Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1256 am EST Wednesday Dec 18 2013
high pressure will remain over the Carolinas through Thursday...then
move off the Carolina coast. Expect a cold front to approach our
region from the Mississippi and Ohio Valley Friday and
Saturday...crossing from the northwest Sunday and reaching the
Carolina coast Monday. Ahead of the front...temperatures will be
much above normal.
Near term /through today/...
1230 am update...winds continue to gust frequently across the mountains and
occasionally in the foothills...but trend continues to be downward.
Winds are veering as they relax...and as cold air advection weakens. Midnight
temperatures were much too cold in the forecast which is evidently due
to winds keeping bdy layer mixed. Cooling rate should increase over
the next hour or two but I doubt we would have been able to reach previous
forecast mins despite low dewpoints. Thus warmed lows and even temperatures for
a couple hours after daybreak. Confidence modest on these temperatures.
Meanwhile...infrared cloud top temperatures over upper east tenn indicate some
snowflakes could form within them...and there are some additional
clouds upstream which may replenish clouds in the northern mountains over the
next couple hours. Took out all probability of precipitation for measurable precipitation but added a
flurry mention there.
At 900 PM EST Tuesday...an upper trough was over the eastern USA
this evening...with a shortwave rounding its base. A surface cold
front was exiting the southern Appalachians and moving toward the
Piedmont. The tight pressure gradient associated with this front was
responsible for gusty SW winds ahead of the boundary...and even
gustier northwest winds behind it. Wind speeds are expected to remain below
Upslope flow precipitation in moisture behind the front will
generally be limited to the northern NC mountains...but small probability of precipitation
will be maintained along the central and southern Tennessee border area of
NC...where a change over to snow is expected...mainly at higher
elevations...before drying ensues overnight. Accumulations are
expected to be minimal...mainly at higher elevations...and mainly in
the northern NC mountains. Clouds east of the mountains are expected
to be mostly over NC...and will follow the front rapidly east. Winds
will decrease toward daybreak as the pressure gradient relaxes with
high pressure building in from the west. Even so...wind chill values
may reach zero on some of the very highest mountains beaks by dawn.
Minimum temperatures are expected to be slightly below normal.
By morning the gradient will have relaxed somewhat allowing for
reduction in winds speeds/gusts. Skies will continue to clear
through the day as high pressure builds across the region allowing
for at least some modest radiational heating. With that...expecting
temperatures to warm back into the lower 40s across the Piedmont
with middle to upper 30s across the higher terrain.
Short term /tonight through Friday/...
as of 130 PM EST Tuesday...a flat upper ridge will build over the area
Wednesday night through Thursday. Good radiational cooling is expected Wednesday
night as surface high pressure moves across the area under clear skies. This
will result in min temperatures in the 20s across the mountains and the upper
20s and lower 30s east of the mountains maxes will reach at least two
categories above normal Thursday afternoon.
The upper ridge axis will move offshore Thursday night through Friday...with
deep layer SW flow setting up over the forecast area. Expect an
increasing clouds trend from the SW Thursday night with min temperatures
rebounding about 10 degrees over Wednesday night values. Afternoon maximum temperatures
will add another category over Thursday values in the SW flow. The deeper
moisture will just begin arriving in the SW mountains late day Friday.
Despite the warming temperatures...instability still looks too
limited for late day thunder Friday afternoon in the SW mountains...so will
contain afternoon chance pop mention to showers.
Long term /Friday night through Tuesday/...
as of 130 PM EST Tuesday...the medium range forecast picks up at 00z on
Saturday with a relatively flat upper ridge over the southeast and a
closed...southern stream 500 mb low over the extreme SW Continental U.S.. the ridge
is expected to remain over the region into Sunday with the closed 500 mb
low opening back up as it moves eastward and merges back into a
northern stream longwave trough. This trough is expected to approach the
Carolinas on Monday and gradually move across the area late Monday
At the surface...with high pressure centered well to our east...southerly low
level flow will persist and deeper layer moisture will increase over
the County warning forecast area on Sat. The low that was expected to develop on Friday over
the Central Plains and rapidly track NE appears to be taking a more
northerly Route with its front remaining farther to the northwest of the forecast
area where it stalls out. Nonetheless...there should be enough deep
layer moisture...low level upslope flow...and upper level dynamic support
to produce widespread showers over a good portion of the County warning forecast area. The
models then generate another low over the miss River Valley on sun
and quickly track it NE. The associated cold front will probably
move across the forecast area late sun into early Monday with another round
of showers and possibly some thunderstorms. The latest 12z run of
the GFS is about 12 hours faster with the approach of the front
compared to the 12z European model (ecmwf). Cold and dry Canadian high pressure is
then expected to build back in the fronts wake for the end of the
period. No major changes were made to the pop forecast with a solid to
likely chance carried over most of the County warning forecast area from Sat through Monday
afternoon. Probability of precipitation taper off Monday evening and Tuesday as the high builds
back in. Temperatures will start out well above normal on Sat and climb a
bit more on sun with an abrupt cool down on Monday and Tuesday as the
front moves through. By Tuesday...temperatures could be below climatology again. We
could also see some convection develop on sun ahead of the frontal passage if
there is sufficient instability.
Aviation /06z Wednesday through Sunday/...
at kclt...VFR with no operationally significant clouds expected through
the period...and much too breezy and dry for fog formation through
daybreak. Low end northwest gusts might occur very early this morning but
by first morning push winds will be light west-northwest. These continue until
this afternoon when high pressure center approaches from the west and
winds back to SW quadrant...but will further weaken as gradient
Elsewhere...VFR. A few low VFR clouds may develop as northwest flow
continues against the mountains this am...but ceilings not expected at kavl
if any of these clouds are seen at all there. Gusts will continue in
the mountains through midday when they finally should diminish with calm
high pressure building in. A weakening trend is already noted over
the Piedmont but can/T rule out a gust here and there. Calm or very
light SW winds will prevail by sunset.
Outlook...dry/VFR conditions expected to persist through Friday
morning. Moisture will return late Friday through the weekend...with
restrictions becoming increasingly likely.
06-12z 12-18z 18-24z 00-06z
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)