Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241134

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
634 AM EST Fri Feb 24 2017

A cold front will move through the area tonight with showers and
thunderstorms mainly for the mountains. Temperatures cool to near
normal late in the weekend then warm up again early in the week.
Another stronger cold front will move through the area mid week,
increasing rain chances again.


As of 630 AM EST Friday:  Low stratus has been a bit slow to
develop, however finally looks to be getting its act together
as multiple sites are now reporting low ceilings amidst a fully
decoupled and nearly saturated boundary layer.  Thus, still think
any widespread dense fog is unlikely, but a few patchy areas of
around 2-3sm visibility cannot be ruled out over the next few hours.
As for the fcst, tweaked t/td trends as well as sky to reflect
latest ob trends and left the remainder of the fcst as it was for
this update.

As of 300 AM EST Friday:  Shortwave upper ridging to prevail
across the Southern Appalachians today as the closed H5 cyclone
moves further into the Western Atlantic, while a broad trof with
a series of embedded shortwave impulses slides east across the
Rockies into the Central Plains.  At the surface, cyclogenesis will
continue to deepen a low pressure system that is currently moving
through Northern MO, while a cold front stretches southward into
the Southern Plains, eventually approaching the MS River Valley
this afternoon.  Out ahead, modest and increasing southerly waa
will dominate beneath the upper ridge therefore leading to a
destabilizing warm sector as td`s push into the mid/upper 50s by
mid afternoon.  Low stratus that is expected to develop this morning
will likely linger into late morning, however should scatter as
mixing improves given relatively dry mid/upper levels per latest
fcst soundings.  With that, temperatures will surge to near record
levels across the Piedmont, but are expected to fall just short
given increasing cirrus from the west into late afternoon.

The fcst will remain dry through the day/evening hours before
gradually increasing from the west into late evening/overnight as
sly/swly moist flow yields at least a low end chance for prefrontal
Southern Escarpment upslope showers.  Fcst soundings do exhibit
modest cape amidst a rather substantial midlevel inversion well
ahead of the approaching front, thus although thunderstorms are
certainly possible, think any deep convection is unlikely at that
point.  Said inversion does look to weaken and possibly erode as
the upper trof approaches in phase with the surface cold front,
which will work to steepen lapse rates somewhat thereby increasing
chances for stronger/deeper convection.  Deep layer shear will be
on the order of 50-60 kts thanks to improved H85 flow, thus a few
severe thunderstorms cannot be ruled out, especially along the
frontal QLCS.  For this reason, SPC has placed portions of the
Western NC high terrain in the marginal risk for severe weather
with damaging winds being the primary threat.  Pops will increase
further into the end of the period with likely levels favored over
the mtns, tapering to slight chances eastward.


As of 245 AM Friday, There`s a pretty definitive signal in the
mesoscale and short term guidance that the frontal band will fall
apart across the southern Appalachians tomorrow morning. This makes
sense in light of the stronger deep layer forcing passing north of
the area, meager buoyancy, and the high terrain disrupting the
frontal circulation. As such, likely pops will be confined to the
northern mtns, tapering rapidly to low chance across the SC/GA mtns
and the I-40 corridor in the Piedmont, with only slight chances
along the I-85 corridor. The severe weather threat should pretty
much be confined to the immediate TN border counties at the start of
the period, and even there the threat is marginal at best.

It is quite possible that the frontal band will be reinvigorated
during the afternoon, as the front interacts with instability
resulting from strong insolation across the Piedmont. However, if
that does occur, it will most likely do so northeast of the
forecast area. Otherwise, strong cold advection on gusty NW winds
will overspread the mtns throughout late morning and afternoon, with
temps falling throughout the day. Meanwhile, downslope warming from
strong W/NW flow and strong insolation will offset the cold
advection east of the mtns, where maxes in the 70s again appear
likely. Much drier air will also filter into the area, and in fact,
fire danger could become a concern in some areas Sat afternoon. See
the Fire Wx discussion below for details.

Once the mid-level short wave trough axis passes into the Mid-
Atlantic Sat evening, a moist low level NW flow is expected to
develop across the TN Valley and Srn Appalachians. Although model
guidance depicts the moisture as quite shallow, the temperature
profile may be supportive of shower development, with snow chances
being initially confined to the high peaks and ridge tops near the
TN border, falling to the lower valley floors by late Sat evening
before the moisture depletes. Even on the high peaks, little to no
accumulation is expected.

Otherwise, temps from Sat night through the end of the period will
be seasonably mild/chilly and very dry.


As of 140 AM EST Friday:  High pressure moves off the Carolina coast
early Monday with S to SE low level flow on Monday between the High
and approaching Low from the west enhancing some rainfall against
the mountains. Moisture gets into our area faster on the GFS late
Monday and delayed on the ECMWF until Monday night. No instability
late Monday for our region as that will be over the Lower
Mississippi Valley at that time.  The newest GFS run is much faster
than previous runs and now has the rain arriving late Monday,
transitioning to shower and thunderstorms Tuesday with greater
instability and finally the cold front passing early Wednesday
nearing the coast at 18Z Wed.  The 12Z EC has the cold front passing
mountains at 00Z Thurs and Piedmont at 06Z Thurs.  The difference in
timing is 12 hours or more.  The cold front is well defined on the
GFS with a line of 200 to 400 CAPE crossing foothills and Upper
Savannah at 12Z Wed. 850mb wind speeds along and just ahead of the
surface front from SW at 45 to 55kts on the GFS. Both GFS and EC
agree on 600 to 800 CAPE from Elberton GA to Lakeland areas of SC
Tuesday PM. The EC has fading instability going into Wed evening
crossing from mountains to piedmont while of course GFS has the
front off the coast.

Some NW Flow mountain snowfall Wed night provided the front moves
through fast enough for cold air and some lingering moisture to
combine. High pressure builds in from the west on Thursday.
Temperatures starting Monday will be as much  as 5 degrees above
normal, then 10 to 12 degrees above Tuesday and Wednesday. Temps
drop behind the front on Thursday to near or even slightly below


At KCLT and Elsewhere:  MVFR and IFR conditions still a possibility
this morning as the PBL has fully decoupled and cooled to near
saturation as fcst low stratus has been slow to develop.  That said,
am starting to see a few areas of such with visb restrictions to
accompany.  Thus kept mention at all sites by way of prevailing
MVFR with IFR tempos as trends/guidance favors these conditions
around taf time.  Otherwise, improvement is expected into late
morning before skies sct to high cirrus with a few low VFR cu not
to be ruled out.  Winds will remain light sly through morning,
increasing/veering swly into the afternoon as a frontal complex
approaches from the west, which will yield precip mention at KAVL,
with visb/cigs lowering to MVFR levels at all sites aside for KCLT
given proximity and timing of the front at 12z.

Outlook:  A cold front will move through the area on Saturday
leading to increased shra/tsra chances, with restrictions expected
as well.  Drier air will advect in behind the front as high pressure
builds from the west, likely keeping chances for restrictions
lower for Sunday, before another wave approaches on Monday once
again increasing chances for precipitation/restrictions.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Med   66%     High 100%     High 100%     High 100%
KGSP       Low   55%     High 100%     High 100%     High 100%
KAVL       Low   30%     High  90%     High 100%     High 100%
KHKY       Med   72%     High  90%     High 100%     High 100%
KGMU       Med   66%     High 100%     High 100%     High 100%
KAND       Med   61%     Low   55%     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:


Gusty W/NW winds and much drier air will overspread the area in the
wake of a cold front Saturday. Minimum RH of 25 to 30 percent is
expected across the Piedmont and foothills Sat afternoon, and may
dip below 25 percent in some areas, esp across Upstate SC.
Meanwhile, sustained winds of 10 to 20, with gusts of 20 to 30 mph
are also expected. These critical conditions could overlap for an
hour or two Sat afternoon, esp across the SC Piedmont. If these
trends hold, coordination on a potential Fire Danger Statement, or
perhaps even a Fire Wx Watch may be needed Fri afternoon.



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      77 1930     26 1947     50 1985      6 1967
   KCLT      80 1982     26 1889     56 1992     16 1967
   KGSP      79 1982     33 1901     57 1909     15 1967




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