Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190016

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
716 PM EST Sat Feb 18 2017

A weak upper low moves east across the Carolinas tonight then off
the Carolina coast Sunday. A strong upper ridge moves in to start
the week, with much above normal temperatures. Another period of
light rainfall is possible mid week as a weak upper trough moves
through the region. A cold front approaches at the end of the week
with possible showers and thunderstorms.


As of 7 PM, light precip is pushing east of the forecast area early
this evening, with widespread cloud cover lingering across much of
the forecast area. Timing of the dissipation of this cloud cover is
the primary forecast challenge of the overnight period, as guidance
indicates a good downslope component to the low level flow will not
develop until after midnight, perhaps well after midnight. As such,
the onset of clearing skies east of the mtns has been delayed until
midnight and thereafter. Wet bulb effect from the earlier light rain
has lowered surface dewpoint depressions across much of the
forecast  area, but they generally remain in the 10-15 degree range,
so skies are going to have to clear substantially to allow for the
late night fog formation that is indicated in much of the
statistical guidance.

Otherwise, the center of the old upper low will move across the
Great Valley and Srn Appalachians during the early morning hours and
could provide the weak forcing necessary for some light precip near
the TN border, especially toward daybreak as a WNW flow develops at
low levels. Temps should remain warm enough to keep almost all of
the precip in liquid form, except above about 6k feet. Low temps are
expected to be on the order of ten degrees above normal.

On Sunday, the WNW flow should continue to act on remnant moisture
over Ern TN to produce a few showers on the NC border through about
midday. Otherwise, an upper ridge to the west will build high
pressure into the region. Temps look like they will be well above
normal with highs about 15 degrees above normal, conservatively.


As of 2pm EST Saturday: By Sunday evening, southeast CONUS upper
shortwave will be well east of the area and a transitory upper ridge
will be moving in, with dry downslope northerly flow and clearing
skies.  This ridge will keep things dry and mostly clear on Monday,
with clouds increasing prior to the next frontal/upper wave system
due in Tuesday evening, with a chance from some showers in the
mountains as early as Tuesday evening.  Temperatures will be 10 to
15 degrees above normal Monday and Tuesday.  Winds will continue to
be northerly to northwesterly at 5 to 10 kts early on Monday,
veering to easterly and weakening as center of surface high settles
over the region Monday evening, and then picking-up from the
southwest on Tuesday in advance of the next trough.


As of 230 PM EST Saturday:  The medium range fcst period initializes
on Tuesday evening amidst weakening east coast ridging by way
of a progressive northern stream trough, while a close southern
stream upper cyclone moves out of the Rio Grande Valley into the
central Gulf of Mexico.  At the surface, the primary features of
note will be a broad anticyclone along the east coast and a cold
front extending across the Great Lakes region into the OH Valley.
Synoptic pattern evolution moving through the period will be
highlighted by total breakdown of the amplified east coast
upper ridge leading to migration of the elongated surface high
southward near Bermuda.  The aforementioned cold front looks to
stall across the OH Valley, possibly making it as far east as
the Central Appalachians, however not likely intruding into the
Southern Appalachians thanks to the repositioned surface ridge.
Meanwhile, guidance seems to have converged on some details
regarding the closed H5 cyclone moving through the Gulf, which
now looks a bit weaker, strongest in the ECMWF, with good track
agreement to the southeast toward southern FL.  Thus, little
if any impacts are expected across Northeast GA and the Western
Carolinas.  That said, weak moist upglide atop the offshore high
will at least warrant low end pops both Wednesday and Thursday.
Moving along, as the southern stream H5 low moves out of the
picture, a few days of southerly flow are expected across the
southeast states thanks to the Bermuda high, while a west coast
system gains strength and plows through the plains into the MS
River Valley by Friday.  Surface cyclogenesis beneath a modestly
deep upper low looks to yield a rather strong/progressive surface
cold front that will march across central CONUS and into the
Appalachian region either on Friday evening, possibly sometime
on Saturday.  Timing inconsistencies within the op guidance lead
to much uncertainty regarding the details, nevertheless, expecting
a strong and potentially convectively active fropa next weekend.
Behind said front, broad Canadian high pressure looks to dive
southward into the Plains by yielding drier yet cooler conditions
to round out the weekend.   Temperatures are expected to remain
well above normal, cooling to near normal at periods end.


At KCLT and elsewhere: Cigs are expected to settle in the 025-040
range at most terminals over the first couple of hours of the
forecast period, and should persist through the evening. Low MVFR
cigs will be most likely at KAND, but will also be possible at the
other Upstate SC terminals and KAVL. After this evening, uncertainty
abounds regarding the timing of cloud dissipation and the related
potential for fog development. Guidance is pretty aggressive in
developing MVFR and even some pockets of IFR visby late tonight, but
based upon observed temp/dewpoint spreads as of 00Z (still 10-15
degrees in most areas), the clouds will almost have to dissipate for
any reduced visby to occur, and again the timing of this is quite
uncertain. That being the case, opted to limit the mention of MVFR
conditions to this evening at most terminals, except for a tempo
from 06-09Z for 5SM/SCT030 at KCLT, and categorical low MVFR cigs at
KAVL after midnight. Light S/SW winds will become light NW by late
Sunday morning, as a weak cold front pushes through the area. This
should also eliminate much of any lingering sky cover.

Outlook: Expect VFR conditions to continue through early next week.
Another system may bring chances of precipitation and restrictions
to the area Tuesday into Wednesday.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  97%     High  93%     High  97%     High 100%
KAVL       Med   75%     Med   74%     High  83%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High  97%     High  95%     High  94%     High 100%
KAND       High  80%     Med   63%     High  91%     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 2014     16 2015     54 1951      3 2015
   KCLT      76 2014     26 2015     55 1961      8 1958
   KGSP      75 2014     30 2015     55 1961      9 1958
                1916        1900                    1900


  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




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