York, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

821
FXUS61 KCTP 180429
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1129 PM EST Fri Jan 17 2025

.SYNOPSIS...
High pressure will drift east overnight. A cold front will
push through the state late Saturday, preceded and accompanied
by a period of mainly snow (north) and rain and wet snow
(south).

A deep upper level trough will pivot eastward into Pennsylvania
Sunday, bringing another chance for snow across at least the
eastern half of PA, followed by dangerous cold for several days
next week.

&&

.NEAR TERM /UNTIL 8 AM SATURDAY MORNING/...
Thin high clouds are going to thicken up through the night.
Timing on latest 00Z NAM is solid with the arrival of precip in
the forecast already, and similar to the GFS. Thus, no tweaks
are necessary there. Expect just a few snowflakes west of Rte
219 before daybreak.

&&

.SHORT TERM /8 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Cloud bases will lower quickly from the west Sat morning with a
period of light snow, then light rain spreading across the
south and periods of mainly snow across the region north of
I-80.

The progressive, southern and northern stream shortwaves ahead
of the arctic front will weakly phase as they cross the Mid
Atlantic Region on Saturday, which will keep precip rates and
snowfall generally quite light. Liq Equiv of 0.15-0.25 of an
inch NW and just a few to several hundredths across the SE
zones.

This will equate to 1-2 inches of snow accum across the Northern
Mtns (esp near and to the west of KBFD) and less than one inch
for the most part along the I-80 corridor.

Mixed precip across the south will prevent anything more than a
coating of snow in the morning.

Precip pattern becomes light, mixed, showery and fragmented
late Sat and Sat night as best best synoptic forcing and low-
mid level warm advection scoots off to the NE leaving wet bulb
temps hovering around zero C through early Sat night, before
temps begin to make a steady and steeper drop later Sat night.

Max temps Saturday will range from the low 30s near the PA/NY
border to the low-mid 40s in the Scent Valleys. Lows at daybreak
Sunday will vary from the low to mid teens in the NW to the mid
and upper 20s in the SE.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Sunday: Confidence continues to increasing in a period of accumulating
snow across the south and east Sunday as a wave of low pressure
develops along the thermal boundary along the North Carolina
coastline. There is high confidence in timing (start around
daybreak Sunday and over shortly after sunset Sunday night).
There is much lower confidence in where and how much snow will
occur in the heaviest band. Given how broad the trough is,
there is above normal uncertainty in the exact placement of the
axis of heaviest snow. Have adopted a middle of the pack
solution for this forecast cycle with the heaviest snow falling
along and southeast of the I-81 corridor with a swath of 6"+
expected at this time.

The reasonable worst case scenario (in line with the GFS/GEFS
and Canadian) shows a deepening surface low tracking close to
the coast and generating an efficient band of frontogenesis
along the I-95 corridor with heavy banding just to the northwest
of there. QPF values approaching 0.75" with ratios of 13-15:1
would equate to a narrow stripe of totals exceeding 8". A
reasonable best case scenario (in line with the ECENS and NAM)
would be a less amplified low that crosses a bit farther from
the coast. Such a scenario would result in lower end totals
(perhaps no more than 3" in any portion of Central PA).

A consensus of guidance at this point leads to a conclusion that
the most likely placement of heavy snow will be in the Lower
Susquehanna Valley, which prompted issuance of a Winter Storm
Watch from 7AM to 9PM Sunday. As the system comes into focus
over the next 12 to 24 hours, it is nearly a guarantee that
there will be modifications - and potentially significant ones -
to the forecast. Frontogenesis banding is quite difficult to
forecast at these lead times and it is likely that there will be
a very sharp gradient in snowfall totals with northward extent.
Our current forecast brings the 4" contour up to the I-99/I-80
corridor with lesser amounts farther north, but would expect the
gradient of totals to tighten up in the coming forecast cycles.


Sunday Night into late week.

High confidence continues in an impressive stretch of frigid
air. A reinforcing shot of bitterly cold air arrives Sunday
night into Monday. Temperatures will bottom out Tuesday, with
highs running 20 to 30 degree below average. Considering that
late January is the climatological coldest time of the year,
highs that far below average means that we are in for a
significant bout of arctic air. Tuesday and Wednesday morning
will be the coldest of the stretch with wind chills as low as
-15F to -25F across the Laurels and northern mountains. Have
 issued an Extreme Cold Watch from 4AM Monday through 12PM
 Wednesday for locations north and west of the Allegheny Front.

To put the upcoming cold in historical perspective, if the
forecast holds, about 80 hours with temperatures in the mid
teens or colder at Harrisburg and Williamsport rivals the cold
snap of Feb 1996 in terms of duration. Several years in the
1970s featured long duration cold as well.

The upper-level trough will begin to lift out of the area by
late next week, which should result in slowly moderating
temperatures.

&&

.AVIATION /04Z SATURDAY THROUGH WEDNESDAY/...
VFR conditions will prevail overnight (through at least 12Z) at
all airfields. Some LLWS is possible into Saturday morning,
mainly across our western Mtns airfields (JST to BFD) as a
result of increasing southwest winds aloft, combined with
relatively light surface winds. LLWS is also favored during the
first half of Saturday at MDT and LNS as the aforementioned
southwest winds aloft shift eastward.

Clouds will lower and thicken during the day on Saturday from
west to east. MVFR ceilings will persist at first with a
subsequent drop to IFR expected. Periods of IFR visibility are
likely in snowfall, which is most likely from the BFD vicinity
east across the Northern Mtns of PA. Precipitation type and
duration is a bit more uncertain at the rest of the airfields,
thus impacting the potential for IFR visibility. A couple hours of
snow followed by a transition to rain is favored at JST, AOO,
UNV and IPT with primarily rain at MDT, and LNS.

Outlook...

Sat...Rain/snow mix and evening fog possible in Southeast PA.

Sun...AM low cigs W Mtns and Southeast PA. Snow/IFR conditions
increasingly likely for at least the southern and southeastern
1/2 to 1/3 of the area Sunday PM.

Mon...MVFR to IFR in snow showers across western PA; gusty west
winds.

Tue-Wed...No sig wx expected; very cold.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Extreme Cold Watch from late Sunday night through Wednesday
morning for PAZ004>006-010>012-017-018-024-033-037-041-042.
Winter Storm Watch from Sunday morning through Sunday evening
for PAZ036-056>059-063>066.

&&

$$

SYNOPSIS...Lambert/Dangelo
NEAR TERM...Lambert/Dangelo
SHORT TERM...Lambert/Dangelo
LONG TERM...Banghoff
AVIATION...Lambert/Banghoff