Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 202134

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
534 PM EDT Fri Oct 20 2017

High pressure will remain in place across our region into the start
of the weekend. 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 5:30pm EDT Friday: Little change to going forecast as mostly
clear skies persist through tonight with upper ridge and surface
high edging gradually eastward on Saturday.  Highs tomorrow may be a
degree or 2 cooler than today following lows tomorrow morning 5 or
so degrees warmer than previous day due to influence of recovering
surface dewpoints.

The axis of a high-amplitude ridge of high pressure will begin to
shift east of the area tonight, reaching the East Coast by the end
of the day on Saturday. The associated sfc high will drift from the
central Appalachians to the coastal waters south of New England.
This will keep dry conditions with light winds thru the period. Some
increase in cirrus is expected during the day on Saturday, but
otherwise skies look clear.


As of 2:10 PM EDT Friday: The H5 ridge will slip off the Atlantic
seaboard Saturday night, while a transitory surface high will be
located from New England to the Carolina`s and northeast Georgia.
Overall Saturday night should be quiet weatherwise.

The pattern will be changing Sunday and Monday. Moisture in the
lower levels will make a run at our forecast area (FA) late Sunday
in an east to southeast flow, while high level clouds will race into
the region in a developing southwest flow aloft. The models continue
to suggest some patchy light rain in our far southern FA late Sunday
afternoon/evening. As a result we will keep a slight chance of POPS
at those locations. Elsewhere clouds will be arriving during the day.

Moving forward, we remain in a somewhat hybrid surface wedge Sunday
night perhaps through Monday. Meanwhile the models were showing some
differences Sunday night and Monday, with a split flow aloft
developing. The was allowing the GFS to cut off an upper low around
the Arkansas area (which is slightly north of its earlier guide).
The European models follows such, but gradually opens the system up
and is a little faster. Our NAM guide is on the far south side of
the envelope.

Therefore, we will follow a blend of the GFS and European models
Sunday night, with showers becoming focused in the upslope mountain
areas of western NC, upstate SC and far northeast GA. Elsewhere
Sunday night we kept the POPS on the low side.

As the upper level jet structure becomes more diffluent, and
isentropic lift enhances, we would expect showers to expand in both
coverage and intensity Monday. The overall thunder chances during
the day Monday look low, since we have a slower system, and the best
shear and buoyancy have yet to arrive. Plus we might still be
dealing with the hybrid wedge, Otherwise precipitable water values
take a huge jump during the day Monday, as a result we have
maintained the threat of heavy downpours in our western sections.

We have kept the diurnal temperature cycle moderate with the
potential wedge holding in the area and precipitation possibly
reinforcing the wedge.


As of 230 PM EDT Friday: Global models are still struggling to come
into agreement on the handling of the cutoff low and upper trough
that propagate through the southeast early next week. The medium
range begins Monday night as this feature ejects towards the
southern Appalachians. The new GFS maintains a more southerly low
track and is the overall slower solution, while the ECMWF absorbs
the cutoff low back into the mean flow faster and results in a more
progressive system overall. The GFS sets up something of a warm
conveyor belt over a remnant wedge of surface high pressure Monday
night, with the resulting precip reinforcing the wedge until the low
itself is able to push through the area early Tuesday morning. This
greatly limits instability in the GFS solution until early Tuesday
morning when the main body of the front approaches our area. A
narrow overlap of decent deep-layer and low-level shear with
marginal instability may be possible early Tuesday morning, which
may be enough to provide an isolated brief severe/tornado threat.
The ECMWF erodes the wedge much faster Monday, which results in
slightly greater instability Monday evening as the front passes
through the area. This results in a more concentrated band of pre-
frontal precip and provides a slightly larger window of overlap of
(barely) sufficient instability with excellent deep and low-level
shear. The system is quite dynamic with very favorable upper and low-
level wind profiles and excellent forcing, so instability will be
the limiting factor in any severe/tornado threat. As with many of
these events, very small details in the position and speed of the
system will matter quite a lot to any possible eventual severe
threat, so uncertainty as to how or if any threat may materialize is

A deep upper trough quickly follows the low and cold frontal passage
on Tuesday, ushering in some of the coldest air we`ve seen this
season so far for Wednesday and Thursday. Some moisture will likely
accompany the trough as the axis swings through the area late
Tuesday and into Wednesday as NW flow sets up across the area,
bringing in a slight chance for some isolated brief snow showers to
mix into light rainfall in the highest elevations.  Highs and lows
will be a solid 10 degrees below average Wednesday and Thursday
before the upper level pattern flattens, moderating temperatures to
near normal by next weekend.


At KCLT and Elsewhere: Deep-layer high pressure will continue to
dominate our weather thru Saturday. The air mass in place continues
to warm, while moisture remains steady or even slightly lower than
previous days. So fog potential looks lower, with only the Little
Tennessee Valley and a few lake areas expected to see much fog. All
TAF sites will carry VFR conditions thru the period. Winds will be
light, favoring a NE direction across the Piedmont and more variable
in the mountains.

Outlook:  VFR conditions with light winds will continue thru Sunday.
Moisture will begin to increase as a cold front will approach the
region to start the new work week. Widespread precip and low clouds
expected Monday thru Tuesday, then cooler and drier weather
returning Wednesday thru Thursday.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
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 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  94%     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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