Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
259 PM EST Thursday Dec 12 2013
a cold cyclonic flow of air over the Great Lakes will maintain
below normal temperatures through the end of the week...with lake
effect snows focused over far northwest Pennsylvania in the
perennial snowbelt region. A series of northern stream waves
tracking eastward from the upper Great Lakes along the Canadian
border will reinforce the cold air. The active early season winter
pattern will resume by the weekend...with another storm system
impacting the area Saturday into Saturday night. Cold and brisk
conditions will prevail into early next week.
Near term /through tonight/...
late afternoon visible Sat imagery shows clear skies over most of the
County Warning Area...save for just downwind of Lake Erie where isolated snow
showers continue to persist. If loop shows some clouds are
advancing ahead of next shortwave trough making its way through the
Ohio Valley...but they are thinning upon entering western PA.
As Great Lakes shortwave passes north of PA tonight...model data
indicates veering blyr flow will bring lake effect snow showers
southward again into the northwest mountains after 06z. Quantitative precipitation forecast from higher res
model data supports a light accumulate of up to an inch by dawn across northwest
Warren Colorado. Elsewhere...tonight looks uneventful as high pressure
passes south of the state. Despite a mclear sky across the
southern half of the state...an active west-southwest breeze should keep
temperatures from falling to quickly. Model blend supports min temperatures within
a few degrees of 10f.
Short term /Friday/...
a series of northern stream waves will move along an Arctic front
dropping southward near the PA/New York border on Friday. This should help to
reinvigorate lake effect snow showers over the northwest/north central mountains
with minor accums /0.5-1.0"/ expected through 00z Sat.
Long term /Friday night through Wednesday/...
in general...expect the lake effect snow mechanism to fade into
Friday night with the focus shifting to shearing southern stream wave
progressing steadily to the northeast from the lower MS/OH/TN valley
to off of the northeast coast ahead of well-defined northern
stream trough digging into the upper Great Lakes region.
Isentropic lift/warm advection ahead of a weak low level wave
lifting northeastward along inverted trough will support a broad area of
snow spreading into the area by early Saturday morning. Shortwave
energy and height falls crossing the central applchns will begin
transferring to a surface low that is forecast to develop along
the middle-Atlantic coast and deepen as it tracks northeastward from the
Delaware-Maryland-Virginia/New Jersey coast to Long Island/the southern New England coast
Sat night into Sunday.
Model ptypes seem to be converging primarily toward a snow event
for much of central PA...with a changeover to sleet/freezing rain
possible over the southern 1/3 of the County Warning Area Sat evening into Sat night. Snow
accumulations of 4" or more seem likely along and north of I-80 which
is supported by the sref/gefs means and the 33-member wpc
probabilities/accumulation percentiles. Amounts in excess of 6
inches are possible over this area...but appear less
likely...hampered by a lack of phasing between the lead southern
stream wave ejecting offshore and the northern stream trough moving
across the Great Lakes. The lack of a closed 700/500 mb circulation
is also expected to limit the amount of deep moisture advection/lift
across the region...while a dry Arctic high to the north further
hampers amounts. This is reflected in the wpc probs with about a
25 percent chance for reaching warning criteria. The ensemble means and
wpc probs show a solid 2-4" across the central counties...with snow
possibly mixing with/changing to sleet/freezing rain over the south-central
zones as warm air tries to surge in from the south. Sref/gefs and
wpc probs show light ice amts around 0.10 mainly over The Laurels.
The winter weather should be over by Sunday morning as a dry slot moves
in behind the departing coastal low. The northern stream trough should
cross the lower Great Lakes Sun night and enhance lake effect snows
downwind of Lake Erie.
Weak ridging should provide a cold but dry start to next week. The
Arctic shortwave crossing the Great Lakes/northeast/mid-Atlc Tuesday-
Wednesday will likely produce some light snows with lake enhancement. This
trough may be the finale to the recent cold pattern as the global
models and ensembles project the next northern stream amplification
much further to the west beyond day 7/Thu. Surface high pressure should
arrive by Wednesday-Thursday with the next upstream weather maker beginning
to show its hand by the end of next week.
Aviation /20z Thursday through Tuesday/...
a shallow layer of moisture ascending the West Mountains on westerly flow
will continue to produce isolated snow showers and lowered ceilings
at bfd through the early evening hours. Otherwise widespread VFR
conds under clear skies will persist through much of the taf valid
Weak shortwave move east along the PA/New York border overnight. Flow
along and behind the wave will increase coverage of le snow
showers...mainly over the northern mountains expect persistent MVFR conds at
bfd with frequent drops into IFR category for both visibility and ceilings
through much of the day from middle morning Onward. Remaining taf
sites should be mainly VFR with isolated shsn causing temperature drops into
MVFR or lower territory.
Increasing southwesterly winds aloft ahead of the shortwave over
high pressure/inversion at the surface should lead to a period of low
level wind shear overnight tonight into Friday morning and have
included it in western and northern tafs.
Friday...shsn/reduced visibilities possible at kbfd.
Sat...snow or mixed precipitation with low ceilings/visibilities possible.
Sun...shsn/reduced visibilities poss West Mountains
Monday...no sig weather expected.