Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241731

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1231 PM EST Sat Feb 24 2018

A cold front will approach from the west tonight and then slowly
cross the region Sunday into Monday, with improved chances of
showers and thunderstorms through the period. Cooler and drier air
will settle southward into the region behind the front on Tuesday
before moisture rapidly returns to the area mid to late week.


As of 1230 PM EST Saturday:  Updated discussion for 18z taf
issuance.  Overall the fcst looks good at midday as temp trends are
on schedule amidst sct`ing low stratus.  Some low end gusting is
favored this afternoon across the NC piedmont, and is already
underway at this time.  Otherwise, no sig changes needed/made with
this update.  Full fcst discussion to follow within the next

Previous Discussion:  The persistent synoptic pattern finally begins
to break down thru the near-term as a strong wrn trof is reinforced
with energy diving south out of wrn Canada. This trof will propagate
east and begin to weaken and displace the ST high which has been the
dominant factor in the weather over the area the past several days.
Model soundings show PWAT values increasing arnd 70 percent over
the next 24-36 hrs as moisture increases thru a deepening layer
this afternoon. The 00z 3-km NAM holds on to a dry stable layer
aloft more than the other op models and therefore has less of a
chance of precip and thunder. Even the moister and less capped GFS
only produces arnd 300 J/kg of sbCAPE this afternoon mainly over
the higher terrain. So...PoPs will remain low-end except across
the SW facing srn mtns where mech lift is aided by an increase
in mlvl destabilization and difl aloft ahead of a cold front. An
isol tstm or two is possible across the NC mtns but these will be
short-lived and non/organized.

Max temps will once again reach abt 20 degrees abv normal in good
sw/ly flow with weak WAA noted in the h92-h85 layer. With high
moisture in the low-levels and sw/ly flow maintaining a mixed sfc
layer...mins will also be held abt 20 degrees abv normal.


As of 310 AM EST Saturday: The model guidance suggests that we
can tighten up the time of arrival of the main precip associated
with the approach and passage of a cold front on Sunday. Still
plenty of support for bringing high precip probability across the
fcst area with excellent forcing and deep moisture, but all the
guidance has the front to the west of the mtns at 12Z Sunday. That
means the eastern zones will have a very low chance at daybreak,
although it will still ramp up into the likely range by the early
afternoon. The main concern is still the possibility of severe
thunderstorms with the front on Sunday, given what should be an
active boundary well to our west through Saturday evening. The
model guidance shows excellent low level convergence with the
front approaching from the west Sunday morning, supporting the
idea that some sort of weakening QLCS might be just to the west
at daybreak, suggesting that the western mountains might have
a wind damage threat early in the day as the front arrives. The
weakening trend continues into Sunday as the front crosses the
mtns though, with sfc-based CAPE perhaps only as high as 300 J/kg,
and shear perhaps only on the order of 20-30 kt. As such, not even
the Marginal risk translates eastward into the western Carolinas
for Day 2, and at this point this seems reasonable unless/until
we see either better instability or better shear. Have included
some thunder, but the severe possibilities are too low to mention
in the HWO right now. High temps will be at least 15 degrees or
so above normal. The guidance also agrees that the frontal precip
band will push off to the east late in the day, perhaps resulting
in brief lull Sunday evening before another shot of mid/upper
forcing moves in overnight to redevelop the precip along and west
of the front. Precip prob ramps right back up to at least likely
early Monday morning in response to the excellent agreement seen
in the model guidance. The jet streak lifts out to the northeast
during the day, along with most of the vorticity, taking the
surface boundary off to the east, so a steady tapering off of
precip is still expected Monday afternoon. Those developments
should help to keep temps merely ten degrees above normal. The
upper trof axis should pass Monday evening, followed by a flat
upper ridge supporting sfc high pressure moving in from the west
to dry us out on Tuesday. Temps will be another category cooler,
but still above normal.


As of 1210 AM EST Saturday: Dry wx is still anticipated on Tuesday
as low amplitude upper ridging builds atop the SE CONUS and sfc
ridge axis translates from the mountains to the coast by evening.
Maximum temperatures remain progged to be about 2 categories above
climo.  Medium model solutions are in decent enough agreement with
respect to the sensible wx for Wednesday, with the consensus
developing waa pcpn eastward into the cwfa.  Based on this, will
plan on creeping chances upward to categorical in favored locales,
and with the clouds and pcpn, a smaller diurnal temperature range
should result, maxes topping out near the late February normal.
There is still plenty of time for model timing differences to work
themselves out with respect to the passage of a moderately forced
cold front Thursday and/or Thursday night.  Even at this point, it
is hard to believe there won`t be categorical shower coverage at
some point during this period followed by a dry punch on Friday
within a gusty post-frontal airmass.


At KCLT and Elsewhere:  Conditions continue to improve across the
region from earlier low stratus and/or fog restrictions.  With that,
moisture in the llvs will remain sufficient for redevelopment of cu
this afternoon beneath mid level cigs, thus all sites favor such.
Winds will remain sly with some low end gusting possible across
the NC piedmont, thus KCLT/KHKY tafs feature such this afternoon.
Otherwise, low stratus and/or fog will return tonight amidst a
decoupling BL, all ahead of cold front that is progged to move into
the region late Sunday morning.  With that, MVFR/IFR restrictions
are expected by around daybreak, lasting a few hours before sct`ing,
then returning in association with any frontal convection toward
the end of the taf cycle.  Did include an afternoon prob30 at KCLT
for tsra.  Sly winds will increase ahead of the front on Sunday,
thus all sites feature low end gusting.

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...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       Med   78%     High 100%     High 100%     High  85%
KGSP       High  85%     High 100%     High 100%     High  93%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High  94%     High 100%     Med   78%
KGMU       High  85%     High 100%     High 100%     High  85%
KAND       High 100%     High 100%     High 100%     High  93%

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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