Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
923 PM EST Tuesday Jan 27 2015
dry high pressure will remain across the region through Wednesday.
On Thursday...a cold front will cross the forecast area with another
high pressure system building in Friday through Saturday. A moist
low pressure system is expected to affect the region on Sunday into
Near term /through Wednesday/...
as of 915 PM Tuesday...within the last hour we have gotten some reports
from the Waynesville and Asheville areas that indicate snow is
falling out of the remaining upslope-forced cloud cover. I revised
flurry/shsn mention in the forecast based on cloud top temperatures...with the
result being shsn over the French Broad valley and smokies where the
tops are coldest...with less widespread coverage toward the north
where moisture is evidently more shallow. Flurries were mentioned in
toward the edges of the cloud deck...where the flow is more
downslope anyway. Short term guidance...which has done pretty good
in this pattern...shows quantitative precipitation forecast ending before midnight...so I maintained
a downward trend with time. Little to no additional accums are
expected except perhaps in the highest elevations.
Highly amplified upper pattern continues to prevail across the
United States with a sharp full latitude trough axis sliding offshore
into the western Atlantic...while 500 mb ridging builds across the
Southern Plains. A dry forecast prevails late tonight and tmrw with
skies nearly clear...but the high terrain remaining at least partly
cloudy through sunrise Wednesday. Heights will rise aloft as the
afore mentioned upper ridge slides east with surface high pressure
building just to the north. This will lend itself to slightly
warmer temperatures on Wednesday due to abundant insolation and
downsloping flow...although still a tad below climatology. Relative humidity levels on
Wednesday afternoon will fall towards critical levels as
temperatures rise and dewpoints mix out. That said...as the surface
ridge builds overhead...pressure gradient forces will remain weak
enough to inhibit any gusting therefore these fire weather concerns
remain marginal at this point.
Short term /Wednesday night through Friday/...
as of 230 PM Tuesday...the main story for the short range will be
the passage of a strong cold front Thursday night...and subsequent
cold advection flow behind the front through Friday. Before we get
there...the axis of an upper ridge will move overhead Wednesday
night and Thursday morning...which will support dry high pressure.
On Thursday...the models continue to show a lead short wave moving
up the Ohio Valley and dampening out. Although there is some DPVA
associated with this feature in the afternoon...and some weak upper
divergence that moves overhead...there will be very little moisture
to work with. As a result...none of the operational models respond
with any light precipitation east of the mountains in fact...the models delay the
onset of precipitation over the mountains until the afternoon. So...the precipitation
chances have been pushed back in time...and eliminated over the
A secondary short wave is still expected to drive the cold front
across the region Thursday evening and early Friday morning. The
model guidance shows a strong signal for precipitation with the front and
then behind the front from Thursday night through Friday morning over
the mountains...and in particular...the Tennessee border. Precipitation chances were
nudged upward a bit more. Strong cold advection will bring snow
levels down to the valley floors very quickly Thursday evening such
that the bulk of the precipitation will fall as snow. Present indications
are for an advisory-level accumulation over parts of the Tennessee border
counties...mainly because of the duration. The low level northwest flow
upslope moisture dries up during midday on Friday...limiting the
snow accumulation. The other concern will be the wind. Behind the
front...a very strong pressure gradient will raise the wind
significantly. The NAM in particular shows winds at the top of a
shallow mixed layer at around 50kt. The highest ridgetops will stick
up into a very fast flow. The question is how much of this high
momentum air will reach the ground. The cold advection suggests a
surface-based mixed layer that should allow some of it to happen.
Will mention the high wind potential in the severe weather potential statement across the mountains and
adjacent Blue Ridge for Friday. Temperatures should fall back below normal.
Long term /Friday night through Tuesday/...
as of 230 PM EST Tuesday...flat ridging will build over the region
from the west Friday night into Sat. Little more than middle and high
clouds spilling over the ridge axis are expected on Sat. 1034 mb surface
high pressure will build eastward from the Southern Shores of the Great Lakes
to upstate New York through through the middle of the weekend.
The next clipper digging southeast through the northern plains Sat night will
become more amplified from the Ohio Valley to the MS River Valley by
late Sunday. Good consensus exists on a surface reflection deepening
over eastern Texas late Sat night and rocketing up the frontal
zone...mainly west of the Appalachians sun/Sun night. The GFS remains a
bit faster than the European model (ecmwf) with the main trough and associated cold
front. Following an ensemble compromise brings the deepest moisture
across our area from early Sunday through Sunday evening...with dry
slotting then expected late Sunday night with the fast moving
system. The peak of warm conveyor belt lift and upper divergence
should yield the best period of likely probability of precipitation by Sunday evening.
The farther west surface low track along the front will support more
rapid warming of profiles through the day on Sunday. Still...most
profiles support wintry ptypes at onset given the cool wet
bulbs...with 850 mb temperatures then rising to +3 to +5 degrees c during
the period of best quantitative precipitation forecast later Sunday. Although the high pressure to the
NE moves offshore quickly...a lingering shallow hybrid cad
configuration will likely persist into Sunday night...with
associated cooler than guidance temperatures expected...especially
along the northern tier. Will keep the current rain versus snow ptypes
since any transition zone looks pretty narrow. Some instability
could wrap up around the edge of The Wedge late Sunday night...but
with too small a chance of thunder to mention it in the lower
Piedmont at present.
Brief northwest flow snowfall will be possible late Sunday night into
Monday. In addition...a tight pressure gradient will set up behind the
departing wave. High winds could be a problem with the 850 mb jet
prognosticated to run 50 to 60 knots during the peak of the cold advection
around 12z Monday. An severe weather potential statement mention may be needed. Dry northwest to west flow is
expected Monday through Tuesday. Strong and dry surface high pressure will
build over to the north...with very chilly thicknesses and associated
Aviation /03z Wednesday through Sunday/...
at kclt...VFR with only a few midlevel clouds seen this evening.
Winds will remain backed this evening due to surface low pressure to the
east...but north-northwest winds will prevail most of the night and Wednesday morning.
Lee troughing might back winds in the early afternoon.
Elsewhere...northwest flow snowfall will taper off near the tenn border
this evening...but a cloud deck near the VFR-MVFR threshold will
linger through much of the night. Kavl may see MVFR ceilings at times until
the clouds break up appreciably...but no precipitation expected. Kavl winds
will remain solidly northwest with some gusts through the night. Some
gustiness will persist through the evening at kgsp/kgmu due to mountain gap
effects. Otherwise all terminals should stay VFR and mostly sky clear through
the period...with northerly winds veering late tmrw afternoon as high pressure
Outlook...a pair of clipper systems will bring chances for rain or
snow showers to the NC mountains Thursday and Friday. Most of these showers will
remain west and north of kavl...but brief periods of restrictions
cannot be ruled out. Otherwise...mainly VFR conditions are expected
across the area until moisture begins to return Sunday.
02-08z 08-14z 14-20z 20-00z
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)