Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201503

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1003 AM EST Tue Feb 20 2018

High pressure off the East Coast will provide a warm, moist
southerly flow through the middle of the week. Another cold front
will descend from Tennessee by Thursday. This front will stall out
and linger just north of our area next weekend. Chances for rain and
well above average temperatures will remain through the weekend.


As of 1000 AM EST Tuesday: With this update, have extended the
current Dense Fog Advisory until noon. Have also made minor tweaks
to going temperatures to coincide with current trends as the rest of
the near term forecast remains on track.

Dense fog persists across much of the Upstate and Piedmont this
morning as the insitu CAD wedge lingers across the FA, with current
temperatures in the mid 50s/lower 60s. Latest guidance is suggesting
mixing will occur closer to noon to help lift the fog, this
timeframe being a bit later than previously anticipated. Thus
have extended the current Dense Fog Advisory in effect through

Otherwise, the main story today will be how quickly and to what
extent the lingering in situ CAD wedge erodes thru the day. Guidance
generally agrees that it should erode much more efficiently than
yesterday, as the inversion in the soundings is weaker and cloud
layer/RH depth is shallow. It will remain mostly cloudy throughout
the day, however, with patchy sprinkles/drizzle still possible,
mainly near the escarpment. Temperatures are expected to warm into
the lower 70s south of I-85, back into the 60s elsewhere.

Tonight, moist SWLY flow will continue atop the CWFA between a large
ridge to our east and deep trough to our west. There`s just enough
vestige of a cold pool across the foothills and piedmont such that
stratus and fog will likely redevelop/expand across the area after
sunset. Dense fog doesn`t look as likely/widespread in the guidance,
but cannot rule out another DFA needed for some portions of the area
thru early Wednesday. Temps will remain way above normal under
clouds and persistent LLVL WAA flow. Min temps will actually stay
above normal afternoon highs, possibly breaking high minimums for
Wednesday morning. As for PoPs, guidance really wants to ramp up QPF
response within the SWLY flow, but other than some weak upglide and
upslope flow, there isn`t much to force precip. Moisture depth in
the forecast soundings does increase somewhat, so I do allow PoPs to
ramp up into the CHC range by daybreak Wednesday, favoring the areas
near the escarpment. Any precip accums should be very light.


As of 310 AM EST Tuesday: A deep-layer southwesterly fetch will
persist Wednesday through Friday night between upper high pressure
anchored off the southeast coast and a trough over the western
CONUS. Weak short waves ripple through the flow and move near the
forecast area but remain mainly to our north and west. At the
surface, a similar pattern exists with high pressure over the
Atlantic and a cold front slowly moving southeast toward the area. A
moist southerly upslope flow remains in place through the period.
However, the moisture isn`t very deep. Synoptic scale forcing and
low level isentropic lift are weak as well. The upslope forcing is
decent and this should help initiate showers from along the Blue
Ridge north and west across the mountains. Precip chances taper off
to the east and south where moisture and forcing diminish. There may
also be a few thunderstorms each afternoon as weak instability does
develop. QPF is generally light through the period, but some
moderate amounts are possible across the southwestern mountains
Wednesday morning. Highs and lows remain very warm with near record
high temps and lows near record high minimums with readings above
normal high temps.

The complicating factor in this forecast is the afore mentioned cold
front. Guidance agrees that the northern portion of the front will
sag south toward the area in backdoor fashion. However, they don`t
agree on how far south it moves. The GFS and NAM suggest the front
moves into the NW piedmont and northern foothills. However, the
ECMWF and the Canadian show it stalling somewhere near the VA/NC
border. The models are consistent with themselves from run to run
but the dichotomy between solutions remains. Have retained some
features of a backdoor front but don`t mainly show a glancing blow
across the NW Piedmont.


As of 120 AM EST Tuesday: Starting Friday morning, expecting surface
high pressure over Maine ridging down the East Coast and pushing a
backdoor cold front to perhaps halfway across NC. The I-77 corridor
could be affected briefly before the high moves out over the
Atlantic and this front retreats north as a warm front into
Saturday. A mostly stationary front extends from the Ohio Valley to
Texas on Friday.

The 500mb pattern has a high centered east of the Bahamas with SW
flow pumping moisture from the Gulf to across the SE States Friday
into the weekend. The deep 500mb trough over the West on Friday
begins to move east over the weekend. The Bermuda / Bahamas High
will be moving east or suppressed south as ridge breaks down with
this western trough moving toward the East Coast. Moisture gets
deeper progressively but of course best along the front which will
progress east from the Mississippi Valley Saturday. 850mb flow on
both the GFS and EC has SW wind of 40 to 50 kts Saturday night
crossing our area. POPs increase on Sunday as the front crosses our
area. Instability rather low Sunday afternoon with CAPE values over
the Carolina Piedmont around 200 or less. Shortwave energy crosses
the Great Lakes Sunday with the frontal precip fading as it crosses
the Carolinas. The GFS is a bit faster in taking the front through
and EC a bit slower but each have the front out of our area by
Monday morning. The front stalls out early next week from the
northern Gulf Coast to northern Florida. General drying but there is
some potential waves passing along this frontal boundary could
affect at least southern portions of our forecast area in the first
half of next week.

Temps over ten degrees above normal until front passes then closer
to normal early next week. Max temps a little cooler on Friday for
the NC areas north of CLT due to the backdoor cold front.


At KCLT and elsewhere: Not much change in thinking from previous TAF
discussion. LIFR to VLIFR conditions continue thru late morning, as
a weak in-situ wedge persists. The wedge is expected to erode a
little more today than yesterday, resulting in more improvement for
KAVL and KCLT. The Upstate sites and KHKY will be under the low
stratus the longest, and may not even get to MVFR thru sunset. At
least, the fog should lift by midday. Tonight, with the air mass
still very moist, LIFR conditions will likely return with redeveloping
low stratus and fog after sunset. Winds will be light thru the
period, variable within the wedge, but favoring a south direction

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 be
likely through the week, esp during the late night and morning hours.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       Med   64%     Med   72%     Med   61%     High  88%
KGSP       Low   57%     High  85%     Med   66%     High 100%
KAVL       Med   74%     High  83%     Med   66%     High  82%
KHKY       Med   61%     Med   61%     Med   64%     High  88%
KGMU       Low   50%     Med   76%     Med   66%     High 100%
KAND       Med   77%     High  81%     Low   51%     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      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


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      77 1986     12 1896     54 1997      8 1896
   KCLT      75 2011     32 1896     56 1997      6 1896
   KGSP      75 1917     35 1978     55 1997      9 1896


  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


GA...Dense Fog Advisory until noon EST today for GAZ010-017-018-026-
NC...Dense Fog Advisory until noon EST today for NCZ035>037-056-057-
SC...Dense Fog Advisory until noon EST today for SCZ001>014-019.


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