Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 070545

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1245 AM EST Wed Dec 7 2016

A drying trend will continue for several days as high pressure
eases into the area. A reinforcing cold front will arrive tonight
and early Thursday, bringing the coldest airmass so far this
season into the area. High and low temperatures well below normal
will persist Thursday night through Saturday before a moderating
trend commences early next week when the next cold front arrives.


As of 945 PM, latest surface obs indicate residual cold pool
lingering across the forecast area this evening, although a large
area of clearing low clouds was finally observed in satellite data
across the Piedmont, likely in response to incr NW/downslope flow
above the cold pool. That being said, there`s not much in the way of
dry air advection to be had at this point, as 40s dewpoints are
observed as far west as the Cumberland Plateau, and dewpoint
depressions in our area are dropping pretty quickly to near 0 within
areas of clearing. This is a roundabout way of saying that
confidence in the occurrence of at least patchy dense fog is
increasing, especially in the 2-6 hour time window. However,
confidence is still nowhere near high enough to issue a DFA at this
point. A mention of possible dense fog was added to the HWO earlier,
and we`ll just let this ride for the time being. Otherwise,
considering the depth of the moisture, as well as obs and radar
returns, pops were removed/replaced with `areas of drizzle` across
the TN border counties for the remainder of the overnight.

Overnight temps may see a slight rebound, but will remain nearly
steady in most places. Assuming the fog develops, it may dissipate
very quickly Wednesday morning. The balance of the day should be
fair, but the return of some weak upglide may bring some clouds back
in from the SW by the afternoon. Temps will see a nice rebound back
to something near normal.


As of 130 PM Tue: Weak upglide will occur over a stalled boundary
early Thursday, though the moist layer is so shallow over most of the
area that PoPs remain very small. Cloud cover however will remain
plentiful. By daybreak, a reinforcing cold front will march into the
NC mountains from the northwest, progressing across the area over the
course of the day. If the cold air arrives before the moisture
departs, the mountains could see a p-type transition; there is
essentially zero chance of a changeover in the Piedmont as the drying
occurs too soon. NAM, GFS, and GEM prog soundings still reveal
differences in terms of how moist the profiles are as they cool; NAM
is the most supportive of wintry precip, but the consensus suggests a
fleeting chance at best. A warm nose is present on the soundings, as
well as saturation in the snow-growth region.  Therefore the
changeover might include a period of freezing rain and sleet. I`ll
reiterate that is IF precip is still ongoing as the cold air moves
in. Stay tuned as the event moves within the range of hi-res
guidance. At any rate, accumulation should be nil given the warm
ground and brief nature of the mix.

Cold advection on northwesterly low-level winds will keep temps from
rising much at all over the mountain zones during the daylight hours
Thursday; the frigid air really won`t be felt in the Piedmont until
that night. Min temps will be 7-10 degrees below normal Friday
morning. Winds will remain brisk overnight, with gusts upward of 30
kt in the higher elevations. Wind chills below -5 degrees F are
forecast in those areas, though they are not currently over great
enough area to warrant an advisory. Max temps Friday will be nippy:
10-15 degrees below normal.


At 2 PM Tuesday: Nearly zonal upper flow will exist over USA
from Friday evening trough next Tuesday, although the models
disagree on the timing of shortwaves and low amplitude troughing.

At the surface, on Friday evening a cold front will be stalled
between FL and Cuba, extending west to the Yucatan Peninsula, while
high pressure over the mid MS River Valley extends east to Carolinas
and GA. The ridge reaches the Eastern Seaboard on Saturday. The
ridge persists into Sunday, while a cold front approaches from the
west. The front and associated moisture reach our area on Sunday
night or Monday, departing to the east on Monday or Monday night.
Model guidance is in disagreement on whether moist northwest flow
will exist behind the front, but evening the more aggressive ECMWF
would keep the bulk of this moisture, and hence any associated
snowfall, over Southwest Virginia on Monday evening. A strong cold
front dropping south across the Great Plains and Upper Great Lakes
would reach our on Tuesday according to the GFS, but would remain
well to the northwest according to the ECMWF.

Temperature are expected to run around 10 degrees below normal
behind the departing cold front, rising to near normal ahead of the
next front.


At KCLT and elsewhere: Incredible uncertainty especially at the
beginning of the period with cigs and vsbys. Guidance is all over
the place. Downslope winds remain in place but beginning to subside.
Strong up-valley flow continues at KAVL as well. Plenty of low level
moisture remains across the area to support fog/low clouds and
certainly looking at satellite trends it looks like widespread low
cloudiness continues to expand and blossom east of the escarpment.
Have improved trends somewhat for KAVL but for all others kept IFR
to LIFR in overnight. Drier air will push into the area later today
which should scour out any low clouds but then another wave of
moisture associated with a strong but mainly dry front will lead to
increasing clouds again toward the end of the period, and thus some
MVFR has been introduced. Except at KAVL, winds generally lgt/vrb
through the period with some variation in predominant direction
depending on location of surface features.

Outlook: Another front will approach late tonight and early Thursday
with some light precip/restrictions possible. Dry and much colder
through the weekend.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High  81%     Med   64%     Low   56%     Med   66%
KGSP       High  81%     High  89%     Low   56%     Med   66%
KAVL       High  81%     Med   72%     Med   62%     Med   61%
KHKY       Med   75%     High  89%     High 100%     Med   61%
KGMU       High  87%     High  89%     High  87%     Med   66%
KAND       Med   69%     High  89%     Low   56%     Low   55%

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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