Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200526

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
126 AM EDT Mon Mar 20 2017

Dry high pressure will settle over the Southeast states through
Monday. A cold front will approach the forecast area Tuesday into
Wednesday, increasing chances for rain and thunderstorms. Cooler air
will return behind the front to end the work week before another
frontal system approaches the areA next weekend.


0530 UTC Update...No signficant changes with this update, as
forecast is on track.

Otherwise, heights will rise over the next 24 hours, as upper low
continues to spin away from the Carolina coast. Northwest flow will
therefore continue to diminish, and NC mtn stratocu should retreat
to the TN border within the next couple of hours. Clear skies and
developing light winds, along with a dry air mass will support good
radiational cooling conditions tonight, with min temps expected to
average about 5 degrees below climo. This should yield solid frosty
conditions across much of the Piedmont/foothills, with a light
freeze likely across much of the NC foothills and NW Piedmont, where
a frost advisory and freeze warning remain in effect.

With short wave ridging overhead on Monday, temps will begin to
rebound, although increasing high clouds may knock a degree or two
off the heating potential. Temps are nevertheless expected to warm
to a couple of degrees above climo.


As of 130 PM EDT Sunday:  The short term fcst picks up on Monday
evening amidst a low amplitude H5 western Atlantic trough, while
a similar low amplitude H5 ridge axis sits atop the Appalachians.
At the surface, high pressure looks to anchor across the FL/GA/SC
coastline, while a cold front associated with a Canadian cyclone
slides into/through the OH valley.  Guidance favors ongoing
prefrontal convection across the OH valley at fcst initialization,
with a modestly unstable warm sector westward across the Midsouth
and Middle TN.  Said prefrontal activity looks to race southward
into the Appalachians well ahead of the front, however weakening
into the overnight due to diurnal stabilization, likely washing out
across northeast GA and the western Carolinas by Tuesday morning.

At that time the cold front is progged to still be north of the fcst
area while the western Atlantic surface ridge drives a warm front
across GA and the Carolinas into Tuesday afternoon.  With that,
nearly all guidance is now favoring ample destabilization for
surface based convection, however with soundings indicating
modest capping in the llvs.  Latest Nam/Gfs still indicate
surface wave development across TN as the front sags further
south, eventually sliding into cwfa beneath a weak upper impulse.
All in all, think ongoing convection will likely move into the
region from the west, while also firing along the front as it moves
southward across western NC into northeast GA and the SC Upstate.
Fortunately upper forcing will be limited as the best DPVA passes
to the north, however there will be sufficient 0-6km bulk shear
to support organized deep and possibly severe convection into
Tuesday evening with the primary threats being damaging winds
and large hail.  Beyond that, the front looks to drive southward
across the fcst area overnight into Wednesday morning as broad
Canadian high pressure overtakes the Great Lakes region leading
to cooler/drier, yet still partly cloudy conditions into periods
end as models favor wedge development.

As for the fcst, pops ramp up from the northwest during the day
on Tuesday as the warm sector becomes conditionally unstable
given max temps well above normal and vastly improved dewpoints.
Further increases in pops were warranted into the evening/overnight
hours with likely levels favored across much of western NC
in association with enhanced frontal convergence, tapering to
mid/high chances elsewhere.  Pop are finally allowed to lower
into the slight/chance range regionwide into Wednesday morning as
the front clears the region and instability is minimal.  Lastly,
the fcst will dry out through the late morning hours on Wednesday
amidst cooler near normal temperatures.


As of 115 PM EDT SUNDAY: The synoptic pattern will feature a broad
plains ridge and a western Atlantic trough Wednesday night through
Thursday. A passing cold front will settle south of the region
during this period and likely stall near the GA/FL line, with 1038
mb surface high pressure stretching from the Mid-Atlantic region
down through the western Carolinas in the wake of the front. Expect
dry and cool conditions across the region through Thursday.

The approaching upper ridge axis will amplify as it moves across the
Appalachian mountain chain Thursday night, and to the eastern
seaboard on Friday, in concert with a full latitude trough
stretching from the western Great Lakes to the Texas Gulf coast. Any
mid and high level moisture topping the ridge axis east of this
developing trough/low system should pass north of the region on
Friday. However, low level moist upglide will steadily return late
Thursday night through Friday around increasingly offshore Atlantic
high pressure. This could produce some light rain and a chilly cold
hybrid or in-situ cold air damming day across the region on Friday
as the nose of the offshore surface high lingers over our area.

The approaching trough/low will then bring the best chance of
precipitation, and possibly convection, for next weekend. Will
generally follow a WPC/mean ensemble blend on timing now that the
previously faster GFS/GEFS have trended strongly toward the slower
ECMWF/ensemble camp on the 12Z run, with a more closed off upper
feature to the west. The best combination of deeper moisture and
forcing should impact our forecast area in the Saturday afternoon
through Sunday time frame. Expect any cold air damming layer still
lingering into the weekend to steadily retreat northward on Saturday
and begin to uncover improving warm sector instability through
Sunday. The southerly low level jet should ramp up through Saturday
night, and southern mountain upslope flow areas could see some
locally heavy rainfall if the pattern holds. Convection chances will
improve through Sunday with falling temperatures aloft as the upper
system moves overhead. A thunder mention seems warranted despite
this being the day 7 forecast.


At KCLT and Elsewhere: VFR conditions are expected thru the period,
as dry high pressure slowly traverses east. Winds will remain light,
shifting from N/NELY to S/SWLY around 15z, except closer to 20z at
KAVL. A frost advisory is effect for the SC sites and KCLT thru
daybreak. Thus...frost may accumulate on airplanes and runways
during this time. Otherwise, expect just a gradual increase in
cirrus this afternoon into tonight.

Outlook: A cold front may bring showers and perhaps a few
thunderstorms on Tuesday, possibly into Wed. Brief drying is
expected Thursday, while moisture should begin returning by the end
of 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:


GA...Frost Advisory until 9 AM EDT this morning for GAZ018-026-028-
NC...Frost Advisory until 9 AM EDT this morning for NCZ070-071-082-
     Freeze Warning until 9 AM EDT this morning for NCZ035>037-056-
SC...Frost Advisory until 9 AM EDT this morning for SCZ004>014-019.


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