Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 061141
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
641 AM EST Tue Dec 6 2016

.SYNOPSIS...
Low pressure will track northeast of our region today with high
pressure building across the area tonight and Wednesday.  Expect a
strong cold front to approach from Tennessee Wednesday night and
cross the Carolinas on Thursday. This front will bring the coldest
air of the season so far for Thursday night through Saturday.
Temperatures returning to near normal early next week ahead of the
next cold front.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 630 AM EST Tuesday: Isentropic and orographic upglide in
response to increasing southeasterly flow just above the cold dome
has led to a blossoming of precip across the area again. Convection
out in E AL and W GA will decrease in intensity but should continue
to see occasional light rain through the morning. Made some tweaks
to pops and temps over the next few hours but nothing major.

Otherwise, the upper low over TX will continue to lift northeast as
it damps today, with the diffluence shifting and passing over the
Southern Appalachians with increasing PVA. Meanwhile as the surface
high slowly shifts east, it has begun ridging down the Eastern
Seaboard with a strong damming signal, with northeasterly winds
picking up across the area to between 10-15kt (some slightly higher
sustained winds are possible with typical low-end gusts). The
surface low associated with the TX upper low will develop into a
Miller B pattern as we have been advertising all week, with the
wedge front strongly in place between the two surface lows.

Atop the cold dome, southeasterly winds at 850mb ahead of the
approaching trough will increase and lead to orographic upglide, so
even before the next round of moisture arrives, we`ll start seeing
some isentropic/orographic light rain redeveloping across the area.
This is apparent in hi-res reflectivity output at rain fills in
again but with more substantial precipitation spreading across the
area around and after 12z. As this falls into the cold dome, it will
serve to reinforce the damming, and as a result high temperatures
today will be held 5-10 degrees below seasonal normals today. Ended
up undercutting guidance temps for today because, as usual, guidance
wants to erode the cold dome too quickly and thus warms temperatures
up a little more this afternoon. Once the upper low, which by that
time will be more of an open wave, passes to our northwest this
afternoon, we`ll lose the isentropic/orographic upglide with
precipitation reinforcing the cold dome and erosion will begin, so
there definitely remains some bust potential with temperatures, but
if we go down, we go down together since afternoon his are pretty
well collaborated with surrounding offices.

As to the question of thunder, just really not feeling it this
morning. The surface inversion is very strong and while it can be
argued that lapse rates above 850/875ish mb might support some
elevated CAPE and indeed SPC has about 2/3 of the forecast area in
general thunder, but we start losing upper level moisture just after
daybreak this morning in southwest zones where elevated instability
seems most likely. Just doesn`t seem to add up so have not included
any thunder in the zones. Never say never especially when dealing
with weather of course.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 200 AM EST Tuesday...A broad longwave trough will persist
across the eastern 2/3rds of the CONUS during the Short Term, with
strong shortwave energy diving in from the west and shifting the
trough axis eastward. This trough will be accompanied by a strong
cold front, which will cross the CWFA on Thursday. Low-level flow
will turn out of the SW with some WAA and moisture to support
variable amounts of cloudiness on Wednesday into Wednesday night.
The front will have little moisture to work with, however. The 00Z
ECMWF is dry with the fropa, while the GFS and NAM have a little
QPF, mainly in the western zones. The GFS is about 6 hours quicker
than the NAM. So all this disagreement makes the PoP forecast
challenging, but still expect the band of precip (if there is any)
to push into the mountains around daybreak Thursday. Even if the
wetter NAM verifies, it looks like a very quick-moving line of
showers with only a few hundredths to about tenth of an inch.
Whatever precip does fall, temps will support rain in the valleys
and rain/snow mix in the high elevations, but with little to no snow
accumulation.

Temps will be near normal on Wednesday and above normal Wednesday
night with increasing clouds. Then noticeably colder in the
mountains Thursday with gusty NW winds. Winds will linger thru
Thursday night as temps continue to fall within strong CAA. So
elevations above about 3500 ft may see wind chills dip below zero,
especially near the TN border. Min temps Friday morning will be the
coldest so far of the season, ranging from the single digits and
teens in the mountains to the 20s across the entire Piedmont.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 130 AM EST Tuesday:  Good cold air advection continuing at the
start of this period Friday morning with NW wind at 850mb varying
from 15 to 35kts. The center of the surface high Friday morning over
the Mississippi Valley then centered over our area Saturday morning
with little or no wind. Temperatures will be 10 to 20 degrees below
normal through Saturday. Wind chills in the single digits for higher
elevations Friday night before the wind dies down.

As the high moves offshore on the mid Atlantic Saturday into Sunday,
return flow from the Gulf feeding into the Mississippi Valley will
begin ahead of the next cold front. Scattered upslope showers may
begin late Sunday and Sunday night from near Clayton GA to Tryon NC.
The bulk of the precip with this system should be on Monday
depending on which model is presenting the evolution. The GFS is the
faster model bringing frontal moisture across the NC Mtns Sunday
night then slowly drying out this moisture as it reaches the
Piedmont by Monday afternoon. The ECMWF brings in deeper moisture
late Monday across our whole area then shifts it east late Monday
night. The GFS has our region dry Monday night. Of course, this is a
week away and many changes will occur as model data evolves in the
next several days.

Temperatures of course 10 to 20 degrees below normal Friday and
Saturday then moderating to around normal Monday.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Widespread IFR to LIFR conditions across the
area this morning with -RA as cold air damming remains firmly
entrenched. NE winds 7-12kt with gusts 15-20kt prevail as well for
all but KAVL (where SE winds prevail). Cigs will remain low with
widespread restrictions through a good chunk of the day, with slow
but brief improvement this afternoon and evening as the damming
erodes. Cigs will improve but late in the period all guidance drops
vsby again. Should see additional improvement beyond the end of the
period. Winds should shift N/NW as the damming erodes, with speeds
dropping to around 5kt.

Outlook: Expect improving conditions on Wednesday behind the 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...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Med   64%     Med   65%     High  95%     High  87%
KGSP       High  89%     Med   65%     High  90%     High  94%
KAVL       Med   72%     Low   55%     High  80%     High  87%
KHKY       High  89%     High 100%     High  90%     Med   71%
KGMU       High  89%     High  90%     High  90%     High  87%
KAND       High  89%     Med   70%     High 100%     High 100%

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...DEO
NEAR TERM...TDP
SHORT TERM...ARK
LONG TERM...DEO
AVIATION...TDP


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