Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210522

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
122 AM EDT Sat Oct 21 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 115 AM EDT Saturday:  Updated discussion for 06z taf issuance.
Clear skies dominate early this morning allowing for good
radiational cooling conditions.  With that, temperatures have fallen
into the 40s/50s across the region, amidst light/calm winds. Given
good decoupling of the BL, am expecting continued valley fog into
sunrise, while some patchy fog is possible elsewhere, albeit with
the best chances near any area lakes. No sig changes needed/made to
the fcst at this time.

As of 10pm EDT Friday:  Saturday should be another beautiful day
with mostly clear skies, light winds, and temperatures above
seasonal normal.  Large eastern CONUS upper trough is currently
centered over the area with a large surface high pressure center
nearby.  This fair weather-making system makes eastward progress on
Saturday with the next upstream major front reaching the Central
Plains late tomorrow.  Dewpoints have been edging upward each day as
airmass recovers and should be a couple degrees higher on Saturday.
This will continue the trend of reducing the diurnal temperature
range with lows Saturday morning 5 degrees warmer than Friday (and
near seasonal normal) and highs a couple degrees lower than Friday
(and 5 to 10 degrees above normal).  With some improvement in
dewpoint, clear skies, and calm winds; some isolated patches of fog
in low-lying areas are possible Saturday morning.


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:  VFR is expected to prevail through the
period at all sites with a few minor exceptions.  Guidance hints at
patchy fog along/south of I85 this morning as the BL has already
decoupled nicely, therefore included a tempo at KCLT for 5sm fog,
albeit with low confidence.  Also included MVFR fog at KAVL and
KAND, both with moderate confidence.  Otherwise, high pressure
to dominate through the taf cycle with calm winds this morning,
increasing out of the se into late morning or 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...

            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 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 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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