Storm Impacting the Northwest U.S.; Fire Weather Conditions in Southern California; Severe Weather in the South

A Pacific storm is bringing areas of low elevation rain, moderate to heavy mountain snow, and high winds to the Northwest. Strong Santa Ana winds and very dry conditions are producing elevated to critical fire weather conditions in southern California. Isolated strong to severe thunderstorms are possible through early Wednesday morning across parts of northeast Texas into western Tennessee. Read More >

Here is the latest Area Forecast Discussion for Central PA

This text statement is the latest forecast reasoning from the NWS in State College, PA

See the links at the bottom of the page for previous issuances/versions of the statement as well as our other text statements.

153
FXUS61 KCTP 022035
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
335 PM EST Thu Jan 2 2025

.SYNOPSIS...
Upper level troughing will remain over Pennsylvania through the
next week. A weak shortwave will dive southeast across the
region Friday, then a stronger wave and associated surface low
will track just south of the state Sunday night into early
Monday. Both should generate snowfall, mainly across the
southern tier. The Sunday night-Monday system will carry much
more moisture, forcing and last longer, leading to more
snowfall.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Weak vort max currently enhancing a very light, but widespread,
snow for the NW half of the CWA. Still some flurries getting
into the SE half, but no accums are expected there, and just a
dusting for the balance of the area.

As the winds at cloud level stay similar, it`s tough to see
much change in the intensity of the bands coming off the lakes.
Some waggle south is certainly possible, but it looks like the
accums should be <=1" for the warning and advy areas tonight.

A very compact low with little reflection at the sfc, more like
a trough, will move into the Upper OH Valley tonight and may
generate some snow by sunrise over the Laurels. But, in the end,
the sensible difference in the character/intensity of the snow
will not be noticeable there until just before sunrise. Will
continue the winter wx flags as they are for the time being.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Main conversation and consternation by the dayshift was
centered around the possible/expected precip and what impacts it
will have on the area on Friday.

Newest guidance from meso models and ensembles has brought some
clarity, but only for the expected precip/snowfall across the
southern third of the area Fri aftn. The late aftn and very
early evening hours (rush) are the most concerning of all. HRRR
and fine mesh NAM output make a burst of heavy precip, most
likely snow at the intensities they are predicting, for places
mainly S of the PA Turnpike. However, the temps will be marginal
at first in the valleys of the Lower Susq for accums to occur
during mid-late day. The interesting hours appear to be 3-6PM
when the low-mid level frontogenesis increase and some meager
CAPE bubbles up. The SNSQ parameter is low during this time
frame. It may be more of a CSI-banded feature that occurs as
the strongest forcing moves thru - the sfc pres drops ~9mb as it
crosses far srn PA on Fri.

Whether it be SQ or CSI band, the precip appears as a relatively
thin band (40mi?) oriented N-S. This could spell a very brief
(30-45 min) period of heavier precip which would likely be snow
as the intensity peaks, even where the temps are initially above
freezing. This temp fluctuation/drop could spell more of a worry
for the roads to ice up as the melted snow freezes up with a
cooldown below freezing during the heaviest snow rates.

While accums may only be an inch or so, and generally sub-advy
(advy amounts are generally >=2" for the SErn cos), the
possibility of a quick inch and the temp drop may spell
commute/travel trouble for the metro areas, esp along and S of
Route 30 and east of I-81. At this point, we`ll hold off on an
advy due to the lack of 2"+ area-wide signals, and expected
brevity of the heaviest snow. This may be better handled by SQWs
than a broad advy. But, we`ll continue to weigh these factors
and may issue an advy in due time. The precip/snow should be
off to the east around 6-7 PM.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Falling inversion heights will translate to dwindling snow/snow
showers by Saturday. However, a cold WNW flow will keep light
lake effect snow going over the NW Mtns. Bukfit soundings
indicate wind gusts will increase to around 30kts again late
Friday night into Saturday, which could result in areas of
blowing/drifting over the Laurel Highlands and NW Mtns.

Upper level troughing will result in below normal temps
Saturday. 850mb temps are supportive of highs from only the
upper teens in the Laurels, to around 30F in the Susq Valley.

Heights will build over the area this weekend, resulting in a
lake effect and upslope snow gradually retreating as a
subsidence inversion cuts off the low level instability. PoPs
fall to less than 30% even across the north and west by Sunday
AM.

Attention then turns to a larger low pressure system moving
east out of the Four Corners region of the Western US, set to
impact parts of the Eastern US Sunday night into Monday. Latest
model consensus tracks the low across the Carolinas, certainly
putting central PA on the cold side of the storm, but the
question will be how far north moisture will make it. It is
worth noting that most long range models keep the wave pretty
flat as it approaches the coast, with little or no strengthening.

Ensemble guidance is relatively tightly clustered, given this
event is still out on Day 5. Cluster analysis reveals that the
most likely scenario, represented by 75% of ensemble members,
keeps the wave between the trough approaching from Mississippi
Valley, the ridge over New England, and the deeper trough over
the Canadian Maritimes (which has origins in Friday`s clipper
system) on the flatter side, yielding less moisture advection
and lower snow totals, with accums ranging from an inch or 2
along I-80 to 3-5 inches along the southern tier.

The worst case (snowiest) scenario is shown by about 25% of
ensemble members (mostly GFS/Canadian members with very little
representation from the European ensemble system). In the
snowier solution, the wave packet is more amplified, allowing
moisture to advect farther northward and jet forcing to be
stronger. This could force the heaviest snow axis to land within
our CWA. In this case, most places south of I-80 would pick up
5+ inches of snow. Remember, according to the ensemble guidance,
the lower- snow scenario is 3x as likely!

All things considered, PoPs (all snow) were raised Sunday night
into early Monday, with snow likely winding down through the
latter half of Monday. This storm is still completely outside
the range of deterministic QPF and snow forecasts.

Behind the departing storm system Tue-Thu, we remain locked in
a pattern with temps 5-10 deg below climo, and lake effect and
upslope snow possible in NW flow.

&&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
Atmos forcing is being temporarily enhanced over the entire
forecast area by a weak shortwave trough. This enhancement is
working with very dry air in most places. But, vsbys have
dipped to IFC in AOO and JST. We think this change to be
possible at UNV and IPT over the next 3hrs (60% chc). Will add a
TEMPO mention there for the time being, not expecting it to be
a long-lasting issue.

The broader pattern is still mainly for lake effect and upslope
snows. That means poor flying conditions for BFD and JST. Not
enough confidence to dip AOO or UNV into IFC during the
overnight. In fact, the conditions will be VFR for most of the
time with partial to full clearing possible in the middle of the
night. But, by morning, clouds move in from the west due to our
next weather-maker. A Clipper-type system zipping along from
IA/IL into the area for Friday. Mainly light snow over most of
the area will bring widespread MVFR to all the srn terminals,
and there is a strong (80%) possibility of LIFR snow for a very
brief period at AOO/MDT and LNS as the little system deepens
rapidly as it crosses PA. The effects of this compact system
will be low in UNV and IPT. JST and BFD will hardly miss a beat
due to the broader pattern.

Improvement occurs for UNV/AOO in mid aftn, and MDT and LNS by 7
PM. Expect then the normal IFR N/W, MVFR middle and VFR SE to
rule for the next two days.

Outlook...

Sat-Sun...Periods of -SHSN and restrictions across primarily
N/W PA. Otherwise, cold with gusty westerly winds areawide.

Mon...Widespread snow possible, especially south with impacts.

Tue...Lake effect NW, mainly MVFR elsewhere.

&&

.CLIMATE...
2024 was one of the warmest years on record for most of
Pennsylvania. Here is where 2024 ended up in terms of average
temperature at several climate sites in our forecast area:

SITE 2024 Avg Temp Rank Previous Record (Year)
Altoona 53.7F 2 54.1F (1998)
Bradford 50.2F 1 49.4F (2023)
Harrisburg (MDT) 56.7F T-1 56.7F (2021, 2023)
State College 53.0F 1 52.9F (2023)
Williamsport (IPT) 54.3F 1 54.0F (2023)

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Lake Effect Snow Warning until 1 PM EST Sunday for PAZ004.
Winter Weather Advisory until 1 PM EST Sunday for PAZ005.
Winter Weather Advisory from 5 AM Friday to 10 AM EST Saturday
for PAZ024-033.

&&

$$

SYNOPSIS...Dangelo
NEAR TERM...Dangelo
SHORT TERM...Dangelo
LONG TERM...Fitzgerald/Martin/Colbert
AVIATION...Dangelo
CLIMATE...Banghoff

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