NWS Forecast Discussion
252
FXUS61 KCTP 210555
AFDCTP
Area Forecast Discussion
National Weather Service State College PA
1255 AM EST Thu Nov 21 2024
.SYNOPSIS...
*A large area of rain over the eastern half of Pennsylvania will
move very slowly to the east overnight as surface low pressure
begins to strengthen near the New Jersey Coast as colder air
filters in changing the rain to snow over the higher terrain
toward daybreak Thursday
*Big pattern change in store for Thursday and Friday
associated with the first winter storm of the season
*Heavy, elevation-driven wet snow accumulation possible in the
Laurel Highlands and Poconos Thursday night-Friday night
&&
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Moderately strong westerly wind gusts peaking in the 30s (KTS)
will continue for the next 2 to 3 hours before subsiding as the
main pressure rise/fall couplet accompanying the Cfropa shifts
toward the Mid Atlantic Coast. A Wind Advisory is in effect for
roughly the southern half of the CWA through 06Z Thursday.
Slowing eastward progression of the stratiform precip area
lagging the Cfropa for 23 hours will gradually pivot to become
more NW/SE oriented as the lead shortwave rotates to the eastern
side of the digging upper low heading at CPA from the Glakes
Region.
A few hours period of lighter stratiform rain diminishing to a
few showers along with decreasing wind becoming southwesterly
in direction will follow for the middle of the overnight, then
some clearing an light wind will occur later tonight (West)
through the mid morning (across the East).
As we approach day break tomorrow a few wet snow flakes begin
to fly around on the highest ridges of Laurel highlands and
Western Poconos/Endless Mtns Region of NEPA. The heaviest
snowfalls won`t begin until late Thursday and into Friday.
Temperatures will fall another 7-11 deg F between 11 pm
Wed-7am Thu. 3-6mb pressure rises in 3hr along with strong CAA
will promote brisk wind gusts between 30-40 mph through 06Z Thu
before the wind slowly decreases through the overnight.
Min temps will be 10-20 degrees lower vs. last night in the
30-40F range by daybreak Thursday with an added wind chill
factor.
&&
.SHORT TERM /6 AM THIS MORNING THROUGH FRIDAY NIGHT/...
Overall confidence is continuing to increase for the first
impactful winter weather storm of the season. The latest
guidance continued to trend towards the same two areas of focus
for higher snowfall amounts Thursday evening through Friday
night. The highest confidence remains in the south west along
the Laurels where strong upslope flow could create areas of
6-8+ inches of snow. The winter storm watch for Cambria and
Somerset counties has been upgraded to a warning as a result.
The position of the overall low shifted south westward with the
most recent guidance, but snowfall totals remain uncertain in
our north eastern counties, thus the winter storm watch for
those counties has been expanded to reflect this latest trend.
Previous Discussion...
Big pattern change unfolds for late week as complex but highly
anomalous upper level closed low spawns the first winter storm
of the season.
Two key areas of concern:
1) Northeast zones adjacent to the Poconos: Elevation driven
heavy wet snow potentially setting up over a 24hr period
Thursday evening into Friday evening as robust deformation zone
pivots over northeast PA. We were keen to issue a winter storm
watch for the higher terrain zones adjacent to WFOs BGM and PHI
with multi model/ensemble trends signaling increasing
confidence in >6" near or above 1500ft.
2) Laurel Highlands: Multi-day upslope flow snow event is set
to occur from Thursday night through Saturday night impacting
the Laurel Highlands. This is our highest confidence heavy snow
risk area given a very predictable long duration NW flow
pattern. Ensemble guidance has been all over this going back
more than a week and now short range blends indicate high odds
for >6" of snow above 2000 feet. A winter storm warning has been
issued for Somerset and Cambria Counties as confidence in heavy
snowfall is high.
A general 1-3 inches is forecast over the northwest Allegheny
Mountains with some potential for local 3-6". Lower elevations
along and to the southeast of I99/I80 corridor could pick up a
slushy coating to an inch or so by the weekend, with very little
to no snow accum expected in the lower Susq Valley. Blustery
winds and heavy wet snow could result in moderate impacts (WSSI)
particularly in the higher elevations, but any accums in the
valleys would likely occur on grass and elevated surfaces.
&&
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Lingering lake effect/orographic snow over the Northern and
Western Mountains Saturday (with scattered valley rain and wet
snow showers) as one or more fast-moving and weak shortwaves
moves SE from the Glakes Region. This light precip will
dwindle by late Sunday afternoon, as subsidence inversion
heights fall and high pressure approaches from the Tennessee
Valley. Milder conditions look likely next Monday ahead of an
approaching, rather potent cold front, which could produce late
afternoon/early nighttime rain showers with the core of energy
aloft progged to move across the Upper Great Lakes region.
Brisk west winds and colder air pushing into the region for
Tuesday with scattered rain showers in the valleys and mixed
rain/snow showers over the higher terrain of the Laurels and NW
Mtns.
&&
.AVIATION /06Z THURSDAY THROUGH MONDAY/...
06z update... A frontal band of rain showers is progressing
quickly across the Commonwealth and it should be through
central PA by 08-09z. MVFR restrictions are the most likely
scenario during this timeframe, mainly for KIPT.
A dry slot behind the front should result primarily in
unrestricted conditions through early to mid-afternoon (17-20z).
Confidence in this scenario is fairly high (70-80%) for most
central PA terminal sites. The exception to this is along the
Allegheny front at KJST, where we expect lower clouds to
redevelop by 11-14z. We have 50-60% confidence in this.
Later this afternoon and evening, conditions should deteriorate
at KIPT (light rain and snow), as well as KBFD and KJST (light
snow). KJST will likely see IFR reductions (70-80% confidence),
with KBFD also probably sinking into IFR conditions by later
this evening. If KIPT keeps a rain/snow mix, MVFR is foreseen,
but prevailing snow would mean lower visibilities and IFR.
KAOO, KUNV, KMDT, and KLNS are expected to stay unrestricted
(60-80% confident) through this evening.
Gusty W-NW surface winds will diminish for a time later this
morning, before picking up again later this afternoon and
tonight (gusts of 20-25 kt common).
Outlook...
Fri-Sat...Restrictions likely with 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. Windy.
Sun-Mon...Lingering showers near KBFD and KJST.
&&
.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 1 PM this afternoon to 7 AM EST
Saturday for PAZ004-005-010>012-017-018.
Winter Storm Watch from this afternoon through Friday
afternoon for PAZ006-037-041-042-058.
Wind Advisory until 1 AM EST early this morning for PAZ017>019-
024>028-033>036-056-057-059-063>066.
Winter Storm Warning from 1 PM this afternoon to 7 AM EST
Saturday for PAZ024-033.
&&
$$
SYNOPSIS...Lambert/Steinbugl
NEAR TERM...Lambert/Steinbugl/Bowen
SHORT TERM...Steinbugl/Bowen
LONG TERM...Lambert
AVIATION...Jurewicz/Lambert/Bauco
NWS CTP Office Area Forecast Discussion
