Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 070009

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
709 PM EST Tue Dec 6 2016

A drying trend will commence tonight and Wednesday as high pressure
eases into the area. A reinforcing cold front will arrive Wednesday
night 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.


As of 655 PM Tuesday, W/NW flow developing in the wake of departing
mid-level short wave trough is very slowly eroding/lifting cloud
cover east of the mtns. Moisture banked against the high terrain
along the TN border warrants some precip mention, although based
upon radar trends and obs, DZ may be a more appropriate descriptor
than -RA. Since near-surface moisture is not expected to go anywhere
anytime soon, there are still concerns that dense fog could develop,
esp after midnight, and if clouds continue to dissipate, as the
downslope flow is expected to shut off during that time. However,
just take a look at guidance and you`ll see that the probability of
this occurring is extremely uncertain attm. A mention of fog has
nevertheless been continued in the forecast.

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: confidence in this TAF set is extremely low,
especially after the first 2 to 3 hours of the forecast period. NW
winds developing above remnant surface wedge will develop through
the evening. Such downslope flow usually takes care of the low
clouds, east of the mtns, but we haven`t seen this happen yet, and
there will otherwise remain plenty of low level moisture to support
low clouds. However, trends have been gradual improvement over the
past few hours, so will allow this trend to generally continue.
However, a tempo for IFR cigs was included in the 03Z-06Z range, if
for no other reason than to reflect the inherent uncertainty. LIFR
conditions were inherited in the forecast at most terminals from
late evening on. It`s plausible that this will occur later tonight,
as surface winds will remain very light, while the downslope flow
will weaken, or even dissipate altogether. The LIFR conditions were
therefore retained, but pushed later by as much as 4 hours at most
terminals. Again, it should be stressed that confidence is low.
Drier air is expected to finally push into the area by late Wed
morning, bringing an end to restrictions at all terminals.

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

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       Med   78%     Med   66%     High  94%     High 100%
KGSP       Med   76%     Med   68%     High  84%     High  81%
KAVL       Med   78%     Med   71%     High  88%     High  95%
KHKY       Med   77%     Med   60%     High  97%     High 100%
KGMU       Med   79%     Low   47%     High  83%     High  81%
KAND       Med   74%     Low   56%     Med   77%     High  81%

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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