Area Forecast Discussion
Issued by NWS State College, PA-- Remove Highlighting --
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FXUS61 KCTP 062151
Area Forecast Discussion
National Weather Service State College PA
451 PM EST Tue Dec 6 2016
Low pressure moving through will continue to bring a mix of
precipitation through central Pennsylvania this afternoon and
evening...with some significant icing likely over the Laurel
Highlands. After a brief period of tranquil weather on Wednesday
as high pressure ridges in from the Ohio River Valley, a cold
front will sweep southeast across the area Thursday. A low
pressure system is likely to track west of Pennsylvania through
the Great Lakes Sunday and Monday.
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Main focus continues to be over the high terrain of the Laurels,
where model soundings continue to show some ice accumulations
probable and latest JST ob has around a tenth of an inch so far.
Based on the near to slightly sub-freezing temperatures and temps
warming aloft to just above freezing in a rather deep layer with
another swath of moderately heavy precip streaming NE across the
region through this evening, we opted to upgrade the Winter
Weather Advisory to an Ice Storm warning. Once the mean south
to serly flow shifts to the west-southwest later this evening,
the precipitation should taper off to just some isolated rain
showers/patchy drizzle in the Warning area with temps drifting
We issued a Winter Weather Advisory the NW mtns through 12z Wed,
where an initial period of mixed precipitation late today could
mix with snow at times, yielding light glazing of ice up to
around 0.10 of an inch from periods of -FZRA and sleet, and
perhaps a coating to inch of snow (with up to 2 or 3 inches of
snow across Potter county) based on model blended QPF.
Based on coordination with BUF/BGM, these higher end snow
amounts should be fairly isolated and given that it will be a
rain/snow mix with mild SFC temperatures, have lowered snow amts
from WPC guidance and held off on a winter wx advisory across
Deep moisture will exit the area with passage of mid level
shortwave late this evening. However, model soundings remain
nearly saturated below 850MB, indicating the likelihood of linger
drizzle/rain and a snow mix late tonight. Superblend and high res
NAM both indicate temps creeping above freezing even across the N
Mtns late tonight.
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Surface high pressure building into the region should supply a dry
Wednesday to Central Pa. However, low level moisture trapped
beneath inversion and weak mixing will likely result in stubborn
low clouds across the NW Mtns. A downsloping westerly breeze
should result in partly to mostly sunny skies east of the Mtns.
Temperatures there should respond by rising into the low-mid 40s,
while cloud cover holds readings to the mid 30s across the NW
.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
The long term will begin with a brief period of tranquil
conditions and deep southwesterly flow, ahead of an arctic front
that will be pushing across the state Thursday morning and early
Clouds will thicken up Wednesday night and early Thursday with snow
showers moving into the far northwest around or shortly after
midnight, then spreading SE and possibly mixing with rain showers
across the Southeast half of the state Thursday morning.
The subsidence inversion base lifts rapidly up to, then well over
7-10 kft agl Thursday afternoon, 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
This scenario is very favorable for heavy, to locally excessive
Lake effect snow and significant amounts of blowing and drifting
snow with occasional wind gusts around 30 mph, and up to several
hundred j/kg of cape being transported SE of interstate 90 leading
to the potential of some Thundersnow with periods of 2+/hr snowfall
rates possible, especially Thursday night through Friday night as
quite anomalously cold air of -2 to -3 sigma in the 700-500 mb
layer moves overhead.
Posted a Winter Storm Watch for Warren and McKean counties from
Noon Thursday through 7 am Saturday, and the significant bands of
LES could even continue through much of the day Saturday. Snow
totals throughout the perennial snowbelt of NW Warren county
could top 18 inches by Saturday, with areas of 6-12 inches
further inland across the SE half of Warren county and at least
the NW half of McKean County.
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.
Expect rather significant snow totals (in excess of 4 inches) to
also occur across the Laurel Highlands, albeit over a more
extended period Thursday night into Saturday, and with lighter
A bubble of high pressure at the sfc (and 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
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 will create an area of steady 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.
One or more, weak and fast moving short waves in the zonal flow
aloft could bring some light mixed precipitation in the Ridge
and Valley Region, and scattered snow showers across the
.AVIATION /22Z TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --21Z TAFS sent.
Large area of rain, with some freezing rain to the west.
Earlier discussion below.
IFR cigs at JST and through the central mountains should continue
through 05Z before the precipitation tapers and cigs should
slightly improve. MVFR cigs will lower from south to north tonight
especially given the mixed precipitation. Confidence in ptypes is
marginal given complex and evolving thermal structure/evaporative
cooling in the boundary layer. Elevation and pcpn rates will also
be a factor. A period of LLWS is possible at BFD/JST from roughly
6/21z-7/03z. Overall, expect flying conditions to be in the sub-
VFR Tuesday night into Wednesday morning before slowly improving
after 09Z. Restrictions should lift Wednesday morning.
Wed...A.M. low cigs becoming mainly VFR.
Thu-Fri...Becoming windy with periods of snow showers. Mainly
MVFR/IFR west to MVFR/VFR east.
Sat...Decreasing winds. Low cigs/-SHSN psbl NW. VFR east.
Sun...Some snow possible. Mainly late across the NW.
-- End Changed Discussion --
Lake Effect Snow Watch from Thursday afternoon through
Saturday morning for PAZ004-005.
Winter Weather Advisory until 7 AM EST Wednesday for PAZ005-
Ice Storm Warning until midnight EST tonight for PAZ024-033.