moving
off the East Coast Wed night will be displaced by an amplifying
ridge  by late Friday, as a series of short waves deepens a trough
across the Great Plains. The first half of the medium range will be
dry with near-normal temperatures.

By early in the weekend, S/SE low level flow will begin a gradual
increase in response to height falls to our west, with an attendant
uptick in Atlantic moisture. Shower chances will therefore begin
increasing near the southern Blue Ridge by Saturday morning,
gradually increasing and expanding through Saturday night, before
increasing to likely in all areas by Sunday, as cold front/possibly
attendant surface wave move across the TN Valley. Cannot completely
rule out a brief, light wintry mountain mix if precip begins early
enough Sat morning, but profiles should quickly become supportive of
all-liquid.

As one would expect with a dynamic system with a large warm sector,
shear parameters ahead of the front are expected to be strong-to-
extreme. As usual, the primary source of uncertainty re: a high
shear/low CAPE severe convective threat is the degree of buoyancy,
if any that will be available. Despite increasing cloud cover, warm
advection should send temps above normal by Sat night.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Between western Atlantic high pressure and
the cyclone lifting north thru the Mississippi Valley, southerly
flow will continue thru the period; winds will be light tonight but
pick up tomorrow as the gradient intensifies ahead of an approaching
cold front. Moisture will increase accordingly. Upslope cloudiness
is expected to develop near the Blue Ridge in the early morning,
with WAA-driven low VFR clouds expanding across the Upstate and
north from there. Profiles look to support some fog at daybreak,
though models have overdone fog lately due to poor handling of
snow cover. Cigs will lower to MVFR after daybreak and later
IFR following onset of precip. A few gusts are possible in the
afternoon though a mention is only warranted after 00z at KCLT.

Outlook: A period of IFR is possible Monday night and early Tuesday
mainly due to SHRA along cold front. Dry high pressure will dominate
the TAF terminals once the front passes, lasting thru the remainder
of the week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High 100%     High  98%
KGSP       High 100%     High  97%     High  92%     High  81%
KAVL       High 100%     High  93%     High  88%     High  99%
KHKY       High 100%     High  96%     High  87%     High  87%
KGMU       High 100%     High 100%     High  93%     High  89%
KAND       High 100%     High  97%     Med   72%     High  84%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JPT
NEAR TERM...Wimberley/WJM
SHORT TERM...CDG
LONG TERM...JDL
AVIATION...Wimberley



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