Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS62 KGSP 240818

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
318 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 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:  VFR/MVFR and IFR conditions expected
through this taf cycle at all sites.  Moisture advecting in from
the Atlantic this morning will settle across the region, leading
to low stratus development toward daybreak.  Guidance favors
MVFR/IFR cigs at all sites, with visbs likely to remain in the
4-5sm range for those sites affected most by the low stratus,
with the exception of the fog prone mtn valleys to affect KAVL.
Therefore all tafs feature restrictions through daybreak, with
improvement expected into late morning before skies sct to low/mid
VFR beneath high cirrus.  Winds look to remain light sly through
morning, increasing/veering swly into the afternoon as a frontal
complex approaches from the west, which will lead to precip and
restrictions within the next taf cycle.

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...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  92%     High  90%     High 100%     High 100%
KGSP       Med   79%     High  93%     High 100%     High 100%
KAVL       Med   61%     High  89%     High 100%     High 100%
KHKY       Med   74%     High  86%     High 100%     High 100%
KGMU       Med   79%     High  93%     High 100%     High 100%
KAND       Med   67%     Med   70%     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




CLIMATE...GSP is the U.S. government's official web portal to all federal, state and local government web resources and services.