Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 120331

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1031 PM EST Wed Jan 11 2017

Expect southerly flow and rising temperatures through Friday.
Cool high pressure is forecast to be centered near New England on
Saturday cooling our region closer to seasonal normals.  Low
pressure will move from Texas Sunday reaching the Tennessee and Ohio
Valleys in the middle of next week.


As of 1030 PM EST Wednesday:  Latest guidance continues to favor
patchy dense fog along the I85 corridor overnight, as well as
northward across portions of the Western NC with the highest
probability of such to occur where lingering/melting snow pack
has yielded micro scale temperature minimas amidst a very moist
surface layer.  Also am expecting areas of fog south of I85 where
visibility could lower into the 2-3sm range where dewpoints have
risen into the mid/upper 40s, with falling temperatures to close
td depressions even further.  Removed all pops from the fcst as
any earlier light shra across the NC high terrain has ceased.
Otherwise, tweaked t/td and sky to account for latest obs/sat and
left the remainder of the fcst as it was.

As of 215 PM: Offshore high pressure will maintain a light and
fairly moist southerly flow over the area through tonight. A
frontal boundary just north of the area should continue to push
north during this time, as a wave of low pressure moves into the
upper Mississippi Valley. In the mid to upper levels, southwesterly
flow associated with a flat ridge will continue to bring patches
of elevated cloud cover over the greater CWFA. One of the primary
challenges for the forecast thru tomorrow morning is to determine
the extent of any advection fog that develops over the damp and/or
snow-covered ground, given the WAA afforded by the southwest flow,
and some degree of radiation even if there is a broken high cloud
deck. NAM continues its trend of producing anomalous surface fields
apparently in response to its snow mask; it suggests widespread
dense fog forming across the NC Piedmont. Most guidance shows low
ceilings forming instead, with surface dewpoint depressions being
too large for those clouds to lower to the ground. I expect that
something in between will verify; the revised forecast contains
moderate vsby restrictions within areas of fog over the NC Piedmont
and the more hilly portions of the Upstate.

Otherwise, a few high-based sprinkles cannot be ruled out over the
mountains this afternoon, with only a small chance of a measurable
shower. These chances diminish this evening, leaving the remainder
of the period dry. As the snow continues to melt and as heights
rise aloft under the building ridge, much warmer conditions are
expected Thursday. Maxes should reach the lower 60s in the mountain
valleys and mid to even upper 60s over the Piedmont.


As of 220 PM EST Wednesday: the forecast for the end of the week
continues to look Spring-like, right down to the cold air damming
wedge which is most common in March. We begin on Friday with
a broad upper anticyclone over the southeast that will support
well above normal temperatures...highs in the 65-70 deg F range
east of the Blue Ridge...more akin to what you would expect in
late March. It won`t last. A large surface high over the Great
Lakes, being pushed along in the northern stream by confluent flow
aloft, will move eastward over eastern Canada Friday night, thus
bridging over the Appalachians. Once this happens, shallow cold
air will plow southward on the east side of the mtns Friday night
and Saturday. This should drive a surface boundary, stretched out
across the Mid-Atlantic region, southward as a back door cold front
through midday Saturday, resulting in a wild temperature swing also
typical of Springtime. The combination of the front, sufficient
low level moisture, and the development of weak isentropic lift
should be enough to account for a chance of light rain and drizzle
over the nascent wedge by Saturday morning. Fortunately, temps
will not be cold enough to worry about any p-type issues. Think
our precip chances will be a bit better than model output would
suggest because of the amount of moisture and the presence of the
low level forcing. The northeast winds, extensive cloud cover,
and light precip will keep temps something on the order of 10-20
degrees cooler on Saturday as compared to Friday. Light precip
should retreat to the Blue Ridge Saturday night while the parent
high collapses and moves off the northeast coast. Low temps will
remain mild.


As of 225 PM Wednesday, The guidance is in good overall agreement on
the weather pattern for the medium range. However, they continue to
show run to run inconsistency keeping confidence on the low side. An
upper ridge builds over the eastern US Sunday and Monday. The ridge
axis slides to the coast on Tuesday as northern and southern stream
short waves move into the central CONUS. The guidance doesn`t agree
on whether the waves phase or one becomes the dominant wave as they
move east toward the area. However, they do agree on a coherent
short wave developing and moving into the TN valley by Wednesday. At
the surface, a stationary front near the NC/VA border washes out on
Sunday as high pressure builds in from the north in a damming like
pattern. The high remains in place Monday before moving east on
Tuesday. Precip chances slowly increase each day as a moist
southerly low level flow develops and strengthens. Although the high
is in a damming pattern, there is little in the way of cold or dry
low level air advecting in from the NE. In fact, temps will warm
above normal Sunday with little in the way of precip over the area.
Temps do cool a little for Monday but rebound Tuesday with the high
moving east. A cold front will move into the TN valley on Wednesday
which is slower than previous model runs. Still, precip chances ramp
up quickly with the moist low level flow in place and increasing
forcing. Instability appears to be minimal given the slower frontal
movement, so any thunder looks to be delayed until after this
period. Temps continue their slow upward climb on Wednesday.


At KCLT/KHKY and the SC Sites:  Initially VFR, lowering to MVFR/IFR,
possible LIFR by around daybreak as moisture continues to advect
into the region from the south.  With that, guidance favors sct/bkn
mid clouds through the first few hours of the taf cycle, before
low stratus builds across the NC/SC Piedmont resulting in MVFR
cigs by early morning, eventually lowering to LIFR near daybreak.
In addition to the cigs, patchy fog resulting in 1-3sm visbs
is favored given continued moisture advection leading to a very
moist/saturated PBL.  A few guidance sources do favor dense fog,
especially at KCLT/KHKY and KAND, however included tempos for such
at all sites with the highest confidence being at the aforementioned
locations.  Beyond that, conditions will slowly improve through
late morning, with low VFR expected at all sites just before noon.
Winds at all sites will remain light sly overnight, increasing
into the daytime hours on Thursday as mixing prevails, possibly
yielding an occasional low end gust.

At KAVL:  Trends similar to that of the sites above, however with
lower confidence in restrictions overnight.  Guidance favors a less
restrictive evening, however opted with a more pessimistic fcst
given the above mentioned moisture advection, as well as elevated
surface moisture levels thanks to melting snow pack.  Therefore the
taf does feature MVFR/IFR restrictions, with a tempo for LIFR by
way of dense fog and 300ft cigs.  Conditions will improve beyond
daybreak to low VFR.  Winds will remain light sly overnight,
increasing into the daytime hours on Thursday as mixing prevails,
possibly yielding an occasional low end gust.

Outlook: Springlike conditions will develop over the area to end
the work week. Cool high pressure ridges down the East Coast over
the weekend, bringing chances for precipitation and restrictions.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       Med   71%     Low   39%     Med   77%     High 100%
KGSP       High  83%     Low   31%     High  85%     High 100%
KAVL       Med   75%     Low   21%     High 100%     High 100%
KHKY       Med   64%     Low   49%     High  89%     High  88%
KGMU       Med   66%     Low   33%     High  86%     High 100%
KAND       High 100%     Low   30%     Med   65%     High  86%

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:




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