Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 061916
AFDGSP

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

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 230 PM EST Tuesday: Water vapor satellite imagery shows nicely
the evolution of the upper system moving across the Srn Appalachians
and Wrn Carolinas. A mid-level dry slot has already surged northeast
across the fcst area E of the Blue Ridge. KGSP radar shows evidence
of the back edge of the isentropic lift also moving NE, as expected,
now roughly aligned with I-26. As this feature continues to move
northeast, it should bring an end to most of the precip across the
fcst area by late afternoon. As the low level flow comes around to
the NW behind the upper system, some precip production is possible
in the upslope areas along the TN border, so a chance of precip will
be maintained there. Meanwhile, low clouds will be slow to erode in
the remnant cool pool left behind in the cold air damming wedge.
Temps may see a slight rebound, but will remain nearly steady in
most places.

Overnight, in the wake of the mid/upper wave, we maintain a broad
WSW flow aloft. A surface boundary/front will attempt to push SE
across the fcst area during the evening hours, which will bring the
flow at low levels around to the NW, with some weak cold advection
at least initially. This scenario sometimes results in dense fog as
the remaining low level moisture (in the form of a low cloud layer)
gets forced down to the ground as the inversion aloft is driven
downward. This is seen in fcst soundings from the NAM after
approximately 03Z at most locations E of the Blue Ridge. Have a
strong suspicion that we will need a dense fog advisory at some
point later this evening for most locations east of the mountains,
but confidence in timing and location is not high enough to issue an
NPW at this point, especially given my track record for issuance
before the fog develops. It is possible that any dense fog will
remain localized and brief if the surface front moves through
quickly and dry advection takes hold. The fog may hold on until
sunrise or a few hours after. 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.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
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.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
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.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: starting out with mostly IFR conditions from
low ceiling and/or visibility, forced mainly by low level isentropic
upglide/upslope flow E of the Blue Ridge. The passage of a short
wave aloft will bring a rapid decrease in this upglide flow during
the mid- to late-afternoon, which should bring an end to the precip.
We may see a brief improvement across the region, at least with the
visibility, although the low clouds will keep IFR/MVFR ceilings in
place. After sunset, the flow at low levels will come around to the
NW. When this happens, whatever remains of the low cloud deck will
essentially get pushed down to the ground and will probably result
in a period of dense fog with visibility down in the LIFR/VLIFR
range. All the TAFs reflect this thinking to varying degrees late
this evening, although timing might not be early enough. The
restriction may persist through the early morning hours and not
improve until just after sunrise, according to the model guidance.
If drier air filters in from the NW, the improvement could happen
much earlier. Either way, a rapid improvement to VFR will take place
on Wednesday morning, with wind coming around to N and NW.

Outlook: Expect improving conditions on Wednesday behind a front
with dry air filtering in to scour out the low level moisture.
Another front will approach on Thursday with some restrictions
possible. Dry and much colder through the weekend.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       Med   72%     Med   65%     Low   48%     High  85%
KGSP       Med   61%     Med   66%     Low   44%     Low   50%
KAVL       Med   67%     Low   56%     Low   42%     Low   54%
KHKY       Med   66%     Low   55%     Low   47%     High  84%
KGMU       Low   57%     Med   65%     Low   47%     Low   49%
KAND       Low   52%     Low   56%     Low   49%     Low   48%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...LEV
NEAR TERM...PM
SHORT TERM...Wimberley
LONG TERM...JAT
AVIATION...PM



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