Scientific Forecaster Discussion
fxus62 kgsp 201842
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
242 PM EDT Tue Mar 20 2018
a complex low pressure system will gradually move off the Atlantic
coast tonight and tomorrow with colder air overspreading the region
in the low's wake. Drier high pressure will briefly return over the
area on Thursday, with another moist low arriving from the west
during the weekend.
Near term /through Wednesday night/... a complicated wx
pattern will continue thru the near term. Current obs have a
stubborn and well defined wedge boundary maintained or slowly
pushing SW across the northeast section of the County warning forecast area. To the
west...the atmos is beginning to destabilized with deeper
cellular cu generating across the mtns, NE Georgia and the wrn
upstate. The cams have become less enthused abt precip coverage
across the upstate and NE Georgia this afternoon while the stronger
cells look to develop across a band of 500-750 j/kg SBCAPE adj
to The Wedge bndry. With strong shear remaining over the
area...a couple of these storms could become stg to svr by late
afternoon into the evening.
A meso-low takes over the pattern this evening as The Wedge retreats
north. This will keep generally unsettled trofines over the forecast area as
another sfc low and a stg ulvl S/W approach and cross the region
overnight. There looks to be a continued feed of upstream moisture
associated with an ulvl low which will create -snsh across the NC
mtns in good mech lift aided by pockets of h5 DPVA overnight. Snow
will continue across the higher terrain thru Wed night with accums
the greatest along the NC/Tennessee spine....where the smokies and nrn mtn
spine areas could receive arnd 8-10 inches. Across the mtn
valleys...generally 1-3 inches is expect during this time.
With the passing stg S/W thru mid-day Wed...there's a chance that
snow will break mtn containment and fall across the NC fthills and
Piedmont. These areas will see the best temperature drop overnight
cooler cp air mixes in. However...guidance consensus has come in
warmer so the best area for snow looks to be arnd the I-40 corridor.
This snow shud be non/accumulating...but a dusting to a tenth is
possible by 16z or so.
Will keep the warning and advisory as is and continue the mention of
non/mtn snow in the severe weather potential statement. Winds still look to gust into 40-50 mph
across the higher elevations of the NC mtns...so the adv levels
winds will be wrapped into the west-southwest products. A cold day on tap Wed
with highs held abt 15 degrees below normal. Min Wed night will also
drop below normal with freezing temps or just below likely across
Short term /Thursday through Friday night/... at the beginning
of the period, gradient winds will be the primary, but
gradually diminishing factor across the high terrain with high
pressure building into the deep south and a <990mb coastal low
moving away and into the northern Atlantic. In the interim,
gusty northwest winds are expected through the late afternoon hours on
Thursday, albeit less significant than those experienced on
Wednesday, with 25-35 mph gusts generally confined to the
highest elevations. With tranquil high pressure building into
the region for Thursday night into Friday, our attention will
shift to the next system to our west...
On Thursday, shortwave energy ejecting to the Lee of The Rockies
will trigger low pressure development to the Lee of the Front
Range and attendant warm-frontogenesis is expected across Ohio
Valley on Thursday. The resultant warm front will extend southeast into
the southern Appalachians late Friday and resultant upglide
will induce some light precip at least by Friday night. The
primary guidance (ecmwf and gfs) is in disagreement (surprise)
about the commencement of precip-inducing upglide on Friday, and
given the very light amounts and 00z to 12z European model (ecmwf) consistency
have opted to only slowly ramp up pops on Friday holding any
measurable quantitative precipitation forecast off until after 00z on Saturday. At this
juncture, temperatures throughout the column are forecast to be
cold enough for snow in the northern mountains and the highest
elevations of the central mountains. However, with a liquid
equivalent of less than 0.05", any accumulations are expected to
be minimal, which makes sense given the weak return flow/lack
of deep moisture flux and lack of significant energy aloft.
Finally, a rain/snow mix is possible across the northern NC
Piedmont as low temperatures dip into the 33-35 degree range
with a subfreezing column just above the surface (>750-1000ft)
so this too will have to be watched. Winter will hold on till
the Bitter End...
Regarding sensible weather, though heights aloft will be building
in advance of the longwave ridge over the eastern Great Plains,
reinforcing shortwave energy embedded within the deep-layer
northwest flow will dominate and force the surface high pressure
ridge to remain stubbornly just to our west. Cloud cover and
developing precip associated with the approaching warm front
on Friday into Saturday morning will influence temps as well.
Therefore, high temperatures on Thursday and Friday are expected to
remain 7-12 degrees below normal regionwide, with low temperatures
Long term /Saturday through Tuesday/...
as of 225 PM tuesday: our forecast area will be stuck between a
ridge to our west and a trough over the northeast Continental U.S. Through much
of the period. This means a series of short waves will rotate
through the northwest flow over the area until Tuesday when the ridge axis
finally moves east and into the area. At the surface, moisture and
isentropic lift develop along a warm front extending toward the area
from a low pressure center over the mid MS valley. This spreads
precip into the area and over the top of a developing cold air
damming high building in from the north. Precip chances ramp up
Saturday and remain relatively high through Sunday as the surface
low skirts the southern boundary of the damming high. The GFS has
come in colder with temps and thermal profiles due to a colder
initial air mass and the surface low moving south of the area
limiting the strength of any warm advection. This keeps the
potential for accumulating snow in place for the higher elevations
of the northern mountains through Saturday morning and again
Saturday and Sunday nights. Temps could be cold enough for snow to
mix in across the rest of the NC mountains and even the I-40
corridor but no accumulations are expected in these locations. That
said, the snow is a low confidence forecast as it is getting very
late in the season even for the northern mountains. Precip tapers
off Sunday night as the low moves east and a drier air mass briefly
moves in. However, there will be a small chance of precip Monday and
Tuesday with lingering moisture and weak insentropic lift over the
warming damming high. Temperatures will be highly dependent on the
strength of the damming high and timing of precip. They will likely
range from around 5 to 10 degrees below normal through the period.
Aviation /19z Tuesday through Sunday/...
at clt and elsewhere...afternoon showers and tstms will develop
mainly across the NC sites and across the nrn upstate. Most of these
storms will remain weak and it will be questionable whether or not
kclt is relieved from a stable sfc wedge. Where showers and storms
do develop...expect gusty conds and lowering cigs to MVFR possible.
Overnight...the atmos winds will remain moderate and generally W/ly
and likely N/ly at kclt, kavl and khky where IFR/MVFR rain/snow
could develop arnd daybreak. A good mixing day is in store Wed with
low-end gusty conds developing aft 15z or so most locales.
Outlook: VFR conds returning Wednesday night through Thursday.
Another moist system will arrive from the west this weekend.
18-24z 00-06z 06-12z 12-18z
kclt high 87% high 88% high 86% high 90%
kgsp high 100% high 95% high 96% high 100%
kavl high 99% high 93% high 96% high 100%
khky high 91% Med 76% high 85% high 87%
kgmu high 100% high 100% high 98% 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:
NC...Winter Weather Advisory from 8 PM this evening to 8 am EDT
Thursday for ncz053-059-062-063-501-503-505.
Winter Storm Warning from 8 PM this evening to 8 am EDT
Thursday for ncz033-048>052-058.