Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
710 PM EST Friday Dec 6 2013
moist southwesterly flow off the Gulf of Mexico will persist across
the region before a cold front slowly crosses the forecast area on
Saturday. Canadian high pressure will then build in from the
northeast Sunday with moist southerly flow aloft maintained into
Monday. Another cold front is expected to cross the area Tuesday
with relatively cold high pressure building back in through
Near term /through Saturday/...
630 PM update...revised probability of precipitation/winds overnight...generally following
latest NAM and rap timing. Scattered rain showers over the mountains at this time should begin
to increase in numbers as moisture convergence improves ahead of
front...which is still taking its time pushing across the mountains rap
and some cam guidance suggests frontal band will weaken in the
mountains/foothills and redevelop in the Piedmont just before daybreak.
Thunder chances seemed a bit too high in the forecast...though more
activity over our County warning forecast area is expected than areas west...where
instability is more limited. MUCAPE best across the south...and kept
mainly a chance mention there.
At 230 PM EST Friday...an upper ridge off the East Coast will
continue to progress...while a broad trough over the western USA
persists. The result will be a increase in wavelength...and minor
deamplification. A shortwave in the middle MS River Valley will weaken
as it lifts out of the trough and crosses the Appalachians tonight.
A cold front associated with this feature will move from the
Appalachians across the western Piedmont by dawn...accompanied by a
broad band of precipitation. Elevated instability along this
boundary is expected to diminish as the front leaves the mountains
and heating abates...reducing the threat of convection.
Precipitation amounts are expected to be lower as the boundary moves
away from the Blue Ridge...and upslope flow ends with veering winds.
Some upslope flow on the Tennessee border will ensue...but cold advection
will have just begun...and no snow is expected. Minimum temperatures
will run around 10 degrees above normal under cloud cover.
Despite the early morning frontal passage...moisture associated with
the boundary will linger behind the front. Although the heaviest
precipitation will move off to the east...clouds cover and scattered
light precipitation may linger through much of the day. Maximums
temperatures are expected to be early in the day and near normal.
Temperatures will fall in the afternoon as cold advection follows
the front. By late in the day a very cold light rain is possible in
the NC mountain bordering Tennessee...but snowfall is not expected.
Short term /Saturday night through Monday/...
as of 225 PM EST Friday...cold air damming will become established
over the region Saturday night. After midnight...isentropic upglide
will begin to increase with rain expected to break out over most of
the region prior to sunrise. This won't be the type of cold air
damming event where the cold and dry air will already be in place as
the precipitation develops. In fact...it will take a precipitation induced low level
ageostrophic adjustment to really drive the cool air into the region
to the point the surface temperatures drop to freezing. Therefore...I/d expect
precipitation to start as rain and gradually change over to freezing rain
along the Escarpment of the Blue Ridge...the northern mountains and
the foothills north of Interstate 40.
Overall the thinking on this shift has not changed much from the middle
shift. However...there has been an increase in the quantitative precipitation forecast into The
Wedge boundary on the latest GFS while the NAM is lighter. Perhaps this
explains why the GFS BUFKIT soundings now have nearly a half inch of
freezing rain at Hickory while the NAM has nothing. Rather than
Chase individual model runs...I/M going to continue to follow the
sref consensus. I/ll also keep freezing rain confined mainly to
areas on the NAM where the surface wet bulb is freezing or below.
Typically this works well for ME with ice events. I will expand the
watch just a little...to include Burke...Catawba Iredell and Davie
counties. I/d say my confidence that at least parts of these
counties will hit ice storm criteria is high enough for a
watch...that being around 50 percent. At the very least...we can
transition these lower confidence zones to an advisory.
Ground temperatures will be very warm with this event...and the main
impact from freezing rain will be ice accumulations on elevated
surfaces. Fortunately this means that roads should be in good shape.
However...we may see some power outages from ice-laden trees. As the
parent high moves farther away Sunday evening...everyone should see
surface temperatures rise to a few degrees above freezing.
Later Sunday night into Monday southwesterly upslope flow will
increase across the region. Based on the orientation of the flow and
a blend of the latest GFS/European model (ecmwf) quantitative precipitation forecast...parts of the Southwest
Mountains could see up to 2 inches of rain Sunday night and Monday.
This could cause some good rises along the Tuckasegee River and it/S
tributaries...but I Don/T think this will be enough precipitation to cause
Long term /Monday night through Friday/...
as of 200 PM EST Friday...good confidence is had in the ext range as
the op models are agreeing well with the overall synoptic pattern
and the mesoscale mass fields will have little impact on the
sensible weather forecast.
A cold front will continue to cross east over the County warning forecast area Tuesday and some
instability ahead of the boundary could help spawn a few general
thunderstorms across the eastern tier counties. Back to the west...there should
be lingering high relative humidity in the cold air to produce low accumulate -snsh
across the western/most mountains through daybreak Tuesday. After Tuesday...the upper
pattern becomes highly conflnt over the eastern Continental U.S. With a high
amplitude western ridge bringing down a 1040 mb Canadian high Wednesday
reinforcing a Continental high already over the area. Thus...will
anticipate a dry period with below normal temperatures maintained in a
relatively low Theta/east airmass through Thursday. Some cirrus/acu will be
present and increase in coverage as the week progresses while a
split flow develops west and instigates weak cyclogenesis over Texas by
late Friday. A moist atl return flow will commence early Friday and isent
lift -shra are probable beginning early south increasing in coverage
north throughout the day. Low layered thicknesses support an early
morning -sn/rain mix Friday across the upstate and NE Georgia...however surface
mins are not very confident at that time range.
Aviation /00z Saturday through Wednesday/...
at kclt...VFR with S to SW winds this evening perhaps with
occasional low end gusts. A few showers may be in the area though
they are not likely to have much impact. Precipitation chances generally
increase toward dawn as cold front approaches. Guidance pretty solid
on MVFR after midnight tonight...with only a couple members
indicating IFR. With widespread IFR over east Tennessee at this time in the frontal
precipitation...did allow a period of IFR during rain showers. Wind shift to northwest
anticipated just before daybreak...going NE thereafter with gusts
mainly during the morning. MVFR ceilings Post front should be able to
lift to VFR by midday...but enough moisture lingers that at least
some low clouds may persist through the end of the period.
Elsewhere...showers will affect the mountains this evening as front
slowly enters the region. Precipitation chances increase in the
foothills/Piedmont overnight as front moves through...Flipping winds to
northwest. MVFR ceilings/visibility may accompany the showers before heavier...more
steady rain arrives with the front. Ceilings look to drop to IFR at most
sites in the heavier precipitation. All sites should see the wind shift
before daybreak and continue to veer into Saturday. Ceilings should be
able to return to MVFR for the early hours of the day and then VFR
across the northern portion of the area...but with the frontal moisture
lingering across the south MVFR likely to last all day.
Outlook...a moist and active pattern will continue across the region
through early next week. Restrictive ceilings each night and perhaps
well through the daylight hours...especially across the mountains can be
expected. A light glaze of ice is possible in the northern NC
mountains/foothills Saturday night. Strong thunderstorms are possible
Monday across mainly upstate SC.
00-06z 06-12z 12-18z 18-24z
kclt high 98% high 81% high 87% medium 71%
kgsp high 94% high 85% medium 76% high 82%
kavl medium 79% medium 69% medium 75% high 89%
khky high 96% high 80% high 100% high 100%
kgmu high 94% high 80% medium 75% high 82%
kand high 98% high 89% high 84% 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: (must be lower case)
record high temperatures for Friday Dec 6th...
Avl...73 in 1998
clt...78 in 1998
gsp...76 in 1998
NC...Winter Storm Watch from late Saturday night through Sunday
evening for ncz033-035>037-049-050-056-501>506.