974
FXUS61 KCTP 220330
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1030 PM EST Thu Nov 21 2024

.SYNOPSIS...
* An upper level low over the Lower Great Lakes will track
  southeast across Pennsylvania tonight into Friday.
* The first impactful winter storm of the season will arrive
  with this low, bringing widespread snow across central
  Pennsylvania.
* The upper low will lift out of the region this weekend, then
  a cold front will likely push southeast across the state
  Monday night into early Tuesday.

&&

.NEAR TERM /THROUGH FRIDAY/...
Little change in the going forecast for now as the approx 990 MB
sfc low just SSE of KHTP will continue to slowly deepen and
pivot to the NW as the left exit region of a strong 140-150 KT
upper jet rounds the base of the 526 DAM upper low over southern
PA this evening.

The nose of a -4 to -5 Sigma NE to Northerly 850 MB jet and axis
of strongest convergence will continue to move about an
additional 75-100 NM Westward overnight through Friday with
increasing/persistent moderately strong 850-700 mb FGEN forcing
supporting a trend toward steadier precip and a transition to
mainly wet snow in all areas late tonight through the mid
morning hours Friday as the precip rates increase and llvls cool
slightly.

The greatest precip rates for much of Central PA and the Western
Mtns will be during the daylight hours Friday.

Previous Disc...

A strong surface low moving across western Michigan combined
with strong upper level diffluence and positive vorticity
advection will spread light rain/snow showers over the area
ahead of the main system. Model sounding are cold enough for
snow showers across the Alleghenies, but amounts should be light
and progged surface/road temps should be just above freezing,
so don`t expect many travel issues through mid afternoon. See no
reason to deviate from NBM temps, which peak this afternoon in
the low to mid 30s across the Laurel Highlands and the mid 40s
in the Susq Valley.

Overall confidence is continuing to increase for the first
impactful winter weather storm of the season to begin tonight,
as the closed 500mb low tracks across Southern PA, causing the
associated surface low south of New England to retrograde
westward through upstate NY. A deep, moist westerly flow on the
northern and western periphery of the upper low should result in
persistent orographically-enhanced precipitation over Central
PA tonight into Friday. Snow will be the primary form of
precipitation during the overnight hours with a mix of rain
possible in the Lower Susquehanna Valley. Marginal boundary
layer temps will result in highly elevation- dependent
accumulations across the Alleghenies and downsloping flow should
yield lower precip rates east of the Alleghenies.

&&

.SHORT TERM /FRIDAY NIGHT/...
Friday morning will see the heaviest snow fall across our CWA.
The highest confidence of heavy snow is focused over high-
elevation Sullivan County, as well as the Laurel Highlands,
where the 12Z HREF supports a likely period of 1-inch/hr rates
between 10-16Z Friday. The greatest impact to travel should be
very early Friday through late Friday morning, but caution
should be exercised when traveling throughout the day Friday as
snow showers will continue into the early afternoon. Most
likely snow totals by late Friday should range from up to a foot
over the highest elevations of Somerset and Sullivan counties.
The 12Z HREF has trended snowfall amounts slightly higher from
previous runs with the mean snowfall for Altoona and
Statecollege now between 2-3 inches. Areas across the Lower
Susquehanna will likely have 1-2 inches of snow given the more
marginal temperature profiles.

Diminishing precipitation is expected in general late Friday
into Friday night, as the upper low lifts out. However,
lingering lake effect will affect the W Mtns. Forecast thermal
profiles moderate with the departure of the upper low to the
point that lake effect snow showers may mix with rain over the
Alleghenies Friday night.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
One last channel of vorticity stretched out to the west of the
center of the upper low will enhance lift on Sat along the
western mtns. That will help precip (mainly rain) linger over
the western hills. It`s a high-PoP but low QPF event. This
light precip will dwindle Sunday AM, as subsidence inversion
height fall and high pressure approaches from the Tennessee
Valley. Milder conditions (+3 to 6F vs Sun) look likely for
Monday. A fast-moving but compact system will develop very
quickly as it near from the OH valley, and pass across the
state Monday-Monday night. Temps should remain mild enough to
be almost all rain except on the very highest (>2000ft)
elevations. No accums are expected at this point, but a slight
timing change (time of day/night) may allow a slushy coating.
Tues looks mainly dry under high pressure. Perhaps a SHSN NW.

The next 2 days (peak travel Wed and Thanksgiving Day itself)
seem highly uncertain. A zonal flow across the ern 2/3rds of the
CONUS will likely lead to a fast, progressive flow. The front
that passed thru on Mon night will be laid out W_E to the south
of our latitude, and moisture should start to return and pool up
along it on return flow from the GOMEX. However, timing of
support/forcing from the west is highly uncertain. GFS Ensembles
have low spread in 5H and thickness plots, with a very, very
dip/trough developing to our west mid-week. 00Z ECMWF is much
(1 day) slower with generating a sfc low and making precip
arrive in PA (not until Turkey Day), vs the GFS which is faster.
Thermal profile for the start of any precip either Wed or into
Thurs would likely be cold enough over some (nrn) portion of
the CWA to make frozen or freezing precip. Following the NBM`s
very slow building/ramp up in PoPs without any notion of support
for deviation at this point. Just not enough strong points to
make any precip arrive slower or faster. Be prepared for
slippery/poor travel during this upcoming long holiday travel
time.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
Light rain and snow showers continue this evening across Central
PA and have been leading to brief LIFR restrictions at JST.
Snow will increase in coverage through the night as low pressure
to the east intensifies and tracks closer to the region.
Ceilings will lower as this happens and visibilities will drop
as snow begins to fall. All sites are expected to drop to IFR or
lower by early Friday morning and their is high confidence in
IFR conditions continuing through the 00Z TAF period.

The main uncertainty in the forecast is in regard to precip
type. Snow will be the predominant precip type into early
afternoon at all sites, but model soundings suggest that rain
will begin to mix in everywhere except BFD and JST after 18Z.
The exact timing of this changeover will impact how long
visibilities remain below 1 mile.

Wind gusts will increase through the night with gusts of 25 to
30 knots likely during the day on Friday. Most guidance also
suggests a period of low-level wind shear will occur at AOO and
UNV before 12Z as winds at 2000 feet increase to around 45
knots.

Outlook...

Sat...Periodic restrictions in rain and snow showers, mainly
across the mountains (KBFD and KJST). MVFR much of the time for
KUNV/KIPT/KAOO. Mainly VFR at KLNS and KMDT. Continued breezy.

Sun-Mon...Lingering restrictions possible at KBFD and KJST,
otherwise mainly VFR.

Tues...MVFR cigs/isold SHSN NW, VFR SE.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 7 AM EST Saturday for PAZ004-005-
010>012-017>019-025-034-045.
Winter Storm Warning until 4 PM EST Friday for PAZ006-037-041-
042-053-058.
Winter Storm Warning until 7 AM EST Saturday for PAZ024-033.
Winter Weather Advisory until 4 PM EST Friday for PAZ046-049-
051-052.

&&

$$

SYNOPSIS...Lambert/Fitzgerald/Bowen
NEAR TERM...Lambert/Fitzgerald/Bowen
SHORT TERM...Fitzgerald/Bowen
LONG TERM...Lambert/Dangelo
AVIATION...Bauco