Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240247

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
947 PM EST Fri Feb 23 2018

Very warm conditions will continue into the weekend. A cold front
will slowly cross the area Sunday into Monday, bringing greater
chances of showers and a few thunderstorms with it. Drier and cooler
high pressure will settle over the region Tuesday.


As of 940 PM: Overall the pattern hasn`t changed all that much
with a large upper anticyclone centered over the western Atlantic
keeping a stalled frontal axis to the west.  With that, another
round of above-normal temperatures is on tap for tonight. Updated
overnight temp trends reflect the adjustments made to sky cover,
that is, more radiative cooling. Another round of early-morning
Piedmont stratus and some fog is expected again, mostly on account
of persistence within this stagnant pattern. Some scattering is
expected later in the day, though a healthy if not shallow cu
field and patchy cirrus will make for mostly cloudy conditions
overall. PoPs are looking less and less likely through the wee hours
of the morning, so new values with this update were trimmed down
below mentionable criteria, away from the upslope areas of the SW
mtns and immediate vicinity. A little before dawn, guidance does
suggest a weak shortwave moving along the stalled front may allow
expansion of any precip to the remainder of the mtns and foothills.
Beyond that, heating into the afternoon hours on Saturday could
yield enough free SBCAPE along the TN line, given that area`s
distance from the aforementioned Atlantic ridge, thus iso/sct TSRA
cannot be ruled out. While much above normal temps will return
for Saturday afternoon, broken max temp records are less likely
simply because the records are a few degrees warmer.


As of 100 PM EST Friday: At the start of the short term period
Saturday evening, the strong upper ridge will still be in place over
the western Atlantic, with a deep upper trough building over the
Rockies. The initial shortwave embedded within the trough will be
approaching the MS Valley at the start of the period, with a
secondary shortwave diving into the northern Rockies. Surface highs
in eastern Canada and over the western Atlantic will be separated by
the quasistationary warm front, while a cold front will drag across
the Lower MS Valley as the surface low over the Mid MS Valley
continues to occlude. Showery activity can be expected Saturday
night especially across typical upslope areas as low level WAA
continues. The cold front will push east on Sunday, losing some of
its oomph, as the upper ridge is forced south and will begin to
retrograde into the Gulf and western Caribbean. With the area
remaining in the warm sector, the synoptic lift (though weakening)
added by the front will likely be enough for at least a few
thunderstorms to develop across the Piedmont and maybe foothills,
with better chances across the Upstate and NE GA. SBCAPE is not
particularly impressive, remaining below 500 J/kg, with deep-layer
shear increasing (50-60kt) as the front approaches. Certainly some
minimal concern for HSLC convection Sunday afternoon, so will
continue to watch forecast evolution. Expect one more day of
afternoon highs 15+ degrees above seasonal normals.

Still seeing some differences in how the ECMWF and GFS are handling
the secondary surge of moisture on Monday, but the 12z ECMWF has
come in slightly farther SE with the QPF axis, closer to what the
GFS has been indicating. Basically that secondary shortwave will
push into the Plains and pull another slug of moisture out of the
Gulf as the upper wave begins to damp. The cold front will stall
across the area and may briefly lift back north with the surge of
moisture, but it`s starting to look like the more significant
rainfall is less likely (and even if so, it would be farther S) with
the surface low working it`s way more across the fall line rather
than I-85. Should see highs on Monday a good 5-8 degrees cooler (but
still 10 or so degrees above normal) than on Sunday, but this will
be highly dependent on how quickly the front moves through. All pops
should be pretty much out of the area by 00z Tuesday.


As of 130 PM EST Friday: Dry high pressure will build in on Tuesday
in the wake of the frontal passage, with highs still above seasonal
normals but at least trending in the right direction. A broad low-
amplitude ridge will build across the Southeast, while a cutoff
midlevel low forms over CA. Split flow pattern will come together
over the Northern Plains, and as the cutoff low slowly moves east,
surface low formation over the central Plains will begin to drag
another slug of moisture out of the Gulf and into the Lower MS
Valley. Our surface high will move off the NC/VA coasts by Tuesday
night, with the moisture advection really picking up shortly
thereafter. Expect some showery activity to begin Tuesday night
across the typical upslope areas, spreading east and north Wednesday
as the warm front surges; however, indications are that the
precipitation into the dry surface layer may result in some insitu
damming Wednesday, which will stall the warm frontal surge into the
Carolinas and keep highs Wednesday pretty close to seasonal normals.
GFS is quite a bit stronger with the development of the surface low,
and also a bit faster, but enough similarities in the general trend
to lend some confidence to precip pushing out on Thursday night.
Damming will erode (per current guidance) on Thursday leaving our
area again in the warm sector and highs Thu about 10 degrees above
normal. Lingering NW flow showers can be expected across the
mountains with the upper system, and behind the front (whenever it
ends up passing) we might actually get some CAA to work with, but
too far out to get into details at this time.


At KCLT and elsewhere: Southwesterly low-level flow will continue
this period with very weak isentropic upglide overnight. The evening
hours will be mostly clear, until cirrus begins to increase ahead
of a weak vort max which will track along the Appalachians in
the early morning. The weak upglide once again is expected to
promote stratus and some fog development. Guidance has mostly
been too pessimistic with cigs in this pattern. However, sfc
radiation may be a bit more of a factor tonight than it was in
previous nights. Therefore confidence is not very high on cigs;
kept IFR out of the TAFs except at KHKY/KAND where sfc moisture is
expected to be a bit greater. The passing vort max will bring small
precip chances to the mountains late tonight and in the morning,
and a small chance of diurnally driven SHRA exists tomorrow.

Outlook: Moist conditions will keep chances for restrictions
elevated through the period, with the best chances for low cigs/vsby
to occur each morning.  Precipitation chances are also slightly
elevated, mainly across the high terrain.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High  93%     High  80%     High 100%
KGSP       High 100%     Med   79%     Med   67%     High 100%
KAVL       High  94%     Med   76%     Med   76%     High 100%
KHKY       High 100%     Med   78%     Med   78%     High  92%
KGMU       High 100%     Med   73%     Med   63%     High 100%
KAND       High 100%     Med   73%     Med   66%     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:



  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


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      78 1930     17 1967     51 1985     -2 1967
   KCLT      82 1930     27 1967     58 1890      7 1967
   KGSP      79 1930     30 1967     57 1985      8 1967




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