Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 261843
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
243 PM EDT Mon Sep 26 2016

.SYNOPSIS...
A cold front will cross our area tonight and be near the Carolina
coast by Tuesday evening.  An upper level low will drop south into
the Ohio Valley by Wednesday and linger over the Central
Appalachians through the end of the week.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 230 PM...convection has fired over the NC mountains, which
the fcst has well in hand. Will make some minor adjustments for
the lack of deep convection at this time east of the Blue Ridge,
otherwise the fcst is in good shape.

For the rest of the near term, surface high pressure ridging down
the Eastern Seaboard has allowed a back door cold front to move into
the region. In the meantime a strong upper low diving out of Canada
and over the Great Lakes will sweep an occluded cold front into
the Ohio Valley and Appalachians today. Downstream ridging aloft
ahead of this upper low will briefly increase today, and with
low-level WAA picking up as flow shifts around to southerly, the
back door front should retreat to the northeast (does that make it a
front door front?) as the Great Lakes system approaches. With this
comes a surge in instability across the area this afternoon with
near-term guidance generally agreeing on a max of about 1500 J/kg,
though the NAM as usual is more aggressive. Expect the convection
across the mountains to spread east across the area this evening.
CAMs disagree on exact timing but the general look is the same. Lack
of deep-layer shear and poor mid-level lapse rates will limit severe
potential, but with strong mechanical lift plus increasing surface
buoyancy, an isolated strong or low-end severe storm cannot be ruled
out. Other concern is for isolated heavy rainfall, with PW values
increasing to 1.6-1.7" - well above the 90% threshold and pretty
close to the daily max. The limiting factor here will be increasing
winds aloft which will allow storms to actually start moving.
Nonetheless, locally heavy rainfall will be possible with storm
total basin-average QPF approaching 1.5" for the Day1 period.

Back to the increasing WAA today and the retreating back door cold
front...as this lifts northeast, despite the increasing moisture and
cloud cover, expect temperatures to rise again today especially with
the increasing amplitude of the downstream ridging aloft over us and
corresponding increase in thicknesses. MET guidance was warmest of
the pack yesterday and still too cool. It is again on the warmer
side today so leaned that direction and brought in a little bias-
correction. Might see a little cooling of overnight lows tonight in
the northern mountains as the front starts to push through, but the
main pattern change will start in the short term.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/...
As of 230 PM Tuesday: Showers and thunderstorms will be scattered
along and southeast of Interstate 85 Tuesday evening as the front
slowly drops south. This activity will slowly diminish but may
linger into Wednesday morning. Most of Wednesday should be dry as
cooler air moves into the region.

A fairly potent upper level low pressure system will drop southeast
across the Ohio Valley and central Appalachians impacting the area
Wednesday night and Thursday. Several disturbances will swing around
the parent low bringing rainfall to the area. The highest chance of
rain will be in the upslope areas of western North Carolina while
the lowest chance will be in northeast Georgia and upstate South
Carolina where downsloping low level flow will exist. All locations
will be cooler as the impacts of the low translate across the area.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 230 PM EDT Monday...An upper low is expected to be over
Bristol TN Thursday evening then lift north to eastern OH Friday
evening. The GFS forecast has been accepted as the better
performing operational model. The ECMWF is better today but still
slower in its evolution of the cut off closed low lifting out and
departing New England. With the lower atmospheric thickness values
associated with this upper low and more cloud cover Thursday night
into Friday, this will keep temperatures several degrees below
normal. Very light rain amounts are possible across the NC Mtns
and north of I-40 Thursday evening then becoming increasingly
limited to the TN border areas until Friday evening when it goes
away to the north of our forecast area.

The closed and cut off upper low will begin to open to the westerly
flow at the north end of the Great Lakes on Saturday. The low will
fill and become absorbed near Montreal Sunday night. We will be at
the southern end of a large area of surface high pressure Sunday
into Monday. The GFS has the high centered over north central Quebec
at 12Z Tuesday ridging down across our area. The orientation of the
low level wind flow with this high may produce upslope wind by
Monday night into Tuesday which may result in light rain in the Lake
Jocassee to Tryon to Lenoir areas.

Temperatures will certainly be several degrees below normal Friday
then slowly rise to near normal as the low moves away from our
region over the weekend. By Monday, temps may even be a little above
normal since the high may not be strong enough and far enough away
to have minimum influence toward wedge conditions.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
At KCLT: Somewhat convectively inactive to start with as we are
dealing with a lull brought about by convective debris in the wake
of morning convection. Cannot rule out an MVFR-level broken ceiling
but think that VFR will prevail, so this detail will be left out in
favor of including TSRA this evening. The CAM guidance, particularly
the 4km WRF, shows a band of convection moving across metro CLT this
evening, so a TEMPO from 00Z to 04Z was included. After this
activity moves east, we may have to deal with some visibility
restrictions in the pre-dawn hours thru daybreak if we get rainfall
at the site. Have hinted at this with a 6SM vis and scattered low
cloud. The cold front should hang up to the west thru the period,
which will keep wind S to SW, and allow for destabilization with
some heating on Tuesday morning. TSRA may develop again in the late
morning, so have included a PROB30 for that possibility.

Elsewhere: Trends similar to that of KCLT, with timing based on the
4km WRF. The exception is KAVL, where a thunderstorm is already in
the vicinity, so they begin with a TEMPO for TSRA. The models are
hitting on the possibility of MVFR/IFR ceiling and vis across the
mtns Tuesday morning, so this was included at KAVL. As with KCLT,
some fog/low stratus possible early Tuesday morning and introduced
this at KHKY/KAND.

Outlook: Cold front will slowly cross the region through Tuesday.
This should result in better chances for convection. Dry high
pressure builds in behind the front on Wednesday, which should bring
VFR conditions in most places.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  93%     High 100%     High 100%     High  97%
KGSP       High 100%     High 100%     High  97%     High  99%
KAVL       High 100%     High  96%     High  83%     High  88%
KHKY       Med   66%     High  82%     High  82%     High  85%
KGMU       High 100%     High 100%     High  97%     High  97%
KAND       High 100%     High 100%     High  97%     High  89%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...PM
SHORT TERM...SW
LONG TERM...DEO
AVIATION...PM



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