417
FXUS61 KCTP 231145
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
645 AM EST Mon Dec 23 2024

.SYNOPSIS...
High pressure will drift east of Pennsylvania today, then a
weak area of low pressure will track north of the state tonight
into early Christmas Eve. A pair of weakening shortwaves are
likely to ride the northern periphery of a building ridge over
PA the second half of next week, as a large surface high remains
anchored north of New England.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
After a frigid start to the day, a return south-southeast flow
behind departing high pressure will help temperatures rebound
slightly today. Early sunshine will fade behind thickening mid
and high level cloudiness ahead of an upstream shortwave over
the W Grt Lks. GEFS 2m temps indicate max temps will remain
several degrees below average, ranging from the mid 20s to low
30s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...
The shortwave an associated weak sfc low is progged to pass
just north of the region tonight into early Tuesday. The
moisture return, as inferred by EPS PWAT anomalies, is fairly
unimpressive. Therefore, expect only a period of light
overrunning snow Monday night into early Tuesday for most of the
area. However, a brief period of moderate snow is possible over
the NW Mtns, as the nose of a 50kt low level jet slides through
between 03Z-07Z.

The 00Z EPS indicates the NW Mtns are best situated to see an
inch or two of accumulation due mainly to orographic enhancement
from an upsloping SSW flow, while most of the forecast area sees
<1 inch.

Model soundings indicate there is some risk for a bit of very
light freezing rain over the Lower Susq Valley around 12Z
Tuesday, but it is uncertain whether measurable precip even
reaches that far south. ForecastBuilder indicates a possible
scenario over that part of the state would be a light snow
accumulation of <0.5 inch topped by a light glaze, which would
cause significantly less travel impact then a light glaze
without the snow. Due to these uncertainties, have opted to
highlight the potential travel impacts in the HWO and reassess
the threat after the 12Z guidance comes in.

The best forcing and deepest moisture should have passed PA by
late Tuesday morning. However, residual low level moisture
ascending the Alleghenies could result in spotty very light
snow/drizzle into the afternoon over the western part of the
forecast area.

Any evening very light snow/drizzle over the Laurel Highlands
should end Tuesday night, as surface high pressure builds in
from the Grt Lks. However, model soundings indicate low level
moisture trapped beneath a subsidence inversion will result in
stubborn low clouds, especially over the Allegheny Plateau.

Fair and seasonable weather is indicated by model guidance Wed,
as surface ridging builds over the state. SREF prob charts and
forecast soundings indicate a shallow layer of upslope stratus
could potentially remain over the Alleghenies, with brighter
skies east (downwind) of the mountains.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
On the large-scale, the amplified, cold eastern
Canadian/northeastern CONUS upper trough dominating at the
present time should be a thing of the past by Christmas Day, as
this feature lifts out into the northern Atlantic. It will
replaced by building heights and a generally more zonal flow
pattern aloft. This translates into more seasonable temperatures
for late December across PA, even trending above climo towards
next weekend.

As for the sensible weather, there is increasing confidence that
surface high pressure from southeastern Canada down into the
northeastern U.S. will keep the weather dry for the Commonwealth
from Christmas Day through Friday. Uncertainty grows into the
weekend, as low- level easterly flow and the approach of weak
short- wave energy from the Midwest could bring bouts of light
precipitation, especially for southern and western sections of
the state. At this early juncture, light rain appears to be the
most probable precipitation type, but pockets of light wintry
mix are not out of the question.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
All sites will begin the day in VFR with clear skies south of
I-80 as of daybreak. Clouds will begin building in over the
northwest ahead of an approaching frontal system that will
impact the area towards the end of tonight and early tomorrow
morning. Cloud bases will lower tonight (esp across the
Northern and Western Mountains of PA) with some periods of light
snow/snow showers developing.

Visibility reductions due to increasing snowshowers are likely
across the northwest. These snow showers will begin near 04Z for
BFD and gradually progress south and east throughout the night.
By early morning Tuesday snow showers are possible at all sites.
The Lower Susquehanna sites of MDT/LNS have the lowest
probability of snow (<30%), but could see a glaze of freezing
rain late Tuesday morning.

LLWS will also be a concern for the western and central
mountains of Pennsylvania. Winds at 2,000ft will likely be
gusting upwards of 40kts starting tonight and progressing well
into Tuesday.

Outlook...


Mon...No sig wx.

Tue...Light snow enters W PA Monday evening and overspreads the
rest of the area overnight Monday night/early Tuesday. MVFR cigs
and vsbys focused over KJST and KBFD.

Wed...No sig wx.

Wed Night...Patchy light freezing rain possible.

Thu...Light rain possible south.

Fri...No sig wx.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...Fitzgerald
NEAR TERM...Fitzgerald
SHORT TERM...Fitzgerald
LONG TERM...Banghoff/Colbert
AVIATION...Bowen