Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 300609
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
109 AM EST Wed Nov 30 2016
Deep moisture will return from the west along an approaching cold
front through the morning. The cold front moves thru this afternoon
and evening. In its wake, dry high pressure fills in tonight,
persisting through Saturday. Moisture will return Sunday into Monday
ahead of another developing low pressure system to the west.
.NEAR TERM /THROUGH TODAY/...
As of 1250 AM Wed: Sfc warm front appears to be located just north
of I-40 across Western NC. The band of showers which developed
along it earlier has followed it north and dissipated. WAA should
keep abundant low cloudiness and patchy fog around the CWFA thru
the morning, but new precip development looks likely to remain
upslope-driven between now and daybreak. Intense convection
continues over the Cumberland Plateau, but is tracking parallel to
the TN/NC border. Mesoanalysis shows a tongue of greater MLCAPE
extending into the SW Mtns and Upper Savannah valley, and the
shear is more than sufficient to promote organization should any
cells form and enter that area.
For today, A potent vorticity maximum will rotate thru the base
of a broad, long-wave trough and lift NE thru the TN/OH Valleys.
This energy will be accompanied by a strong upper jet and associated
upper divergence atop the region. Another cold front will approach
from the west, and be accompanied by a solid band of precip over the
TN Valley to the central Gulf Coast. The CAMs hint at a potential
quasi-linear convective system approaching the NC mountains around
midday, but generally have that fall apart. The GFS has almost no
CAPE across the area in front of that line, while the NAM does have
pocket of about 400-600 J/kg of MUCAPE across the Upper Savannah
Valley. Of course, if anything relatively deep can set up, shear
will be more than adequate to keep things organized Wed aftn. So the
Day 2 Marginal Risk for severe weather looks good. The other issue
is gusty winds, as another SWLY LLJ traverses the area ahead of
the front late tonight thru Wed aftn. A wind advisory is in effect
for most of the NC mountains from midnight tonight thru Wed aftn.
Temperatures will run well above normal.
.SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/...
As of 200 pm EST Tuesday: We`ll see some lingering precip across the
area at the start of the period, generally between 00-06z Thursday,
but the front will be rapidly exiting the area as the upper low
pushes east across southern Canada and the occluded surface low and
front lift northeast. Southwest flow aloft continues over the area
at the start of the period with modified surface high pressure
sliding east across the Southern Plains and beginning to ridge into
the Southeast. The biggest sensible weather impact for Thursday will
be the markedly cooler temperatures behind the front...a good 10
degrees colder than highs tomorrow...but still a little above
seasonal normals. The CAA will continue on Friday as a shortwave
passes across the area early in the day, with highs another handful
of degrees cooler still and below seasonal normals. Continued dry.
Meanwhile a strong system will deepen over the Desert Southwest,
inducing downstream ridging over the Plains that will be beat back
down by a shortwave diving out of Canada into the northern Plains.
This will be our main weather-maker for the extended.
.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 215 pm EST Tuesday: Ah, the extended. This has been quite the
puzzle and more than a little frustrating for the last few days but
the good news is that the ECMWF has actually done a flip with the
12z run and is lining up really well with the GFS, which was the
preferred model yesterday. So what we have now is that the northern
Plains shortwave will slide east, while the Desert Southwest (well,
more like northwest Mexico) upper low continues to remain cutoff,
spinning in place, to affect us toward the very end of the period.
The first wave of moisture will lift into the Southern Plains late
Saturday and spread east overnight and into Sunday. The shortwave
moving through the Ohio Valley will act to suppress moisture to the
south, with some low-end QPF (but high pops) at least spreading into
our GA/SC zones during the day on Sunday. There may be some moisture
attendant with the shortwave itself to affect the northern mountains
and with the cold air in place, continued the mention of rain/snow
mix or snow depending on temperature, but with such low QPF over the
mountains would not expect much. Temperatures will be held well
below normal on Sunday.
Monday into Tuesday, both models have a surface high pressure
crossing the Appalachians and ridging down the Eastern Seaboard,
though the ECMWF is centered much farther north in more of a
classical CAD with the GFS more of a hybrid CAD (with some associated
QPF already across the area again Monday night. With this, the
former upper low will lift northeast and be absorbed by another
trough pushing into the west coast, with a rapid response in
downstream ridging over the Appalachians Monday night, with a
corresponding increase in temperatures on Tuesday as the warm front
lifts north ahead of the trough. At this point, the GFS is a little
faster with lifting the warm frontal precip across the Carolinas,
but the overall pattern is fairly similar. For now have kept chance
pops through Tuesday before the system exits late in the period, and
of course we`ll refine this through time. And it`s that time of
year, so right now we`re looking at at least some mix or snow in the
northern mountains each night. WPC snow probabilities for the
extended are low but non-zero across western NC.
.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: IFR/LIFR cigs remain over most of the area,
with warm frontal boundary bisecting the area. Light showers are
occurring north of the boundary. Some improvement has occurred
in spots over the Upstate following the departure of the showers,
but it is believed any periods of improvement will be short-lived
given continued WAA and moist boundary layer. The setup favors cigs
over fog, but some areas of MVFR will persist. The poor conditions
are expected to persist into the day, with a cold front pushing
in from the west this afternoon. Ahead of the front, instability
will be limited, but a few isolated TSRA are expected (chances
too low for TAF mention at this issuance). Expect frequent gusts
of 20-25 kt with peak winds perhaps exceeding 30 kt, due to strong
flow prior to the fropa. The front moves thru the area during the
late afternoon to mid-evening timeframe; following its departure
VFR returns. Wind shift to NW occurs in the evening at KAVL,
and after 06z at the other sites.
Outlook: Drying occurs behind the front Wed night, with dry high
pressure returning for Thursday thru Saturday.
06-12Z 12-18Z 18-24Z 00-06Z
KCLT Low 47% Med 69% High 91% High 90%
KGSP Med 79% High 80% Med 72% High 99%
KAVL Med 79% High 85% High 92% High 100%
KHKY Med 76% Med 68% Med 74% High 87%
KGMU Med 78% High 80% High 80% High 97%
KAND Med 65% Med 72% Med 73% High 99%
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:
NC...Wind Advisory until 7 PM EST this evening for NCZ033-048>053-