Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 220515
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
115 AM EDT Sun Oct 22 2017

.SYNOPSIS...
Dry high pressure will remain across the region through Sunday.
Meanwhile, a low pressure system will develop across the deep south
and bring moist air to the Carolinas early Monday. A cold front
associated with this system will cross the region Tuesday with
cooler air moving in behind the front through mid-week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 115 AM EDT Sunday:  Updated discussion for 06z taf issuance.
All in all the current fcst trends looks good at this hour as high
cirrus continues to stream overhead.  With that, cooling has been
delayed a bit, and thus so has any fog development.  Think valley
fog in the Little Tn valley is inevitable, with chances in the
other valleys and across the piedmont being somewhat lower.
No sig changes needed/made with this update.

As of 10pm EDT Saturday:  Main axis of 500mb continental ridge
has moved east of the area with front of next upstream system now
passing through the Plains states.  As ridge continues eastward
and front approaches the area on Sunday, winds will gradually
increase from the southeast and begin to increase moisture and
PWAT levels.  Current thin high clouds are expected to be joined
by lower-level clouds Sunday afternoon.  By Sunday evening, front
will have reached Western TN and PWAT levels will be near an inch.
A few scattered showers may occur Sunday evening over the mountains
with upslope assistance, but significant rainfall will hold off
until Monday morning when upper forcing and the front will be
directly impacting the region.

Winds tonight are mostly calm with surface ridge axis still just
east of the mountains, and dewpoints are up slightly over yesterday,
however, some cloud cover may limit fog potential to a few isolated
locations.  Clouds and passage of ridge are also expected to trim
a couple degrees off of Sunday`s high temperatures, but highs and
lows are still looking to be 10 degrees above normal.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...
As of 230 PM EDT Saturday: We have made some significant changes to
the forecast based on the latest guidance from the GFS/NAM and
European models. The culprit is the upper low near the state lines
of Arkansas, Missouri and west Tennessee late Sunday night and early
Monday. Earlier guidance held onto the closed low scenario, which
allowed for a slower movement of the upper features, and also their
accompanying frontal low and cold front.

It appears the models have latched onto the evolution in the
southern stream, where the upper lows are embedded, to open up into
a rather robust trough axis. This trough axis, and surface cold
front, will then race quickly east.

Initially isentropic lift, and an increasingly strong H85 inflow
Sunday night, will allow for an expansion of precipitation as the
column of moisture saturates. Elevated instability suggests that a
small chance of thunder will be possible near daybreak in our south
FA.

The modeling of the upper system and cold front Monday into Monday
night will be dialed into the overall speed. The NAM and GFS were
the fastest, the European a tad slower - but not by much. A 50+ knot
H85 jet will continue to pump moisture into the region, while very
strong DPVA, a coupled jet structure and converge along the front,
all spell a hefty increase in POPS from Monday into Monday night.

Shear values will be very impressive, although the buoyancy factor
is a bit of a wildcard. However, we have noted a trend up with the
latest CAPE values. Another question will they line-up together in
our area? We also will be watching closely how the hybrid wedge
reacts. Our thinking it will ease, but still could be a player.

Precipitable water values surge higher as this unfolds, especially
late Sunday night and Monday. We are still carrying some hefty
rainfall amounts, and although it has been dry, we will need to also
watch high "short fused" rainfall rates.

Following later Monday night and Tuesday, we have cut way back on
POPS, but left some smaller values in the forecast for a 2ndry wind
shift and some elevated CAPE.

Temperatures may become a problem because of a number of factors:
rain cooled air, colder air arriving quickly, breaks in the clouds.
The best course of action we have decided was to trend temperatures
cooler from northwest to southeast across our FA ... right through
this part of the forecast cycle.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 130 PM EDT Saturday: The region is progged to be near the axis
of a deep upper l/wv trough at the start of the period. The post-
frontal northerly pressure gradient lingers, resulting in a well-
mixed, brisk and cool Wednesday featuring max temperatures 10 to 12
deg f below climo. Channeled vort energy rounding the base of upper
trough in concert with NW flow moisture may also be able to generate
a few NC mountian showers and perhaps high elevation snow showers
into Wed nite. Advective freezing temperatures are also possible
across the northern NC mountains early Thursday morning.

Rising upper heights and the development of weak llvl ridging is
still on tap for Thursday followed by a more prounced sw flow
through a deep layer on Friday. Sensible weather for this period
will feature sunshine, but still well below normal maximum
temperatures on Thursday, followed by a milder Friday, although sun
should becoming increasingly filtered by the influx of clouds within
the aforementioned SW flow. Shower chances should increase at some
point next weekend as moistening is onoing ahead of the slow
encroachment of elongated frontal zone.

&&

.AVIATION /05Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere:  VFR conditions expected to prevail through
the first 3/4 of the taf cycle as high pressure gradually retreats
thanks to an approaching upper trof and associated surface
cold front.  With that, expecting llv flow, which is currently
light/nely, to increase and veer ely/sely.  Moisture advection
will prevail amidst this newly veered flow allowing for gradually
increasing low VFR cloudiness through the day, all beneath increased
mid/high cloud cover.  Into the overnight hours of Monday morning,
guidance favors developing shra across the western tier of the cwfa
to affect all sites aside for KCLT/KHKY, thus such is included
via shra prob30s with MVFR/IFR cigs.  Also favored MVFR cigs at
KCLT/KHKY to round out the period.

Outlook:  A cold front will move through the region on Monday
increasing chances for restrictions via low cigs and reduced visbs,
as well as showers and possible thunderstorms.  Drier and cooler
conditions return for the rest of the work week.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  83%     High 100%     High 100%
KHKY       High 100%     High  83%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High  83%     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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...SBK
NEAR TERM...CDG/WJM
SHORT TERM...TS
LONG TERM...CSH
AVIATION...CDG



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