Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 231933

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
233 PM EST Thu Feb 23 2017

A cold front will move through the area late Friday, increasing
chances for rain and thunderstorms. Another stronger front will move
through the area early next week, increasing rain chances again.
Temperatures will remain above average through next week.


As of 200 PM: The region will remain under the influence of a
somewhat nebulous pressure pattern through tonight, with a shortwave
upper ridge centered over the Southeast, while one sfc low continues
to meander eastward out of Florida, and another swings into the
mid-Mississippi Valley. The net result will be a continuation of
the light southerly flow we are currently seeing, and a midlevel
inversion which will serve to trap low-level moisture and also to
permit only shallow convection. Low stratus is expected to move in
during the predawn hours, while some fog may develop and produce
appreciable vsby restrictions (particularly if the stratus does
not arrive as early as anticipated). Other than a few sprinkles
near the Blue Ridge, the fcst remains dry through morning. Mins
will remain a remarkable 15 to 20 degrees above normal.

As heights fall to our west with the eastward movement of
the Mississippi Valley low, low level winds pick up and veer
slightly, providing slightly better upsloping. By late afternoon
this may generate some very light precip along the southern-facing
Escarpment, but only a low slight-chance PoP is warranted. Inversion
looks much too strong for any chance of thunder at that time, so
that mention has been removed from the fcst. Temperatures should
trend upward from today, reaching the upper 70s over much of the
Piedmont, and low 70s even in the mountain valleys. The current
fcst is within a couple degrees of the records at GSP and Charlotte,
but well shy at Asheville.


As of 2pm Thursday:  Friday evening, upper long wave/surface cold
front will be approaching the area from the west, with front
reaching TN/NC stateline early Saturday morning, and passing through
the CWA by noon on Saturday.  Airmass is strongly capped ahead of
the front.  Cap erodes with FROPA, though BUFKIT soundings indicate
only marginal instability will be present, as front passes through
at a diurnally unfavorable time for CAPE.  Line of possibly severe
storms will be approaching with the front, but latest NAM/GFS both
deamplify line dramatically as it crosses the mountains. It is
possible some severe winds will persist for awhile as line moves
into the mountains and the CWA Saturday morning, but storms should
break-up as they move much further east. With 850mb winds of 30kts
ahead of the surface trough, Winds will increase with the FROPA,
especially over the higher terrain, though they should remain below
any advisory need. Dewpoints plummet behind the front with dry/cool
advection, which will further limit shower potential, with activity
mostly completed by noon on Saturday.

There is some potential for very light snow over the higher mountain
elevations Saturday evening with some upslope enhancement and cold
air, but this will be very limited due to very dry airmass.

Surface high pressure will build-in behind the front with clear
skies and light winds on Sunday.  Sunday morning is the cold spot in
the forecast with clear skies/light winds/ and dry air all
contributing to below freezing temperatures over western North
Carolina and northeast Georgia, with low to mid 30s and possible
frost in Piedmont areas. High moves east of the area Sunday evening,
with some southwesterly surface flow developing, that will begin the
process of increasing moisture again.


As of 215 PM EST looks like the middle part of next
week will be fairly active across the region. By the time high
pressure moves off the east coast Sunday night, the next system
should already be organizing to our west. The first issue has to do
with the arrival of moisture and forcing on Monday, and the new
ECMWF does not clear any of that up. In fact, the trend is opposite,
with the GFS being a bit faster while the ECMWF is slower as it
delays the arrival of moisture and precip until late Monday. The new
guidance at least gives some better confidence that rain will not
develop until after sunrise on Monday, so the fcst reflects that
thinking with a dry Sunday night. The fcst trends more GFS-like for
Monday with a chance of showers developing from W to E by late in
the day. From that point onward, although the details are different,
the guidance suggests a similar conclusion, that being a boundary
laying down across the area late Monday through at least Tuesday.
Low level moisture and isentropic upglide suggest at least a chance
of precip across the entire region through Monday night and Tuesday.
The location of the boundary might be problematic for temps. If it
sets up to the south, it may become the edge of an in-situ cold air
damming wedge on Monday that persists into Tuesday, in which case
the high temp fcst of well above normal values is called into
question. Will not depart significantly from the consensus...yet.
Will the boundary wash out Tuesday night as in the ECMWF or will it
remain active through Wednesday as in the GFS? Through the middle
part of the week, the passage of two short waves to the north may
bring two low level jet surges across the region, but the timing is
uncertain, as is the degree of instability.  However, now that the
atmosphere thinks it is April, we have to be on the lookout for
severe weather, and it seems like in this pattern we will get some
sort of overlap between higher shear and low CAPE at some point
between Monday and Wednesday night. The front should move across the
region early Thursday to bring an end to most problems. There could
be some NW flow precip early Thursday, with a changeover to snow
showers at the high elevations along the TN border, before the
moisture pulls out. The passage of the front should knock our temps
back on the order of ten degrees.


At KCLT and elsewhere: A low VFR cumulus field will last through
the remainder of the afternoon over most of the area. Light SE
winds will continue between mid-Atlantic sfc ridge and low pressure
entering the Ohio Valley. This moist flow appears likely to bring
in IFR stratus and fog in the early morning hours; cigs are expected
to be more impactful than vsby. (Guidance members showing LIFR vsby
tonight seem to do so due to persistence within a similar flow
regime to what was seen this morning; it looks like the stratus
is likely to form early enough to limit the formation of a fog
layer.) An isolated shower can`t be ruled out over the foothills
during the period but chance at TAF sites is too small to mention.

Outlook: Moist southerly to easterly low-level flow will persist
across the area until a cold front pushes thru on Saturday. This
will keep high chances of morning stratus and possibly fog again
Saturday morning. Drier conditions set up on Sunday.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     High  82%     Med   76%
KGSP       High 100%     High 100%     Med   72%     Med   78%
KAVL       High 100%     High  90%     Med   66%     High  82%
KHKY       High 100%     High  93%     Med   67%     High  89%
KGMU       High 100%     High 100%     Med   73%     Med   76%
KAND       High 100%     High 100%     Med   71%     Med   74%

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      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




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