Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
239 am EST sun Dec 21 2014
low level moisture will linger across the region today through
Monday. Deeper moisture will arrive from the west on Tuesday ahead
of a strong cold front. The cold front will cross the region on
Wednesday. Cold and windy conditions will set up in the mountains
through Thursday...with drier conditions expected east of the
mountains through late week.
Near term /through tonight/...
at 230 am EST Sunday...a mean upper trough axis was located over the
center of the nation early this morning...with low amplitude ridging
downstream over the southeast and lower Great Lakes. Weak shortwave
energy was moving out of the base of the trough over the Gulf
states. At the surface...a high pressure ridge extended form Quebec
to the Gulf states. A stationary front was located from off the
Carolina and Georgia coasts...across North Florida...to the central Gulf of Mexico.
Light moist upslope flow into the Blue Ridge this morning may result
in some very light precipitation...some of which may fall as
freezing rain of freezing drizzle...but accumulations are expected
to be very light and rather spotty.
The models show a weak surface wave developing along the coastal
front today...which will bring moist isentropic upglide northwest over the
surface ridge and across our area. This will keep cloud cover in
place over the area...and support light rain by this evening...
especially over the Piedmont. High temperatures will run slightly
below normal due to clouds...despite above normal heights aloft.
Minimum temperatures tonight will run around seven degrees above
normal. There may be a few areas at high elevations of the mountains
that see some freezing rain toward dawn Monday...but the duration
will be too short for much ice accumulations.
Short term /Monday through Tuesday night/...
as of 220 am EST Sunday...weak surface cad will be established across
the region on Monday...with 1026 mb high pressure nosing southward east of the
Appalachians...and srly 850 mb flow developing ahead of falling
heights back across the plains states. The precipitation will likely
be very light...with drizzle expected at times. Will feature below
guidance maximum temperatures for Monday afternoon.
Very little movement in the surface cad layer is expected through Monday
night. Upglide and southern tier mountain upslope flow will steadily improve
Tuesday through Tuesday night...with widespread rain/showers gradually
developing. Considerable Gulf Coast and southeast coastal plain convection
is still featured in the models Tuesday to Tuesday night...which may serve
to inhibit the transport of deeper moisture and associated higher
quantitative precipitation forecast into the southern Appalachians and surrounding areas. Moderate to
occasionally heavy rainfall could still result...but little or no
Hydro problems are expected. This period should also see steady
erosion/pinching of the surface cad layer back toward the northern NC Blue
Ridge through late Tuesday night...with non diurnal warming of Piedmont
Long term /Wednesday through Saturday/...
as of 230 am EST Sunday...the potential for high shear Low Cape
convection along the immediate cold frontal passage Wednesday will depend on several
factors. A slow steady erosion of the surface cad is expected...but surface
based instability may only bubble up to a couple hundred j/kg of
cape by midday Wednesday in the uncovered Piedmont. There are timing
differences...with the best deep layer forcing on the GFS toward Wednesday
evening...while the European model (ecmwf) is faster toward midday. Finally the Gulf
coastal convection may disrupt any developing shallow convective
line segments farther inland. The potential for isolated severe
convection will have to be monitored...but confidence is too low for
any severe weather potential statement mention at this point.
The slowest operational GFS has surface waves developing along the front
and delaying frontal passage into Wednesday evening...while other solutions are a
bit faster with earlier cooling across the mountains Wednesday night. A brief
period of upslope snow showers should develop Post frontal passage in the western
mountains through Thursday morning...but a less than favorable westerly
direction to the low level flow should curtail the forcing a bit.
Snow accums Christmas morning are expected to be sub advisory given
the limited duration of the deeper moisture and the less than
favorable wind component.
Otherwise...anticipate zonal flow in the wake of the passing trough
Thursday through Thursday night. A gradual return to deeper layer SW flow is
expected Friday/Friday night as another trough deepens over the plains and
moves east through MS River Valley. The associated frontal system
should reach the forecast area on Sat...but with more limited
moisture than the middle week system.
Aviation /08z Sunday through Thursday/...
at kclt...guidance favors keeping ceiling at MVFR until around
midday...then favors low VFR into tonight. With a low dew point
depression...the forecast leaned toward more pessimistic guidance
with regard to visibility...carrying MVFR in fog around daybreak. Light
rain should move in late tonight as a low moves up the coast.
At kavl...low IFR ceiling should persist...with visibility lowering from MVFR
to IFR around daybreak. Some light drizzle is possible overnight in
light upslope flow...but odds are too low to mention this. Guidance
favors VFR this afternoon...with a return to MVFR this evening...and
light rain late this evening...as low pressure moves up the coast.
Elsewhere... with many ceilings just above IFR...chances seem good for
IFR around daybreak. Dew point depressions seem ripe for IFR fog as
well. The models support improvement to VFR late this afternoon...
with a return to MVFR this evening with rain...as a low moves up the
Outlook...lingering low level moisture is likely to keep low stratus
and night time fog around through Monday...with some improvement during
the daylight hours. A cold front will approach on Tuesday from the
west and move through on Wednesday. Widespread showers with restrictions
should arrive Tuesday afternoon and will continue until the front
clears the area late Wednesday.
07-13z 13-19z 19-01z 01-06z
kclt high 88% medium 78% high 100% high 94%
kgsp medium 73% medium 78% high 85% high 88%
kavl medium 78% medium 76% high 91% high 90%
khky medium 76% high 88% high 83% high 90%
kgmu medium 73% high 81% high 83% high 88%
kand high 83% medium 78% high 85% high 90%
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)