Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
411 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 /until 6 am Friday morning/...
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 /6 am Friday morning through 6 PM 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 Thursday/...
surface high pressure will slide across northern glakes Friday night
in wake of Arctic front...reinforcing a cold air surge mainly
north of Pennsylvania. At the same time...southern stream wave
exits the southern rockies and progresses steadily toward the
northeast coast ahead of well-defined northern stream trough
digging into the upper Great Lakes region. Isentropic lift/warm
advection ahead of the weak low level wave lifting northeastward along
inverted trough will support a broad area of snow spreading into
the area by early Saturday morning...mainly across
northern/western counties. 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.
With colder air entrenched over central PA...models continue to
forecast a primarily 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 later Sat into Sat night as some warmer air noses in from the
southwest. 24-hour snow accumulations of around 6" seem likely
along and north of I-80 with 850mb temperatures remaining -3c or colder. Sref
also supports this. Though quantitative precipitation forecast totals of about 0.50-0.60 inches
forecast...totals may be hampered by lack of phasing between the
lead southern stream wave ejecting offshore and the northern stream
trough moving across the Great Lakes about 12 hours later. The lack
of a closed 700/500 mb circulation is also expected to limit the
amount of deep moisture advection/lift across the region. But
drier Arctic air to the north may produce a fluffier higher snow
to water ratio which could bring amounts a bit higher. But with
longer duration of event and somewhat gradual accumulation
rates...will hold off on issuing a watch with this package...
though advisories will likely be required as event approaches.
Further south...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 and lower susq valley.
System moves out quickly and the winter weather should be mainly over
by Sunday morning as a dry slot moves in behind the departing
coastal low. The northern stream trough will then cross the lower Great
Lakes Sun night and begin a period of enhanced lake effect
snows/extensive clouds downwind of Lake Erie.
Weak ridging should provide a cold but dry start to next week. An
Arctic shortwave crossing the Great Lakes/northeast/mid-Atlc Tuesday-
Wednesday will likely produce some light snows with lake enhancement.
After that...models begin to diverge significantly /after being in
quite good agreement until then/. GFS keeps broad trough over the
eastern U.S. With trough digging over west rockies...while European model (ecmwf) digs
trough over central U.S. Bringing warmer SW flow to PA. Best
forecast is for a gradual moderation of temperatures with poss a more
pronounced warmup heading into next weekend.
Aviation /21z 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.