Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230253

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
953 PM EST Thu Feb 22 2018

Warm and moist conditions will continue into the weekend until a
cold front moves through on Sunday. Showers will then linger Sunday
into Monday before dry high pressure builds in on Tuesday.


As of 945 PM: A rather stagnant pattern continues to prevail
across the eastern half of the CONUS as strong subtropical ridging
dominates over the western Atlantic, with troffing out west.  At the
surface, the primary feature continues to be a quasi-stationary
frontal axis draped along the OH and mid MS valleys, which continues
to promote modest QPF for those areas. Closer to home, sly flow
around the parent surface high consequent of the upper anticyclone
mentioned above is promoting sustained warm/moist advection across
the southeast states. Decoupling this evening should allow for low
stratus development tonight, somewhat like what happened the past
few nights. Sky grids reflect clearer conditions this evening,
with the stratus not becoming widespread until the predawn hours.
Calmer winds tonight should permit more fog development than last
night, though vsbys may not drop as low due to dewpoints having
mixed out somewhat this afternoon.

The latest adjustments to temp trends were to cool a little more
rapidly given the expectation of clearer skies for most of the
night, bringing mins a bit lower. Nonetheless record high mins still
are forecast at all three climate sites. Overnight PoPs still look
very small and are too low to mention away from the south-facing
Escarpment. The 00z cycle of the NAM family of models still depict
a back door front (wedge boundary) settling into the I-40 corridor
of the NC Piedmont Friday morning. However, with no support from
GFS/EC and with minimal model QPF, the idea of the wedge making
inroads this far south seems questionable. Only a slight-chance
PoP is included for that possibility, and this mention has been
pushed back by a couple hours with this update.

The low stratus will lift into the mid/late morning hours with
partly cloudy skies prevailing by period`s end. Expecting record
highs again Friday afternoon.


As of 200 PM EST Thursday: The persistent upper ridge will remain in
place over the western Atlantic with corresponding surface high
allowing low-level WAA to continue across the Southeast. As we start
the period on Friday night, a sharp shortwave pushing across the
Rockies will begin to lift into the Plains, while a warm front
remains in place from TX to the OH Valley. This will result in a
continued focus of prolonged excessive rain across that area.
Impacts to our area will be pretty minimal, but continue upslope
flow toward the mountains will allow for intermittent showery
conditions to continue, with an uptick starting Saturday afternoon.
With the warm and humid conditions, there may be enough surface
instability to get a few thunderstorms especially across southern
zones, but nothing too significant. The bigger story will be the
continued ridiculously warm temperatures, with lows remaining closer
to seasonal highs, and highs 15-20 degrees above normal, both around
record territory. This will continue on Sunday, but could see highs
Sunday just a couple of degrees cooler with increased cloud cover
(see below).

During the day Saturday, another shortwave will dive down the NW flow
behind the Plains wave into the Rockies, acting to finally kick the
systems east. The upper ridge will be forced SW and will continue to
slowly retrograde as we push into the extended. However, for Sunday,
the cold front will stretch across our area and elongate as it
stalls, while that secondary shortwave pulls another slug of
moisture out of the Gulf and toward the Southern Appalachians,
keeping pops in the area through at least Monday morning, though the
ECMWF is a little slower with the final kick and doesn`t move pops
out until Monday evening. Where the front ends up across the area
Sunday into Monday will of course significantly impact temperatures
and confidence is low, but for now given the pattern figured it`s
better to err on the warm side so generally trended toward the
slower ECMWF. Even so, Monday highs will be much more reasonable
(still above normal), with improvement expected into the extended as
the front finally scours the airmass out.


As of 230 PM EST Thursday: Behind the front Monday night into
Tuesday, conditions look much more seasonable (still slightly above
normal temperatures) and overall pleasant with dry high pressure
dominating the area into Tuesday night. The upper ridge will be over
the western Caribbean by this point, but a deepening trough across
the Desert SW (cutoff low on the ECMWF but sharp/deep trough on GFS)
will start raising some ruckus for the end of the period. Another
round of moisture will lift toward the area during the day on
Wednesday as, once again, low-level WAA increases around the western
periphery of the surface high that will be centered just off the
NC/VA coast. Differences in the operational models become more
unsurprisingly more significant toward the end of the period, but
generally still looks like moisture will be lingering with
temperatures remaining above normal.


At KCLT and elsewhere: A moist southwesterly flow pattern continues
across the region. While skies are mostly clear as of 00z, weak
isentropic upglide should permit low stratus to fill back in during
the early morning hours. Guidance proved too pessimistic last
night, with IFR verifying much more limited in temporal/spatial
extent than depicted. However it seems GLAMP picked up on the
trend and depicted low MVFR to IFR not forming until just before
dawn across the Piedmont, lingering until mid-morning. KAVL is the
only site expected to escape restrictions tonight. High pressure
will set up on the Eastern Seaboard overnight and will bring a
cool northeasterly flow to parts of the Mid-Atlantic. NAM family
of models, plus Canadian Regional, depict the backdoor wedge front
reaching KHKY if not KCLT in the morning. However, with so little
precip depicted near the wedge boundary, that seems unlikely,
and the forecast will reflect a continuation of southerly flow,
with improving cigs by midday.

Outlook: Unseasonably warm and humid conditions are expected to
persist through the end of the week. Shower chances will be highest
across the mtns, while periodic CIG and VSBY restrictions will
remain likely during the late night and morning hours. A frontal
passage on Sunday is likely to bring SHRA and possibly TSRA, with
more seasonable weather possibly returning Monday in the wake of
the front.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       High 100%     Med   66%     High  87%     High 100%
KGSP       High 100%     Med   75%     High  85%     High 100%
KAVL       High 100%     High  85%     High  89%     High 100%
KHKY       High  98%     Med   79%     High  87%     High  94%
KGMU       High 100%     Med   68%     High  81%     High 100%
KAND       High 100%     Med   62%     Med   77%     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      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


  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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