Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
656 am EST Thursday Dec 12 2013
a cold cyclonic flow 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/...
regional radar loop showing dwindling shsn/flurries across central
PA...as dry Arctic air mass flows into the region. Dewpoints have
already fallen into the single digits across northern PA as of 09z.
Weak multi-bands downwind of Lake Erie expected to give way to a more
intense...long-fetch le band across western New York state later
today...as blyr flow backs to the west-southwest. Can/T rule out northwest Warren Colorado
briefly getting into this developing le band later this morning.
However...a call to penndot indicates little overnight snowfall
across Warren Colorado and any additional accums should be light...so have
cancelled the lake effect Snow Advisory.
Shortwave rounding base of trough over Ohio at 09z will swing through
central PA this morning with little impact...due to dryness of air
mass. Lg scale subsidence in the wake of this feature should ensure
mostly sunny skies across central PA by afternoon. However...warm air advection in advance of
next shortwave cross the Great Lakes will spread increasing clouds into
the northern counties.
Despite the return of sunshine...temperatures will be slow to rise given
low sun angle and 850/925mb temperatures around 2sd blw normal. Gefs mean 925
temperatures near -13c are supportive of highs no better than the mteens
over the Allegheny plateau and the m20s over the susq valley.
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
Short term /Friday through Sunday night/...
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
enhance lake effect snow showers over the northwest/north central mountains with
minor accums /0.5-1.0"/ expected through 00z Sat.
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 betwn 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.
Long term /Monday through Wednesday/...
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 /12z Thursday through Monday/...
a shallow layer of moisture ascending the West Mountains will produce patchy
MVFR ceilings and scattered -shsn at kbfd and kjst early this morning.
High pressure building in from the Ohio Valley will bring clearing
skies across most of central PA by late morning with widespread
VFR conds expected by afternoon. Model soundings indicate westerly winds
will gust to around 20kts at times from late am through the afternoon.
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.