Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
958 PM EDT Thursday Mar 26 2015
a cold front will cross the western Carolinas and northeast Georgia
tonight. A trailing upper level disturbance will then cross the area
Friday through Friday night. Dry and cool high pressure will settle
in over the weekend. A low pressure clipper system and associated
cold front will pass over the region on Monday.
Near term /through Friday/...
2015 UTC update...probability of precipitation were lowered the next few hours based on
radar trends...and thunder chances based on the latest model data.
Sky cover was updated from visible satellite imagery.
As of 225 PM...clouds have thinned and broken up around the area
which should allow temperatures to rise into the 70s across much of the
Piedmont and upper 60s in mountain valleys. Held onto a low pop through
early evening...with latest soundings having a small amount of
instability across the Piedmont...albeit capped off in the midlevels.
The chance of thunder in these areas looks near zero through the rest
of the daytime hours.
Looking upstream...deep trough continues to dig into the middle
Mississippi Valley...preceded by a fairly potent cold front and an
wave of low pressure over the central Appalachians. A band of
convective precipitation and embedded thunderstorms is seen over WV and eastern
Kentucky...just ahead of the surface front. Latest hrrr and 4km Storm Prediction Center WRF take
this feature northeastward...while more activity develops along the front
over the tenn valley. The front is expected to slowly move across the
mountains overnight as the trough digs and the surface wave moves to the middle
Atlantic coast. Both the convection allowing models as well as the
GFS/NAM show quantitative precipitation forecast in our County warning forecast area in vicinity of the front...though this appears
due to redevelopment rather than advection of the current activity.
Indeed most guidance shows modest instability of up to a few hundred
j/kg...mainly elevated in nature...present in the County warning forecast area prior to the
front. Thunder chances are mainly confined to the mountains and areas
south of I-85. Though 0-3km shear would suggest some organization or
damaging wind threat with thunderstorms and rain...the effective shear is small and severe weather
is not anticipated. Storm Prediction Center does not have the County warning forecast area in any probabilistic
threat areas through 12z. Despite the front arriving late...mins will
be a couple categories above climatology.
Cold air begins to sweep in behind the front...and northwesterly winds will
still have some moisture to work with. Small chances of northwest flow snow
are included near daybreak...though in reality if the cold air
arrives early enough a mix of ptypes could occur as it cuts
underneath the very warm midlevel air still present...creating a
warm nose. Precipitation chances will generally trend downward in the wake
of the front as the day progresses...but some degree of upper
dynamic forcing will be present with the trough still just to our
west. Midlevel lapse rates improve sufficiently to allow slight surface
based instability by afternoon. These ingredients could produce some
spotty showers...with temperatures again becoming supportive of snow or a
rain/snow mix in the high elevations by sunset. Early afternoon maximum temperatures
will mainly be in the 50s in the Piedmont and mountain valleys...with
upper 30s and lower 40s in the high elevations.
Short term /Friday night through Sunday/...
as of 145 PM Thursday..a deep upper trough centered west of the area
Friday evening is forecast to swing through the area by early
Saturday and move into the Atlantic early Sunday. In the wake of the
trough...weak ridging will occur across the southeast states on Sunday.
On Friday night...there may be some lingering showers across the
Piedmont along the axis of a departing vorticity maximum...so have maintained
a mention of chance probability of precipitation there before midnight. Also...some northwest flow
snow will likely be on going as another vorticity maximum crosses the
mountains ahead of the approaching base of the upper trough.
Meanwhile...850 mb temperatures will fall to the -8c to -12c range by 12z
Saturday. Hence...a period of decent northwest flow snow should occur
Friday night. In regard to snow accums...the latest model consensus
of quantitative precipitation forecast has ticked upward somewhat so that advisory type snowfall may
occur in the smokies. Snow accumulations will be on the order of 2-3
inches in smokies with generally 1-2 inches elsewhere in the Tennessee
In addition...it is looking like the mountains will experience an
advective freeze Friday night...with low temperatures in the 20s. The
Piedmont will see temperatures in the 30s...with isolated freezing temperatures and
possibly some patchy frost where winds can decouple in sheltered
Northwest flow and cold air advention will continue on Saturday as the cp
high remains west of the area. However...enough dry air arrives to
cut off the snow shower activity in the mountains early Sat. Highs
Saturday will remain chilly with highs 15-20 degrees below climatology.
The cp high will settle over the area Friday night producing ideal
radiative cooling conditions. A hard freeze is expected across all
of the mountains...with freezing temperatures also expected outside the
mountains. Min temperatures Sunday morning look to be in the teens to middle
20s in the mountains and middle 20s to lower 30s across the Piedmont. These min
temperatures will approach record lows across the Piedmont (see climate
Even with plenty of sunshine and weak warm advection returing on
Sunday...maximum temperatures are still expected to be a couple categories
Long term /Sunday night through Thursday/...
as of 140 PM Thursday...the medium range begins with a progressive
trough and attendant frontal boundary moving across the eastern
states in the Sunday night/Monday time frame. This will support
chance probability of precipitation across the mountains by 12z Monday. Upper forcing is expected
to pass north of the region...as frontal zone begins to slow down
across the southern Appalachians. The result in veering low level
flow in the vicinity of the front...with resultant weakly
convergent/downslope flow limiting the precipitation potential outside the
mountains...and probability of precipitation will generally be maintained in the slight chance
For the remainder of the period...western height falls and eastern
rises in the wake of the departing trough will result in
establishment of fast/quasi-zonal flow across much of the Continental U.S.
Through the middle part of next week. Meanwhile...the aforementioned
baroclinic zone will become quasi-stationary within this flow
pattern. It is therefore not surprising that global model details
diverge during this time frame...as they struggle to resolve weak
short waves rippling through this low amplitude pattern. The GFS
offers the wetter solution...with at least a couple of weak waves
spreading light precipitation into the region prior to the arrival of
another frontal system late in the week. The European model (ecmwf) is quite a bit
drier. Considering the pattern...it probably behooves US to maintain
at least a slight chance pop during most periods of the medium
range...with the highest probability of precipitation reserved for day 7 for the potential
approach of the next frontal system.
Aviation /02z Friday through Tuesday/...
at kclt...precipitation chances will ramp up overnight in advance of
a cold front. With some instability present...thunder cannot be
discounted. Guidance favors an MVFR ceiling toward daybreak...that
persist through Friday. Guidance also favors MVFR visibility around dawn.
Precipitation...though decreasing on Friday...will be slow to clear
the area. SW winds will veer northwest overnight as the cold front moves
through....then north on Friday.
Elsewhere...precipitation now in east Tennessee will move in this evening...
accompanied by lowering ceilings...and perhaps some thunder. Guidance
suggests in IFR ceiling at kavl late this evening...and bouts of MVFR to
IFR at kand around dawn...with low VFR setting up at other sites.
Guidance favors MVFR visibility at kgsp and kgmu around dawn. Kavl and
kand restrictions should end by middle morning...but low VFR is
supported at all sites through Friday afternoon...even as
precipitation diminishes. Winds will veer from SW to NE with the
frontal passage overnight.
Outlook...a middle level system will bring a second round of light
precipitation across the area Friday nigh...with light snow showers
possible at kavl. Dry high pressure follows through Sunday...then
another front moves in with precipitation on Monday. Restriction
chances are best at kavl with the two fronts.
02-08z 08-14z 14-20z 20-00z
kclt high 100% high 85% high 88% high 80%
kgsp high 100% medium 77% high 85% high 100%
kavl high 92% medium 62% high 81% high 100%
khky high 100% medium 65% high 80% high 100%
kgmu high 100% medium 75% high 88% high 100%
kand high 100% low 42% high 81% high 95%
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 minimum temperatures for March 28th...
Gsp 26 1982
clt 25 1982
avl 11 1887
Record minimum temperatures for March 29th...
Gsp 26 1899
clt 26 2013
avl 19 1982