Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211751

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
151 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 1000 AM: Fairly expansive thin cirrus continues to stream
across the area, as axis of upper ridge shifts east. The clouds will
filter a little sun, and may keep temps down a deg or so, but
looking at the inherited forecast, I think that is already taken
into account. Otherwise, just tweaks to hourly T and Td grids were
made to line up with latest obs.

An increasingly amplified pattern highlights the synoptic scale
setup today as deep ridging continues to dominate across the extreme
southeast states, while broad troffing and associated height falls
approach from the west as the center of the trof axis swings into
the plains. This pattern looks to keep the southern Appalachians dry
and warm again today, however with moisture increasing through the
period thanks to sly/SWLY veering flow.  With that, think max temps
today will once again be a few cats above normal, yet a degree/two
cooler than yesterday as a bit more high based cirrus is expected to
stream in aloft.  Overnight tonight the llvl flow should be veered
sly allowing for improved moisture advection per latest SIG level rh
analysis on the GFS/NAM.  Therefore high cirrus could eventually be
obscured by intruding low stratocu from the south near periods end
on Sunday morning.  Min temps on Sunday morning will remain above
normal levels as well thanks to reduced radiational cooling per the
aforementioned sky cover.


As of 315 AM EDT Saturday: A trough axis over the central CONUS on
Sunday will develop a split flow pattern as a southern stream low
cuts off just west of the lower Mississippi River Valley through
Sunday night. Developing low-level southeasterly flow ahead of this
system will permit cloudiness to increase through the day on Sunday.
With the surface high center to the north already well east of New
England, and very limited precipitation potential through the day on
Sunday, cold air damming conditions look less likely to set up and
persist during the short-term period. Upslope moisture will allow
PoPs to increase through Sunday night, with dewpoints quickly
recovering around the region.

The southern tier system will gradually open and get re-absorbed
over TN into a deepening low pressure system over the upper Midwest
Monday through Monday night. Ahead of this phasing system, a period
of deeper moisture, forcing, shear, and marginal instability will
occur over the southeast. A slightly backed 40 to 45 kt low level
jet will likely lift across the region, centered sometime around
Monday evening, and coincident with the best period of forcing as a
90 kt upper jetlet rounds the southern stream trough. The lifting
trough axis will likely cross the forecast area on Monday night,
although the latest GFS/NAM solutions are a bit slower - with
associated pre-frontal convection lingering into early Tuesday. The
faster ECM/Canadian camp are preferred with the phasing system,
which would bring the strongest convection through the area mainly
Monday evening, with strong dry slotting occurring from the
southwest early Tuesday morning. The models remain quite reserved on
instability Monday afternoon and evening, with little more than a
few hundred J/kg of sbCAPE during the best southerly jet and low
level shear. It seems prudent to mention thunder for at least the
southern/eastern halves, but the SPC Day 3 Outlook currently has
only a general risk.

Regarding heavy rainfall, QPF will focus on the southeasterly
upslope areas of the southern mountains Sunday night through Monday
night, but with dry antecedent conditions making flooding rather


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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