Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 262106

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

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.


As of 455 PM, a sold line of showers and scattered thunderstorms
currently extends from the northern NC foothills, across the NC mtns
and into north-central and NW Georgia. This activity is steadily
moving east into an area that has seen good insolation throughout
the afternoon, and CAPE currently ranges from 1000-2000 J/kg across
the Piedmont and foothills according to the latest SPC mesoanalysis.
Based upon these trends, along with the latest guidance from
convection-allowing models, see no reason not to increase pops to
70-80 percent across the mtns and foothills for the next couple of
hours, and to 60-70 percent across much of the Piedmont in the 2-4
hour time frame. Convection has been rather tame thus far, but still
would not rule out an isolated/brief severe storm as activity moves
into the more unstable air. The bigger threat should continue to be
locally heavy rainfall. However, activity has generally been moving
steadily along, and while some rates as high as 2 inches per hour
have been noted, FFGs remain around 3 inches in one hour and 5-6
inches in 6 hours. These amounts are not going to be realized on
anything more than a highly localized basis, so a token flash flood
mention in the HWO still appears to be the way to go.

Otherwise, over the next 24 hours, big-picture-wise a large
mid/upper low is expected to slowly drop down from western Ontario
to the upper Great Lakes, which will bring mid/upper troffing down
across the western Carolinas. This should drive a surface cold front
from the TN valley eastward across the Srn Appalachians tonight and
Tuesday morning. The front will probably get hung up across the mtns
thru most of the day on Tuesday. Weak upper divergence will get
strung out across the region tonight and Tuesday as the right
entrance region of the upper jet moves overhead, but mid-level
forcing is not that impressive. Most of what the front does will be
the result of deep convection firing along and ahead of the feature
this afternoon and evening, and then the weak forcing overnight. The
guidance suggests holding onto a likely precip prob mainly out
across the NC foothills and wrn Piedmont into the overnight hours.
Min temps tonight should be mild. On Tuesday, with the front in our
midst, precip chances should ramp up especially over the eastern
zones where a likely was included. Temps will be at or above normal


As of 245 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.


As of 315 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.


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...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     High 100%     High  98%
KGSP       High 100%     High  98%     High  92%     High 100%
KAVL       High 100%     High  94%     High  89%     High  91%
KHKY       Med   70%     High 100%     High  87%     High  98%
KGMU       High 100%     High 100%     High  94%     High  98%
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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