Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1013 am EST Wednesday Nov 26 2014
low pressure will intensify along a stalled cold front off the
Atlantic Seaboard and move up the coast today. An upper level system
will cross western North Carolina early Thursday. Dry high pressure
will return Friday and linger into the weekend.
Near term /until 6 PM this evening/...
as of 1000 am EST...all morning winter hazards have been cancelled
as the back edge of the precipitation now stretches from near Boone
to Hickory and continues to lift NE. With temperatures above freezing at
all but the higher elevations...major Road problems through the day
are not expected...and even the higher elevations will warm quickly
through the 30s this afternoon. Good insolation is expected for the
bulk of the afternoon away from the Tennessee border and temperatures have been raised
a couple of degrees most areas...especially in the SW.
Short term /6 PM this evening through Friday night/...
as of 300 am Wednesday...just as quickly as the coastal low pulls away from
the Piedmont this afternoon...the next system enters from the west in the
form of a clipper shortwave rolling through the region tonight. Models
continue to show a brief but potent shot of q-vector convergence and
Omega with this wave along with fairly deep moisture. Profiles will
be cold enough for all snow in higher elevations at the onset of the
forcing and probability of precipitation. Most of the area however will still be too warm for
snow and remain so through the event.
Trends in the low level flow are a little complex with a very Short
Ridge and weak warm air advection coming between the two troughs. There was some
indication from previous model runs that this warm air advection might cause a small
warm nose to develop and allow for sleet/freezing rain in some of the
foothills. Latest runs indicate this warm air advection is too weak to have such an
effect and we will advertise an all rain/snow event. Snow levels are
expected to drop somewhat steadily overnight with wet bulb effects
providing only marginal enhancement. With or just after the clipper
axis passes early Thanksgiving morning...winds veer to northwest and remain
brisk through the lowest few thousand feet which should provide a jump in
precipitation rates. The northwest winds continue to be strong through Thursday albeit
with moisture slowly becoming more shallow.
Despite the strong forcing model quantitative precipitation forecast is not that strong but I did
favor the higher and more terrain-oriented NAM quantitative precipitation forecast as part of a blend
of raw model and wpc quantitative precipitation forecast. Total accums of 1 to 2 inches are expected
in the smokies and balsams...and 2 to locally 3 inches in the mountains
north of the French Broad. Half of that will probably fall in a short
period when the two forcing mechanisms overlap. Given the Holiday and
potential travel impacts a Winter Weather Advisory will be hoisted for our
tenn border zones in addition to northern Jackson County midnight to noon
Thanksgiving day. Enough warming should occur during the day that
most valleys will see rain mix in or a complete transition. Light
shsn could continue through Friday morning though moisture is insufficient to
expect much additional accumulate.
Dry high pressure will filter in Thursday night into Friday. Min temperatures Friday
morning will be in the l20s mountains and u20s to around 30 in the Piedmont.
Skies will be mostly clear by this time...but continued northerly winds
will keep temperatures from rising out of the 40s over most of the area Friday
afternoon. Slight return flow sets up Friday night which will keep temperatures from
plunging as much...also introducing a little cloud cover into the
Long term /Saturday through Tuesday/...
as of 220 am Wednesday...the medium range forecast picks up at 12z on
Saturday with a relatively zonal upper pattern over most of the
Continental U.S.. as we move into sun...another broad 500 mb trough digs down over
the far northern states and then over the Great Lakes. At the same
time...flat upper ridging will persist spread over the Gulf of mex
and persist over the region through most of the period. The long range
guidance actually has the ridge amplifying towards the very end of
the medium range.
At the surface...high pressure will be centered over the forecast area at
the start of the period with warmer southerly flow increasing thicknesses
and temperatures over the weekend. The next frontal system will approach
the area by early Monday and lay down across our northern fringe.
Strong high pressure behind the front will slide eastward and
appears to set up a potentially strong cad pattern to our NE by day
7. The European model (ecmwf) and Canadian operational models remain more moist with
this front and subsequent wedge pattern...yet as we have seen many
times before...models tend to overestimate the extent of the deeper
layer moisture in these scenarios. With that said...the only probability of precipitation I
have are slight to low end solid chance from late sun Onward. The
best chances are still over the higher terrain and along the NC/tenn
border region. Temperatures will warm through Monday with values exceeding
climatology. Values should cool again as we go into middle week and the
flow becomes more northeasterly.
Aviation /15z Wednesday through Sunday/...
at kclt...IFR ceilings should linger through 16z before rapidly
improving 16z to 18z with drying from the SW. VFR clouds should then
linger later today into the overnight...as moisture persists in
advance of a secondary upper air disturbance. This may bring a stray
rain shower near the terminal tonight...but probability appears too
remote to include in the taf at this time. Winds will steadily turn more northwest
and increase to near 10 knots as surface low pressure moves up the
Elsewhere...rapidly improving conditions are expected throughout as
drying arrives from the SW through 18z. Tempo IFR to MVFR ceilings are
likely until 16z or 17z...but VFR should quickly follow. VFR clouds
should then linger into the overnight...as moisture persists in
advance of a secondary upper air disturbance. This may bring a stray
rain shower near the terminals...snow showers possible at
kavl...overnight but the probability appears too remote to include
in the taf at this time.
Outlook...a dry air mass will then return to the area Thursday night and
continue through the weekend.
15-21z 21-03z 03-09z 09-12z
kclt high 83% high 100% high 100% high 100%
kgsp high 86% high 100% high 100% high 100%
kavl high 88% high 100% high 100% high 100%
khky medium 75% high 100% high 100% high 89%
kgmu high 83% high 100% high 100% high 100%
kand high 80% 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)
NC...Winter Weather Advisory from midnight tonight to noon EST
Thursday for ncz033-048>052-058-059.