095
FXUS61 KCTP 081956
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
256 PM EST Wed Jan 8 2025

.SYNOPSIS...
Windy conditions will prevail through Thursday with single
digit to subzero wind chills. Bands of lake effect and upslope
enhanced snow showers will persist across the Northwest
Mountains and Laurel Highlands into Thursday morning. Cooler-
than-average temperatures continue through the weekend as a
storm system brings another chance for light snow to all of
Central PA. Lake effect follows after that system.

&&

.NEAR TERM /THROUGH TONIGHT/...
As with the previous passage of the band of snow on radar, the
local effects were a drop to ~3/4SM vsby but no accums. With air
temps well below freezing and most road temps below freezing, we
will continue to monitor these lake effect bands, but they don`t
need any further attention at this time.

In fact...the longest band that has the good Huron connection is
breaking up and diminishing in intensity at the same time. This
trend is all in the plan, and we`ll continue to mention high
PoPs but little accums for all the wrn mtns and areas NW of
N38-UNV-AOO. Nighttime and increasing stability usually helps to
stratify things more than during the daylight hours when the
convection helps break things up. Thus, we are thinking that
some better linear organization to the bands will occur as it
gets dark. However, the exit of the best forcing from a strong
short wave trough dropping across from the north will spell a
lowering/strengthening of the subsidence inversion and the
beginning of the end for the lake effect.

Temps and the gusty NW wind will combine to take about half of
Somerset Co down to about -10F wind chills in the morning. With
just an hour or two at those numbers, we`ll hold off on issuing
(one of the new) cold weather advy products. The northern mtns
do drop to -10F wind chills in spots, as well. But, their advy
criteria is -15F, so that`s not as tough of a call on holding
off on the cold wx advy. Min temps around +5F are possible in
the nrn mtns and central Somerset Co with the lower
elevations/cities of the Lower Susq holding in the mid teens.
Those numbers are 3-8F below normal.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY/...
The approach of high pressure Thursday will result in a further
lowering of the subsidence inversion with mixing heights in mid
aftn about 2500ft in the NW and 3500ft SE. Little inland moist
transport is expected and cloud depth will become too poor to
support SHSN for anyone but Warren Co by the aftn. The wind
keeps up, though. We will see more in the way of sunshine as
well. But, the max temps will be very close to Wed`s cold
values.

The sfc high slides in through the night and should be directly
overhead Friday during the day. Expect the wind to lighten up,
but not go calm. MinT will be very similar to Thurs AM. But,
with less wind, no worries exist for cold wx advys. By late
aftn Fri, a southerly breeze should exist, and temps do rise vs
Thursday. Maxes NW will be 7-8F higher than Thurs, and 3-4F
higher SE. High clouds do start to stream in from the next storm
system slated to make light snow Fri PM-Sat.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
A longwave trough over the eastern US will dominate the
extended period, keeping temps about 5F below normal through the
entire long term period. A series of shortwaves traversing along
the prevailing longwave trough will generate potential for
increased precipitation and, if properly phased with low
pressure at the surface, could result in more significant snow.
No major snow in the forecast at this point, but there will be
plenty of cold air in place if the ingredients can come
together.

A minor ridge aloft moves overhead for Friday bringing sunshine
to start the day, but is quickly replaced by a progressive
trough and increasing clouds by afternoon. That trough will
drive a storm to develop over the Mid Atlantic and Gulf Coast
states. The sfc low from this system will likely pass 3-4 states
south of PA (GA- SC), but an inverted trough will help draw
moisture into PA. Good height falls are expected, so confidence
is high that enough forcing will be present to generate at least
light precip. The extent of the moisture feed remains the main
forecast challenge of the long term. Recent runs continue to
support a brief period of main light, fluffy snowfall mainly Fri
night and early Sat AM. Have bumped up PoPs vs NBM guidance
across the srn tier, but left them alone in the nrn tier where
less moisture is expected to reach. A few members of the GEFS
and ECENS continue to phase the northern and southern stream
troughs generating significant snowfall somewhere along the
east coast, but the majority of members don`t develop much of
anything.

The remainder of the long range holds a short period of lake
effect flow, then a minor sfc ridge moves overhead for later
Sun-Sun night. A moisture-starved clipper may move through the
Great Lakes Monday-ish, and generate a broad light snow, mainly
for the NW half of the CWA. Then, another multi-day period of
cold with lake effect flow occurs for days 7-8 and maybe 9, too.
1000-500Hpa thickness values get close to 500 those days, which
would be just about the coldest airmass of the winter. With many
days of sub-freezing air for Lake Erie, ice ought to start
expanding across more of the lake. As the cold persists,
expanding ice should hinder the lake effect/upslope snow machine
some.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
NW flow dominates the state. A good Huron connection is making
for enhanced lake effect snow showers (used to be a good/long-
lived band) for places just S of DUJ-FIG-UNV. However, it has
broken up into more-cellular showers. In general, most recent
meso models continue to indicate a very steady-state set of
conditions is expected overall during the next 18 hrs. MVFR NW
with frequent but very brief dips to IFR in the snow showers for
BFD and JST. Prob of IFR cig at JST and BFD is 40-50pct, so no
good signal exists to point one way or the other (above/below
1000ft). Mainly VFR for IPT, MDT and LNS with a good downslope
flow and it`s drying/subsidence.

The gusty NW wind continues into the night, and only weakens
slightly, increasing back to current strength for Thursday
daylight. The possibility of a 40-45KT wind just at the top of
the inversion will be enough to mention LLWS at a couple of
sites (JST-AOO), despite already having mentions of gusts in
the wind groups. Most of the SHSN will be gone by mid-day Thurs
with the inversion lowering and capping the cloud tops at
3500ft SE and perhaps only 2500ft for BFD. High pressure starts
to float in from the west Thursday night, and should squash any
remaining SHSN while also decreasing the wind.

Friday will hold light wind that should turn srly for the W.
However, high clouds will be increasing and lowering during the
afternoon as the next storm moves in from the west.

Outlook...

Fri...Early AM low cigs poss NW Mtns. Late evening widespread
light snow/IFR vsbys possible, esp W Mtns.

Sat...AM light snow/IFR vsbys likely. Improvement late AM/PM.
Lake effect kicks in PM.

Sun-Mon...Snow showers/tempo IFR vsbys possible W Mtns.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon EST Thursday for PAZ004-005-
024-033.

&&

$$

SYNOPSIS...Dangelo
NEAR TERM...Dangelo
SHORT TERM...Dangelo
LONG TERM...Dangelo
AVIATION...Dangelo