Winter Storm Warning 8, NWS State College, PA

Area Forecast Discussion
National Weather Service State College PA
1125 AM EST Wed Nov 20 2024

.SYNOPSIS...
*Brief period of moderate to locally heavy rain spreads from
 west to east this evening into early tonight
*Big pattern change for late week 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 /THROUGH TONIGHT/...
Low thick clouds and fog continue to hang on across much of the
central and northern mountains in the latter half of this
morning. Sun has broken out across the southern half of the
Commonwealth, and temperatures are beginning to rise. Most of
central PA should break out into partly sunny skies ahead of the
occluded front expected to reach us later this evening, which
will promote the weak instability for the convective rain
showers tonight.

Previous Discussion...
Expect late day highs in the mid 50s to low 60s or +10-15
degrees above daily climo.

Hires models show a band of soaking rain accompanying an
occluded front sweeping through the area this evening through
early tonight. A brief period of convectively enhanced/locally
heavy rainfall is possible given weak/slightly elevated
instability particularly over portions of south central PA. The
short duration downpours could cause some minor street or
roadway ponding especially due to leaf-clogged storm drains.

Conditions will turn windy and sharply colder in the wake of the
frontal passage with temperatures falling about 20 degrees
in 12 hours between 7pm-7am. 3-6mb pressure rises in 3hr along
with strong CAA will promote brisk wind gusts between 30-40 mph.
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 /THURSDAY THROUGH FRIDAY NIGHT/...
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 watch was also
issued for Somerset and Cambria Counties.

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. There
are still some details to resolve, but feel watch issuance was a
solid step in the headline progression at this stage/fcst
length.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Lingering lake effect over the W Mtns should dwindle by Sunday
afternoon, as inversion heights fall and high pressure
approaches from the west. Milder conditions look likely next
Monday ahead of an approaching cold front, which could produce
late day rain showers.

&&

.AVIATION /16Z WEDNESDAY THROUGH SUNDAY/...
11 AM update.

15Z TAFS sent.

IFR to low MVFR CIGS will be tough to break/mix out this
afternoon across much of Central and Northern PA under light
llvl wind.

VFR and a nice open area in the low cloud deck across SOuthern
PA late this morning will be fleeting and expect high end MVFR
to VFR BKN-OVC to advect back in from the southwest during the
early to mid afternoon hours.

Timing of a strong cold frontal passage with a narrow line of
brief heavy showers and even a few embedded, low-topped TSRA
with brief strong WSW-westerly wind gusts will impact the
Western airfield from KJST to KAOO, KFIG and KBFD starting
around 22-23Z and 02-03Z in the east. Peak wind gusts will be
about 10 kts lower at Susq Valley TAF sites and points east.

Precipitation will only last a few hours along and in the wake
of the cfront, before an 8-12 hour period of improving MVFR to
VFR in the east occur. Any improvement across the Laurel
Highlands and NW Mtns will be much more brief (only 2-4 hours)
and will likely stay MVFR during that time late tonight through
the mid morning Thursday before the deepening cold air and moist
upslope flow lowers cigs into IFR/LIFR with snow developing by
later Thursday morning.

Outlook...

Thu-Sat...Restrictions likely with rain and snow showers,
mainly across the mountains (BFD and JST). MVFR much of the time
for UNV/IPT/AOO. Mainly VFR at LNS and MDT. Windy.

Sun...No sig wx.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Thursday afternoon through Saturday
evening for PAZ024-033.
Winter Storm Watch from Thursday afternoon through Friday
afternoon for PAZ037-042-058.

&&

SYNOPSIS...Steinbugl
NEAR TERM...Steinbugl/Bowen
SHORT TERM...Steinbugl
LONG TERM...Lambert
AVIATION...Lambert/Martin

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