Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190223

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
923 PM EST Sun Feb 18 2018

Rain chances will increase to start the work week as
moisture overspreads a wedge of high pressure. Temperatures will
climb through the middle of the week as another cold front arrives
from Tennessee by Thursday. This front will stall out and linger
just north of our area next weekend. Chances for rain remain in the
forecast each day.


930 PM Update...With llvl stcu building in quickly this temps have been held abv the fcst curve in many
locales. Thus...have adj the T/Td grids. PoP grids are in good shape
with light precip moving over the SW zones. This area of rain will
continue and spread further across the wrn half of the FA during the
next few hrs.

600 PM Update...Moisture from a srn frontal zone continues to slowly
advance northward as sfc ridging builds in from the northeast.
Expect the development and an increase of low cloud coverage thru
the next update.

The flow across the Conus will amplify considerably during the near
term, as a series of short waves carve out a deep trough across the
West, forcing a building ridge along the East Coast, while
tightening gradient between these two features will result in a
strengthening upper jet across the Midwest. Developing Great Plains
cyclone responding to large scale height falls will result in
strengthening S->SW flow across the Southeast, with moisture return/
warm frontal activation already underway from the SC Coastal Plain
through ctrl GA/AL. Moisture will begin overspreading our forecast
area from the SW by early evening, with increasing/thickening clouds
giving way to increasing chances for mainly -RA by late evening.
Pops increase to likely in most locations at some point between 06
and 12Z. Precip amounts will be light, only around a tenth of an
inch or less in most locations, with perhaps some locally higher
amounts with any terrain enhancement. Cloud cover will limit cooling
such that min temps will be 10-15 degrees above climo.

Precip chances will begin tapering off around sunrise Monday, as
deeper isentropic lift shifts into the Mid-Atlantic and central/
eastern NC. However, early morning precip falling into initially
relatively dry air mass should leave behind in-situ cold air
damming, and shallow upglide will allow for abundant low clouds and
at least patchy drizzle to linger through the morning, if not the
entire day. That being the case, Monday`s max temps will be cooler
than today`s (or very close to climo), especially along and north of
I-85, as the CAD may begin to erode by afternoon across southern
Piedmont areas, if it develops in those areas at all.


As of 145 PM Sunday: A deep upper ridge, characterized by 500mb
heights near 595 dam, will remain situated off the Southeast
coast through the short term. We will remain in the warm sector
of a frontal system centered over the Midwest, with the ridge
hindering the eastward progression of that system. Warm and humid
conditions thus are in order for our CWFA. In terms of forcing,
the pattern is not very exciting; WAA is relatively weak through
Tuesday. Models generate occasional QPF in the southerly upslope
areas near the Blue Ridge through that time, so our PoP trends
will reflect that. Tuesday night, a weak warm front moves into the
area, further increasing moisture and PoPs, which expand into the
Piedmont. These elevated chances persist through Wednesday. The
GFS brings a few hundred joules of SBCAPE into the lower Piedmont
Wednesday afternoon, with the NAM depicting lower values; a
midlevel subsidence inversion looks likely to stifle most deep
convection. The chance of thunder looks too small to mention in
most areas despite what the GFS shows.

In the warm pattern of late, a bias-corrected blend of raw
model values has verified well for max temps, generally being at
the warm end of the envelope. The Piedmont should easily reach
the mid 70s both days, trending upward for Wednesday when the
climatologically warmest spots may flirt with 80. Min temps will
be within a couple degrees of normal max temps on Tuesday morning,
and will be several degrees warmer Wednesday morning under the
influence of that warm front. Both max temp and high-min records
are in jeopardy for our climate sites. Please see the Climate
section below for full details.

As previous shift noted, with a saturated low-level inversion and
support from MOS guidance, areas of fog are a reasonably good bet
early Tuesday and Wednesday mornings.


As of 230 PM EST Sunday:  The medium range fcst period tips off
on Wednesday evening amidst strong subtropical H5 ridging over the
western Atlantic, while an upper troffing prevails over the west.
At the surface, high pressure will dominate the pattern over
the southeast states, reinforced by the aforementioned Atl upper
anticyclone.  Further west a cold front will be moving into the
MS river valley, eventually sliding east into the TN valley.

At fcst initialization the prevailing sly flow regime will lead
to a moist and unsettled airmass across the southeast favoring
elevated pops.  Therefore pops will be featured in the likely range
across the high terrain, with chance levels highlighted elsewhere.
At that point guidance tends to diverge with regard to the frontal
intrusion leading to high uncertainty.  Specifically, the ECMWF/CMC
are much less aggressive than the GFS which pushes the front
through half the fcst area on Friday, stalling it out near/along
I85 leading to enhanced focusing for convection.  Not to mention
the possibility of partial wedging given the northeastward movement
of the new reinforcing parent high.  The former solutions really
never favor frontal intrusion at all, keeping it westward across
the TN valley and thereby holding northeast GA and the western
Carolinas in the warm sector.  This later solution would favor
lower pops, and looks plausible given the strength of the Atl ridge
to the east.  Therefore the fcst will favor such trends with pops
lowering through Friday to slight/chance levels.  Continuing with
the ECMWF, the upper ridge looks to flatten a bit and shift westward
on Saturday allowing for lower heights across the entire fcst area.
This pattern looks to hold through the remainder of the period
with no major synoptic features expected to move into the region.
Therefore pops will remain rather stagnant amidst this warm/moist
pattern with chances generally atop the high terrain, and slight
chances elsewhere, with occasional thunder possible each afternoon
through Sunday.  Temperature through the period will remain well
above normal levels.


At KCLT and elsewhere: Surface high pressure will continue to wedge
in from the northeast over the period...while a highly moist frontal
band crosses aloft. Expect precip and lowering CIGS to begin arnd
midnight at KCLT and a little earlier over the Upstate and higher
elevations. CIGS will continue to drop in a soupy airmass with IFR
conds developing over all terminals thru the overnight period.
Precip shud taper off by the afternoon...but IFR conds continue thru
the daytime as the wedge is reinforced with continued moist s/ly
lift. VSBY will generally remain MVFR with some sites possibly
dropping into the IFR range thru the period.

Outlook: Unseasonably warm and humid conditions are expected to
develop by Tuesday, and persist through the end of the week. Shower
chances will be highest across the mtns, while periodic cig and
visby restrictions will be likely through the week, esp during the
late night and morning hours.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  97%     High  95%     Med   69%     High  87%
KGSP       High  93%     High  95%     Med   74%     High  86%
KAVL       High  95%     High  95%     High  87%     High 100%
KHKY       High  97%     High  82%     Low   59%     High  84%
KGMU       High  94%     High 100%     Med   68%     Med   78%
KAND       High  89%     High  91%     Med   61%     High  81%

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:



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      75 1939     26 1908     50 1951      4 2015
   KCLT      77 2014     31 2015     59 1939      7 2015
   KGSP      75 2014     33 2015     57 1939      9 2015


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      77 1986     12 1896     54 1997      8 1896
   KCLT      75 2011     32 1896     56 1997      6 1896
   KGSP      75 1917     35 1978     55 1997      9 1896


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      72 1897     27 1939     54 1897      2 1963
   KCLT      74 1990     33 1963     62 1897     12 1963
   KGSP      76 2011     35 1969     55 1980     10 1963


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      76 1982     29 1989     54 1922     10 1939
   KCLT      76 2012     29 1901     56 1944     19 2009
                1980                                1978
                1962                                1963
   KGSP      76 2012     36 1966     57 1922     15 1963
                1996        1939


  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




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