Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211839

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
239 PM EDT Sat Oct 21 2017

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.


As of 215 PM: a highly amplified upper trough will cross the Plains
tonight and approach the MS Valley Sunday. This will result in
height falls encroaching on the region toward the end of the near
term. At the sfc and low levels, high pressure will shift east off
the Mid-Atlantic coast and bring flow around more out of the SE late
tonight thru Sunday. This flow will begin to advect some low-level
moisture into the Carolinas and GA. Guidance is mixed, but seem to
be trending toward some stratocu development, at least in the SELY
upslope areas late. The combination of increasing cirrus aloft and
potential low stratocu early Sunday morning should limit radiational
cooling somewhat. So min temps are expected to be 2-3 categories
above normal. Some fog may develop in the Little TN Valley, but
little elsewhere.

Sunday, the early morning stratocu that does develop will likely
just expand in coverage, but should lift slightly in altitude by
early AFTN. So expecting more cloud cover around and this should cap
temps a few degrees where it persists, namely near the eastern Blue
Ridge escarpment. The lower piedmont and lower valleys near the TN
border should see similar temps to today, maybe a deg or two cooler.
As for PoPs, forecast soundings show the moist layer to be shallow
with lingering inversion around 800 mb. So I am going with a dry
fcst. Cannot rule out a few sprinkles late AFTN in the western
Upstate and adjacent mountains just to the north, as moisture depth
starts to increase.


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

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.


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.


At KCLT and elsewhere: The high pressure system that has dominated
our weather over the past few days is sliding east, allowing
increasing cirrus aloft and possibly some low-level stratocu late
tonight thru Sunday. Despite increasing sfc dewpoints, fog potential
looks low, as increasing clouds will limit radiational cooling. Some
mountain valley fog will likely develop again, but mainly in the
Little Tennessee Valley. Guidance times arrival of the low-level
moisture around 12z, with patchy stratus possibly developing near
the eastern Blue Ridge escarpment. For these TAFs, will show sct
MVFR-level clouds early, then going BKN for 3500 to 5000 ft by 18z
Sunday. Winds will be light, mainly SE this AFTN, then returning to
ENE this evening or going variable, before going back to SE by
midday Sunday.

Outlook:  Increasing low clouds possible Sunday night, as a cold
front approaches from the west. The cold front will cross the region
on Monday, bringing a round of widespread showers and possible
thunderstorms.  Drier and cooler conditions return for the rest of
the workweek.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
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 100%
KHKY       High 100%     High 100%     High  97%     High  93%
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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