Area Forecast Discussion
Issued by NWS State College, PA

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000
FXUS61 KCTP 080633
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
133 AM EST Thu Dec 8 2016

.SYNOPSIS...
A cold front will sweep southeast across the area on Thursday.
Colder air moving over the eastern Great Lakes will produce
significant lake effect snow accumulation over the snowbelt region
of northwest PA late Thursday through early Saturday. A weak low
pressure system is likely to track west of Pennsylvania through
the Great Lakes later in the weekend, bringing a chance for snow
to the entire state.

&&

.NEAR TERM /UNTIL 8 AM THIS MORNING/...
Regional radar mosaic at 06Z showing some weak returns aloft
assoc with lift along jet streak, which extends from the Ohio
Valley through Pa. PIT 00Z sounding showing plenty of dry air in
the lowest 5kft, which will likely preclude precip reaching the
ground in most places. However, wouldn`t be surprised to see a
few flurries toward dawn across the the northern half of the
state, as mid level shortwave moves through.

Plenty of mid level cloudiness and an active westerly breeze
should keep temps from falling much from 06Z readings. Lows should
generally range from around 25F across the Allegheny Plateau, to
around 30F across the Susq Valley.

&&

.SHORT TERM /8 AM THIS MORNING THROUGH 6 PM FRIDAY/...
Shortwave and assoc batch of mid level cloudiness should push east
of the region by around midday. In it`s wake, arrival of colder
air will result in developing lake effect snow downwind of Lake
Erie today. All model data currently indicating the boundary
layer flow will remain WSW for much of the day, keeping the bulk
of the accumulating snow north of the border. However, a gradual
veering of the flow is anticipated by late afternoon, bringing
snow into Warren/Mckean counties. LE snow warning remains up for
Warren and an advisory for Mckean beginning this afternoon.

Confidence has increased to the point where we feel heavy lake
effect snow bands are a given tonight through Fri night.
Proximity to the lake and fetch for the first ~12 hours of the
event point to NW Warren and far nrn McKean Co as most likely to
see a few inches by sunset Thurs, and many more by sunrise Friday.
Accumulations could be quick overnight with a gradual veering to
the winds and multiplication of the bands as the cross- lake fetch
becomes shorter. We may also see LH & Georgian Bay get involved
and contribute moisture to our region.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
The long term will begin with arrival of a deep trough extending
from a low pressure system situated over eastern Canada. This
trough will usher in cold northwesterly flow. Latest model runs
have 850 mb temperatures in the -10 to -12 range, especially
through the northwest mountains. The initial wave of precipitation
from the front should generate decent snowfall however the main
focus of this event will be the lake effect pattern that will set
up after frontal passage. The latest ensembles have actually
lessened the amounts into the NW mtns. From the model soundings
there should be a subsidence inversion but bases should lift
rapidly up to, then well over 7-10 kft agl Thursday night right
through Friday night, as the mean low to mid level flow becomes
well- aligned from the West- Northwest. The subsidence inversion
base will drop gradually down to below 7 kft agl during the day
Saturday as the mean 925-700 mb flow backs to more of a west, then
swrly direction by around 12Z Sunday.

This scenario of inversion heights near 700mb, combined with the
anomalously cold air at 850mb and warm lake waters of around 50F,
support locally heavy snowfall (mainly NW Warren Co) between Thu
night and Saturday. Strong lapse rates combined with a fairly
gusty wind should hold snow/water ratios to no more than 20 to 1.
Based on QPF input from WPC and latest higher resolution models
first guess is for 36-hour totals of 12+ across the snowbelt of NW
Warren by Sat AM.

Further south, scattered narrow bands of snow showers will stream
SE into the Central Ridge and valley Region occasionally, with a
coating to an inch possible in a some locations. Orographic
forcing over the Laurels should lead to an inch or two of accum
Thu night through Fri night. A generally westerly flow is
unfavorable to draw lake moisture that far south.

Diminishing Lake effect snow is expected Saturday as inversion
heights fall upon approach of high pressure.

A bubble of high pressure at the sfc with a weak ridge aloft will
bring a brief period of tranquility, though very cold overnight
min temps, as it slides east across the forecast area late
Saturday into early Sunday. Expect partial clearing and perhaps
mostly clear skies across the SE part of the CWA for the middle
third of the weekend.

Deep, mean-layer flow backs to the southwest with a slug of low-mid
level warm advection sliding up and over the deep/retreating cold
airmass. This could create an area of light snow or snow showers
across mainly the northwest half of PA Sunday afternoon and Sunday
night.

The boundary layer could become warm enough by Monday to mix in
or changing the scattered light precipitation to rain showers,
throughout the Lower Susquehanna Valley.

Seasonable temps indicated early next week, but both the 012Z
ECENS and NAEFS indicate a significant cold front for mid week
with below normal temps likely late next week.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
A developing cold W-NW flow pattern in the wake of a low moving
across Quebec into Atlantic Canada will support a prolonged period
of MVFR/IFR conditions over the western 1/3 airspace into the
weekend, with frequent lake effect/upslope snow showers. MVFR
cigs and ocnl visby reductions are possible over the central
terminals with VFR prevailing across the eastern sites. Expect
frequent sfc wind gusts 20-25+kt by this afternoon resulting in
low level mechanical turbulence during takeoff and final approach.
Lake effect snow showers should decrease in coverage/intensity by
later Saturday as area of high pressure briefly slides over the
airspace.

Outlook...

Sun-Mon...MVFR/IFR with mixed rain/snow likely.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Lake Effect Snow Advisory from noon today to noon EST Saturday
for PAZ005.
Lake Effect Snow Warning from noon today to noon EST Saturday
for PAZ004.

&&

$$
SYNOPSIS...DeVoir
NEAR TERM...Fitzgerald
SHORT TERM...Dangelo/Fitzgerald
LONG TERM...Dangelo/Fitzgerald/Ceru
AVIATION...Steinbugl



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