Area Forecast Discussion
Issued by NWS State College, PA
FXUS61 KCTP 071156
Area Forecast Discussion
National Weather Service State College PA
656 AM EST Wed Dec 7 2016
A ridge of high pressure over the Ohio Valley will build into
Pennsylvania later today and tonight. A cold front will sweep
southeast across the area on Thursday. A weak low pressure system
is likely to track west of Pennsylvania through the Great Lakes
Sunday and Monday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Nearly saturated 0-1km layer remains across Central Pa at 09Z,
resulting in areas of drizzle and fog over the high terrain.
However, occluded front pushing into the Allegheny Plateau at 09Z
will sweep across the rest of Central Pa by around 14Z, ushering
in a drier westerly flow and causing drizzle to end from west to
east between 5-9AM. Can`t find any sub-freezing readings at 09Z
in mesonet and expect a brief rise in temps with arrival of front
as low level cold air is scoured out, so not anticipating any
significant icing issues this morning.
Surface high pressure building into the region should supply a
dry Wednesday to Central PA. However, low level moisture trapped
beneath inversion and upslope flow will likely result in stubborn
low clouds across the NW Mtns. A downsloping westerly breeze should
result in clearing 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 Mtns.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
High pressure ridge will remain over the region tonight, producing
generally fair and tranquil conditions. A fast-moving shortwave
lifting out of the Mississippi Valley will track across NW Pa
around dawn, likely producing no more than an increase in mid
level cloudiness. However, colder air working into NW Pa in the
wake of this feature will result in developing lake effect snow
downwind of Lake Erie on Thursday. All model data currently
indicating the boundary layer flow will remain WSW for much of
Thursday, keeping the accumulating snow north of the border.
However, a gradual veering of the flow will likely bring snow
into at least NW Warren County by Thursday evening. Lake effect
watch likely begins a bit too early, but will leave start time at
18Z for now.
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
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
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.
.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...
Expect most areas to become VFR by late morning as the storm
moves away from the area.
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.
Temp/dewpoint sensor at KIPT appears to be malfunctioning. It
continues to read too warm compared to meso obs and surrounding
METARs. Techs have been advised and repair is estimated to be
done on Wed.
Lake Effect Snow Watch from Thursday afternoon through
Saturday morning for PAZ004-005.