Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
348 am EST Friday Dec 13 2013
dry high pressure will continue over the region today and then move
off the Carolina coast by late tonight. A low pressure system is
expected to arrive from the west on Saturday bringing widespread
precipitation to the region over the weekend. Expect another high
pressure system to bring dry weather through the middle of next week.
Near term /through 06z Saturday/...
as of 230 am...cirrus will increase and coverage and thicken through
the day as moisture moves in in the nearly zonal flow over the
area. This will keep highs a couple of degrees below normal
despite the warm thicknesses over the area and southwesterly flow around
the departing surface high.
This evening...an upper low over the southwestern Continental U.S. Opens up and moves
toward the area. However...good low level moisture and isentropic
lift hold off until after midnight. Therefore...expect the County warning forecast area to
remain dry until after midnight. Winds over the mountains become
breezy...especially at the ridge tops...while winds begin to turn
northeasterly outside of the mountains as a non-classical cad develops.
Short term /06z Saturday through Sunday night/...
as of 3 am Friday...picking up at 06z Saturday...broad high pressure
will be centered off the Atlantic coast under a nearly zonal but
slightly confluent upper pattern. Strong shortwave will begin
lifting from the Southern Plains into the Mississippi Delta Region as a
larger trough begins to occupy the middle third of the Continental U.S.. short wave
along with attendant surface low will shoot up the Ohio Valley during
the day. Low to middle level flow rapidly turns southerly as it
approaches...and the resultant isentropic upglide and moisture return
over the cool high pressure will bring a quick round of precipitation
before initial shortwave pushes across the County warning forecast area...followed by dry
slot. Upsloping in the southern mountains and along the southern facing
Escarpment will cause probability of precipitation to ramp up starting just after 06z
Sat...spreading east and covering practically all of the forecast area by
midday before diminishing from the west.
Setup suggests a brief in situ wedge could remain in place early
Saturday...leading to potential ptype concerns. Model consensus low
temperatures Sat morning are generally very close to the freezing mark...but
raw model wet bulb temperatures are approx 3-5f below 2m temperatures. Ice nuclei
should be available per forecast soundings from both NAM/GFS...and
despite the strong warm air advection the warm nose is cool enough in the northern mountains
that some snow or sleet could result at the ground. Further south
any areas that are below freezing would be at risk for freezing rain. Temperatures
do warm enough by midday to return all precipitation to liquid...though
chances are diminishing in the areas of concern by that time anyway.
Model quantitative precipitation forecast is fairly modest despite the strong low level jet and upper
divergence...with the system moving fast and initially overcoming
dry llvls. Slight convective enhancement not out of the question
though instability above the warm nose is not enough to be of much
consequence...and certainly looks too weak to include any thunder in
forecast. All in all...ice and snow amounts are below warning
criteria...and given slight differences in model soundings will
allow day shift to make an advisory decision.
Surface low undergoes secondary cyclogenesis in Miller b scenario
Saturday night...with larger upper trough and cold air beginning to
push in. Concern now becomes for northwest flow frozen precipitation in the mountains
once again...amounts look light as saturation not reached over a deep
layer. Relatively warm temperature of the surface based moist layer initially
suggests fzdz or light freezing rain...but the top of the layer cools as winds
taper off on Sunday suggesting a changeover to snow before the event
Saturday night and Sunday...as cold advection takes hold winds will
pick up especially in higher elevations. A few gusts into the 30s knots
are likely there...with frequent gusts of 25-30. No headlines
necessary at this time.
Highs will be cooler Sat vs sun in the Piedmont given the erosion of
The Wedge...but cooler sun in the mountains due to the aforementioned cold air advection.
Temperatures dip below freezing throughout the County warning forecast area by Monday morning.
Long term /Monday through Thursday/...
as of 130 am EST Friday...expect dry weather and near seasonal
average temperatures throughout the medium range period.
Models continue to agree that a broad low amplitude upper trough
will swing across the eastern states Monday through Tuesday with its
trough axis pushing off the Atlantic coast by early Wednesday. By
Thursday...an upper trough will deepen across the western states
while 590 dm anticyclone will build south of Florida. This will result in
moist southwesterly flow aloft across the region through Thursday
At the surface...high pressure centered across the region Monday
will gradually shift east of the area through Tuesday morning as a
dry cold front approaches from the northwest. The front will then cross the
region late Tuesday with another high building in from the west on
Wednesday. Wednesday night...a dry back door cold front will slip
through the region as a Canadian high over the Great Lakes region
migrates into New England. Thursday...the high centered across the
NE will ridge sward into the forecast area resulting in continued
dry conditions. Temperatures will be a few degrees below normal on
Monday and warm up to near normal by Tuesday.
Aviation /09z Friday through Tuesday/...
at kclt and elsewhere...surface high pressure over the area early this
morning moves east of the area this afternoon. Calm winds become
light SW today. Cirrus clouds will increase and thicken during the
day. A storm system approaches the area tonight with lowering VFR
clouds during the evening. Winds turn southeast then NE outside of the mountains
and southeasterly at kavl during the evening.
Outlook...the storm system will move from the Southern Plains to the southern
Appalachians during the day Sat...when restrictions in rainfall will
become most likely. Conditions will dry out from the SW on Sunday...
with dry high pressure returning for Monday.
08-14z 14-20z 20-02z 02-08z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% high 100% 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: (must be lower case)