538
FXUS61 KCTP 250139
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
839 PM EST Sun Nov 24 2024

.SYNOPSIS...
* Mostly cloudy and breezy with rain showers lingering over the
  northern tier of central Pennsylvania into tonight
* Periods of rain Monday night followed by gusty winds and lake
  effect rain and snow showers into Tuesday
* Potential travel disruptions for Thanksgiving Day followed by
  cold temperatures and lake effect snow through the holiday
  weekend

&&

.NEAR TERM /UNTIL 7 AM MONDAY MORNING/...
Any lingering light rain showers/sprinkles over the northern
tier of PA should end by late evening, as high pressure ridging
builds into the area. Low clouds will continue to break up from
south to north overnight, as inversion heights fall.

Clearing skies, a light wind and relatively low dewpoint
depressions going into the evening support the development of
patchy valley fog later tonight. We adjusted min temps slightly
higher than NBM guidance along the NY border, due to an
expectation of lingering cloud cover up there. Expect daybreak
readings to range from 30-35F over most of the forecast area.

&&

.SHORT TERM /7 AM MONDAY MORNING THROUGH WEDNESDAY NIGHT/...
Partly to mostly sunny skies through the first half of Monday,
combined with strong WAA and southerly flow ahead of an
approaching frontal system, should support max temps +5-10F
above climo with highs ranging from the mid to upper 40s in the
Endless Mtns to the mid 50s in the south central valleys near
the MD line.

Models in good agreement spreading rain showers into the
western part of the forecast area Monday afternoon, as a 30kt
southwesterly low level jet and associated plume of anomalous
pwats work into the state ahead of an upstream trough. Strong
large scale forcing and a decent plume of pre-frontal pwats
supports high POPs Monday night into early Tuesday associated
with the cold front passage. Most likely rain totals based on
latest HREF/EPS/GEFS QPF range from 0.10-0.30" with the highest
totals likely over the Laurel Highlands. Thickening cloud cover
and warm advection ahead of the approaching cold front should
result in a mild Monday night with fcst lows adjusted higher to
the 40-45F range, or +10-20F above the historical average for
late November.

The cold front and any associated rain showers should exit the
eastern edge of the forecast area by midday Tuesday. Large
scale subsidence behind the front should result in blustery
conditions with breaking clouds over most of Central PA. A
progged west-southwest boundary layer flow should keep
developing lake effect snow showers north of the border.

Pressure rises/isallobaric couplet and CAA behind the front
will result in windy conditions Tuesday with fcst soundings
supporting peak gusts 30-40 mph. Diurnal temps may go neutral to
even falling during the day across the western mtns through
Tuesday afternoon. Surface high pressure building east from the
Ohio Valley should supply fair and seasonable conditions with
diminishing winds Tuesday night. Latest guidance indicates a
west-southwest flow will continue to keep lake effect snow
showers north of the border.

Fair weather and light winds look likely Wed, as surface
high pressure slides just south of PA. However, thickening high
clouds are possible in the PM hours ahead of an area of low
pressure approaching from the Miss Valley. 24/12Z model spread
(GFS/CMC vs. ECMWF) has increased in a big way Wednesday night
into Thanksgiving - more to come in the long term section.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
* Winter Weather may impact travel on Thanksgiving Day

The latest guidance cycle presents similar themes as before. These
include good agreement on the large scale pattern evolution toward
an amplified eastern trough. However, the guidance continues to
have difficulty in resolving important details/storm track, etc.
for low pressure affecting the region during the busy holiday
travel period spanning the latter half of the week.

Needless to say, the combined differences which have increased
over the last 2-3 cycles results in lower than average
predictability for surface low evolution. Including the new 24/12Z
run, the ECMWF has been on the northern and then western side
of the envelope (and fairly strong) for surface development due
to its stronger leading shortwave. The CMC and GFS are closer
together and generally lean away from the most recent EC run.
WPC noted that most machine learning models show a more
suppressed low track aligned closer to the operational GFS/CMC
with development occurring much farther offshore and leading to
reduced impacts (drier on balance) over cetnral PA. We had to
make significant manual adjustments to POP grids to account for
a solution closer to the ECMWF as 24/13Z NBM guidance incorporated
more than a desired amount of the drier scenario from the GFS/GEFS.
The key takeaway is that there is still high degree of uncertainty
in a potential impactful storm system for Thanksgiving.

There is much greater confidence in blustery and cold weather
conditions (stretch of below average temperatures) developing
behind the late week system with lake effect snow over the NW
Mtns remaining very likely Friday into the weekend.

I did up POPS some across the west and north Friday into Sunday,
given the tendency for a large scale pattern change to a colder
northwest flow pattern, as a western states trend toward a large
scale upper level ridge. Lower Great Lakes are quite warm yet,
compared to average. This will trend to support snow events, but
at the same time, lakes can result in the winds being more
southwest vs. northwest. Thus the location can shift around.
Details in the upslope, lake effect, shortwave wave snow events
will be sensitive to the flow backing more to the west,
southwest at times. Best chance for that will be on Sunday, and
perhaps again by midweek.

&&

.AVIATION /01Z MONDAY THROUGH FRIDAY/...
Northwest flow will persist overnight with some, but only slow
improvement to cig situ. KMDT and KLNS are VFR and should
remain that way into Monday. Remaining isolated light rain
showers, confined to northwest corner of the airspace will
continue to diminish in coverage

Late Monday afternoon, we will see increasing mid and high
clouds, before rain moves into the area ahead of a cold front
after 00Z Tue.

Outlook...

Tue...Strong CFROPA Monday night with widespread rain and
potential LLWS followed by strong winds and snow showers over
the western half of the airspace.

Wed...Not as windy with pcpn ending and MVFR to VFR cigs; mixed
precip could reach the southwest airspace by 12Z Thu.

Thu...Increasing likelihood of widespread IFR in rain across
the southern half of PA. Snow possible across the northwest.

Fri...Becoming windy. Snow showers across the north and west.

&&

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

&&

$$

SYNOPSIS...Steinbugl
NEAR TERM...Fitzgerald
SHORT TERM...Fitzgerald/Steinbugl
LONG TERM...Steinbugl/Martin
AVIATION...Martin/Gartner