Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
249 PM EST sun Jan 25 2015
low pressure will track across the central Appalachians
tonight...then intensify into a strong East Coast low Monday through
Tuesday. Dry high pressure builds in from the west on
Wednesday...then a cold front will cross the region late Thursday.
Near term /through Monday/...
as of 240 PM...another tricky forecast as a strong upper low with a
strong lead short wave rotating around the low move into the area
tonight then east of the area Monday as channeled vorticity moves in
Monday afternoon in the northwesterly flow behind the departing low. The
associated surface low moves north of the area tonight dragging a cold
front across the County warning forecast area. The low then reforms near the NC/Virginia coast
late in the day. There will be warm air advection flow...isentropic lift ahead of
the system with developing deep moisture...but little in the way of
upper divergence tonight. The best deep layer forcing will be
tonight with the lead short wave. Both the forcing and moisture will
be a quick shot...so while there is a chance of rain outside of the
mountains...quantitative precipitation forecast will be very light. The moisture and forcing will be
better across the mountains...so have retained Cat pop along the Tennessee
border. Temperatures fall enough the precipitation will quickly change to snow
across the higher elevations...but may only mix with snow across the
valleys. Temperatures may drop enough for some snow to fall across the NC
foothills and the mountains of the upstate and NE Georgia. However...the
boundary layer remains quite warm...limiting the amount of snow that
can survive to the surface.
The upper level cold pool associated with the upper low slides
across the area Monday...keeping a chance of precipitation across the NC
foothills and Piedmont...and the eastern upstate. Again...any precipitation
would be very light. Although there is some low level instability...
the boundary layer remains relatively warm. This again will limit
any snow or graupel that could form in this region...leading to
Across the mountains...copious low level moisture...developing northwesterly flow
and low level instability will keep elevation dependent rain/snow
going through the morning. The precipitation will taper off in the afternoon.
Little to no snow will mix with the rain across the NE Georgia/upstate
mountains however...snow will continue across the high elevations of the
NC mountains with some snow mix across the valleys. Given the forecast
quantitative precipitation forecast...temperatures and precipitation duration...looks like the high elevations
along the Tennessee border from Haywood south could see advisory level
accums. The high elevations from Madison north could approach advection
level...but the bulk of the area should remain below. Therefore...will
issue a winter weather advection for the southern portion above 3500 feet.
Short term /Monday night through Wednesday/...
as of 230 PM Sunday...for Monday night through Tuesday...the County warning forecast area will
be within the northwesterly backside of the deep noreaster that will be
tracking NE up along the coast. The 12z NAM and GFS have trended a
little less bullish on moisture and snow potential along the Tennessee
border. However...still looks like at least periods of scattered to
numerous snow showers will exist during these two periods. An
additional 1 to 2" of snow can be expected in the usual favored
spots...which may approach low-end advisory criteria. Will continue
to mention in the severe weather potential statement. Otherwise...gradual clearing is expected east
of the mountains...with more clouds lingering east of I-77 closer to the
coastal low. Temperatures will be near normal overnight...then stay a
category or two below normal on Tuesday.
Tuesday night and Wednesday...as the noreaster pulls away...dry high
pressure will build from the west...with the surface ridge axis crossing the
area Wednesday afternoon. Forecast soundings show very dry air atop the bl...so
expect temperatures to dip below normal Tuesday night. On Wednesday...temperatures
rebound a category or two...especially in the high terrain. May see near
critical relative humidity for fire weather concerns...but winds will be light. Probability of precipitation
should taper off along the Tennessee border by around midnight...with dry
weather/sunny skies for Wednesday.
Long term /Wednesday night through Sunday/...
as of 145 PM EST Sunday...some measure of spread in the handling of
the 500 mb pattern shows up on Thursday. The GFS and gefs mean
depict a series of fairly weak clippers crossing the central
Appalachians Thursday into Friday. The 00z run of the European model (ecmwf) has a more
amplified system digging through the Ohio Valley late in the
week...which showed decent consistency over the past few runs. Both
models feature a relatively narrow band of moisture ahead of the
waves along a cold front for Thursday...but with better moisture
accompanying the more amplified wave in the cold advection flow on
the European model (ecmwf) Thursday night into Friday. The amplified...but slightly more
muted...12z European model (ecmwf) solution looks to be a reasonable compromise. This
will keep continued mountain precipitation chances Thursday night into Friday...mainly
as light snow during the best cold advection.
Once the late week waves move east...the solutions surprisingly
appear more consistent for the weekend...with an active northern stream
bringing a weak clipper type wave through the Appalachians Sat
night...while a strong SW system near the Baja California peninsula stalls
through the weekend. Some moisture should work back into the
forecast area from the west on the sheared flow between these
systems on Saturday...but deep moisture and forcing look unlikely at
present. This will yield mainly chance rain/snow probability of precipitation primarily across
the western mountains through the latter half of the weekend. Also
expect a tight pressure gradient to set up in the northwest flow across the
southern Appalachians on Sunday...with Windy Mountain conditions and
downslope drying east of the mountains through late weekend. Expect near
climatology mins but below climatology maxes through the medium range period.
Aviation /20z Sunday through Friday/...
at kclt and elsewhere...VFR conditions expected through the next 12
hours or so. S to SW winds will increase and may become gusty this
afternoon. Middle level VFR ceilings expected to overspread the area
quickly during the evening in advance of the next weather system.
Precipitation chances will also increase by around 06z...mainly across the
mountains...and a -ra mention has been introduced at kavl. Rain chances
are non-zero...but much lower at other terminals. Increasing
moisture is expected to result in development of MVFR ceilings between
06z and 12z at most terminals. Some guidance is extremely
pessimistic during this time...featuring widespread IFR/LIFR ceilings.
Although I wouldn/T rule this out in areas that see at least a
couple of hours of rain...confidence in that is very low at this time...so
have opted to hold conditions at the MVFR level for the time being.
MVFR ceilings will linger through at least middle morning across the NC sites
while ceilings lift or even scatter out before noon across the SC sites.
Cannot rule out some rain at khky/kclt during the day...but low chance
and high uncertainty. Kavl could see rain or snow developing during
the day...but again...low chance high uncertainty. Winds will turn SW
to west-southwest overnight...northwest at kavl...then west to west-northwest after daybreak Monday.
Gusts are likely at kavl/kand and are possible elsewhere.
Outlook...a series of clipper systems will bring periodic chances
for snow showers...mainly to the mountains through the week. Most of
these snow showers will remain west and north of kavl...but brief
periods of restrictions and snow showers cannot be ruled out there.
Otherwise...mainly VFR conditions are expected.
19-01z 01-07z 07-13z 13-18z
kclt high 100% high 100% medium 73% high 82%
kgsp high 100% high 100% high 80% high 100%
kavl high 100% high 100% medium 77% medium 68%
khky high 100% high 100% medium 67% medium 68%
kgmu high 100% high 100% medium 73% high 100%
kand high 100% high 100% medium 67% high 94%
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)
NC...Winter Weather Advisory from midnight tonight to noon EST
Monday for ncz051-052-058-059.