Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211450

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1050 AM EDT Sat Oct 21 2017

High pressure will remain in place over our region today into
Sunday. Meanwhile, a low pressure system will develop over the lower
Mississippi River Valley late in the weekend and move across the
southeast early next week. A cold front associated with this system
will cross the region Tuesday, and cooler air will spill into the
area 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 325 AM EDT Saturday: A prominent, full-latitude trough axis
will get carved out from the Great Lakes to the Deep South Tuesday
through Tuesday night. Westerly flow will develop over our region,
along with less moisture, following the Tuesday cold frontal
passage, however, scattered showers will remain possible under the
trough due to very steep mid level lapse rates. Scattered northwest
flow snow showers may start up in the highest terrain Tuesday night
as the colder air spills in.

The trough will migrate east across the forecast area on Wednesday
before deep layer NW flow sets up into the southern Appalachians
behind the departing trough and reinforce the cooler air Wednesday
night into Thursday. A brief uptick in moisture is expected as a
shortwave rounds the trough axis Wednesday evening, mainly for the
northern tier. Advective freezing temperatures will be possible
across the northern mountains early Thursday morning. A shallow
ridge will cross the region late Thursday into Friday, just as
another deep trough develops over the plains.


At KCLT and elsewhere:  VFR is expected to prevail through the
period at all sites.  High pressure will dominate through the taf
cycle with calm winds this morning giving way to increased/veered
sely flow by early afternoon as the center of the ridge shifts east.
Soundings favor a bit more upper moisture than days past, thus
did hit the high cirrus a little bit harder as the period evolves.

Outlook:  No restrictions expected through the weekend aside for
possible morning mtn valley fog to affect KAVL.  A cold front
will move into/through the region to start the new work week,
thereby increasing chances for widespread showers and possible
thunderstorms.  Drier and cooler conditions to round out the week.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
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 100%     High  94%
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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