Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 071942

Area Forecast Discussion
National Weather Service State College PA
242 PM EST Wed Dec 7 2016

A ridge of high pressure over the Ohio Valley will build into
Pennsylvania through tonight. 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
accumlulation 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
Sunday and Monday...bringing a chance for snow showers statewide.


Rather pleasant early winter day over central PA in the wake
of this mornings occluded frontal surface high
pressure builds into the region. Downsloping westerly breeze
is allowing maxes to warm through the upper 40s over the southeast
while temperatures elsewhere range from the mid 30s north to the
mid 40s.

5h shortwave streaks ENE from Ohio overnight increasing mid level
cloud cover from west to east...along with an increase in lake
cloudiness as colder air aloft works over the warmer waters of
Lake Erie. Therefore...generally increasing clouds are seen west
and north especially after midnight. This will keep min temps in
the mid 20s north...ranging to the lower 30s southeast.


Lake clouds will continue to enhance on Thursday...with lake
effect snow showers developing downwind of Lake Erie by late
morning and afternoon. Model data continues to indicate boundary
layer flow will remain WSW for much of Thursday, keeping the
accumulating snow north of the border for most of the day. However,
a gradual veering of the flow will likely bring snow into at least
NW Warren County by late in the day. Lake effect watch currently
begins at 18z Thu...but significant accums over my northwest
counties aren`t expected until Thursday evening and lasting
through Friday.


Main focus at the start of the long term will be the expected lake
effect snow event across NW Pa. The subsidence inversion base
lifts 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
anomalously cold air at 850mb and relatively warm lake waters of
around 50F, support locally heavy snowfall (mainly NW Warren Co)
between Thu night and Saturday. Examination of omega time sections
indicates the max lift should be just below the DGZ. This 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 LES expected Saturday as inversion heights fall upon
approach of high pressure.

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
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

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 00Z
ECENS and NAEFS indicate a significant cold front for mid week
with below normal temps likely late next week.


Skies becoming VFR this morning as high pressure builds across
central PA...continuing tonight through the first part of Thursday.
Local MVFR cigs may linger over portions of the NW mountains.

Some snow showers will likely work into the north and west late
Thursday into early Saturday afternoon, as winds shift more to the
west and northwest off the Great Lakes.

A weak storm system tracking northeast across the Great Lakes on
Sunday may bring a few snow showers to the area late.


Thu-Fri...Becoming windy with periods of snow showers, mainly late
Thursday into Friday. 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.


Lake Effect Snow Watch from Thursday afternoon through
Saturday morning for PAZ004-005.


LONG TERM...Fitzgerald/Lambert
AVIATION...DeVoir/Gartner is the U.S. government's official web portal to all federal, state and local government web resources and services.