327
FXUS61 KPBZ 081749
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
1249 PM EST Wed Jan 8 2025

.SYNOPSIS...
Scattered snow showers continue through the first half of
Thursday with some notable accumulations in the high terrain
and within snow bands in Venango and Forest Counties. Dry
weather returns late Thursday ahead of another system that may
bring snow Friday into Saturday. Temperatures remain below
average.

&&

.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:

- Continued snow showers with highest totals in the terrain and
  in localized banding north of Pittsburgh.
- Localized snow totals of to 3-6" for the high terrain and in bands.
- Moderate confidence in exact placement of snow band and
  totals in eastern Tucker County through tonight.
--------------------------------------------------------------------

Update focused on handling band of light to locally moderate
snow showers across the central CWA, in a band that likely has
its origins from WNW flow off of southern Lake Michigan. 12Z PBZ
sounding shows saturation through most of the dendritic growth
zone beneath an inversion at around 760mb. Many of the CAMs did
not handle this well, with the NAM3km nest being the one model
that has something close to reality.

As daytime heating commences and low-level lapse rates start to
steepen, expect activity to become more cellular in nature,
with upslope flow and lake streamers becoming more predominant.
Placement of the lake bands remains moderate confidence at best,
with Venango/Forest Counties still the most likely main resting
area. Eastern Ohio is still a possible location for another lake
band by late afternoon/evening; this potential will continue to
be monitored.

Previous discussion...

Light snow showers continue to plague the area in a stagnant pattern
in place over the past 24 hours characterized by shallow Great Lakes
moisture in adiabatic profiles off the surface beneath a subsidence
inversion. Said inversion has sunk just a bit on our 00z sounding
compared to yesterday`s 12z which has shrunk the DGZ slightly
as expected, and light snow showers and flurries prevail. With
little change to the thermodynamic profile and the destabilizing
effects of daytime heating steepening low-level lapse rates
with lingering moisture trapped beneath a subsidence inversion,
lake-effect streamers are likely into the day today. The DGZ
will remain a bit lower with colder temperatures which may lead
to higher areal coverage of snowfall and continued efficient
snowfall into the high terrain. Wind fields do not modulate much
with uniform northwesterly flow within the cloud-bearing layer,
so the snow will likely intensify in a banding nature off of
Huron/Erie extending down to Venango/Forest.

Predictability of exact location of the band(s) is moderate
confidence; the latest HREF has risen a signal for a band to
wiggle around Forest and into Venango. Increasing the
neighborhood probability (in essence allows for better capture
of spatial differences among the CAMs) for >0.15" of liquid,
which is a more appropriate representation of 3" of snow given
ratios more along the lines of 20:1 as opposed to its advertised
10:1, supports that the occurrence of said band producing 3"
amounts somewhere in these two counties is high confidence.
With that said, have opted to expand the current Winter Weather
Advisory into Venango, but amounts are likely to be highly
localized and some of the counties may see very little snow
accumulation at all.

Accumulation of at least 3" in the higher elevations of PA and WV
are likely to be a bit more widespread given continued orographic
enhancement and saturation of the DGZ. Light to occasionally
moderate snow will be the theme through a lot of the day today with
likely the best push of snow tonight as a mid-level wave drops south
over the area. With optimal snow growth being capitalized on,
accumulations are expected to be towards the higher end of guidance.
This could put storm totals close to the 3-6" range, highest in
the WV ridges. There, totals could be enhanced by a downstream
moisture fetch from wrap around Lake Michigan/Superior moisture
and the most orographic influence. Given the constructive
interference of the synoptic lift, orographic lift, dendritic
growth, and adiabatic saturate profiles, it is expected that
eastern Tucker preforms towards the higher end of guidance (an
additional 4" to 8"), justifying an upgrade to a Winter Storm
Warning with the peak storm rates from roughly 8pm to 4am
tonight.

Overall coverage outside of the bands and higher elevations will
likely be isolated to scattered through most of the daytime hours,
but as the aforementioned wave dives through, enhancement in
coverage of snow showers is likely and some may produce up to a
half inch of localized accumulation.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
KEY MESSAGES:

- Snow showers winding down on Thursday.
- Lows in the single digits Thursday night.
- Potential for return of snow Friday night.
--------------------------------------------------------------------

Rising heights with finally some dry air intrusion in the
low-levels as surface high pressure builds closer and flow
gradually backs off of the lakes will likely shut down lake-
enhancement on Thursday, though some orographically-forced snow
is favored to continue into the late afternoon hours with
sufficient alignment of the flow to the terrain. Some scattering
of the cloud deck becomes higher probability given the
disconnect of lake moisture, so this and lighter wind points
toward overnight lows tanking to the single digits for most. The
exception may be north and east of PIT where cloud cover could
linger longer, though if it does clear then these locales will
turn colder. This will be the coldest temperatures of the winter
thus far.

The axis of an upper ridge will shift east on Friday downstream of a
digging trough across the Plains. Surface high pressure will lose
its grasp, but is favored to keep at least the first half of the day
dry. The latest ensembles still hold with the trend of previous runs
in which a less likely, but still plausible, solution presents with
a faster approaching trough could introduce precipitation chances
into eastern Ohio by the afternoon hours should it come to fruition.
Either way, precip chances ramp up overnight Friday night with
approaching low pressure.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
KEY MESSAGES:

- Snow may continue into Saturday.
- Cold with a dry end to the weekend but snow chances increase
  again to begin next week.
--------------------------------------------------------------------

Ensemble clusters are still exhibiting notable deviation with the
evolution of the trough and its surface low reflection as they
progress across the eastern seaboard. Primary uncertainty arises
from timing but also trough amplitude to a lesser extent. Some
deterministic members have continued to advertise a blockbuster
nor`easter evolving from this feature, but it`s important to not
take these solutions at face value at least until they would have
more unified ensemble support which is still not the case. A
solution with a deeper, slower, more positively tilted trough that
phases with a northern stream wave favors a stronger low tracking up
the East Coast, and while this is noted in one potential ensemble
cluster driven in majority by the GEFS, several other outcomes
remain possible at this point as well. Deterministic and clustered
guidance has trended away from this solution over the last 24 hours.

Another camp with a flatter and quicker trough points toward weaker
and more progressive surface low development bringing much less
impactful weather. The cluster with the most equal distribution of
ensemble members indicates a mix between these two with a slightly
flatter, negatively tilted trough resulting in low pressure tracking
off to our south and offshore of the Mid- Atlantic before
strengthening notably. This would bring the chance for some snow
accumulation locally, and in fact all of the present clusters do
signal at least part of our area seeing measurable snow, but
widespread significant accumulation is low probability. The higher
amounts would be favored south of Pittsburgh, especially in the WV
ridges, based on this low track, but fluctuations are likely over
the next couple days.

Ridging, albeit short lived, is favored to build back in behind the
departing trough returning dry weather for the latter half of the
weekend with below normal temperatures. Another upper low looks to
take shape well off to our north for the start of next week with a
more zonal progression to the flow establishing.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...

Northwest flow/lake-effect snow showers will persist through
tonight, with the greatest concentrations north of I-80 and
along the ridges. Intensity has ticked down a bit from earlier
this morning, when banded activity briefly caused IFR to LIFR
visibility. MVFR or low-end VFR ceilings will persist through
the period for the most part. From here, visibilities in snow
showers will predominantly be MVFR, with occasional drops to IFR
possible in the heavier cells. These drops will be most likely
at FKL/DUJ, where impact from lake enhancement will be the
highest. Brief drops to IFR are possible from upslope influences
at LBE and MGW as well into tonight, as a shortwave trough
digging across the region concentrates more snow showers to the
near and south of PIT. Narrow lake-effect streamers remain
possible this afternoon and evening that could create more
impactful conditions locally, but any impact on one terminal
should be brief.

By sunrise, as 500mb heights begin to rise, snow shower activity
will begin to become more confined to the ridges in lingering
upslope flow, with increasing probability of improvement to VFR
conditions to the west. Increased subsidence/lowering inversion
heights will help to reduce snow showers to isolated coverage
by the end of the TAF period.

Outlook...
The Upper OH Valley region will remain in a cold NW flow
pattern, under broad upper troughing through much of the week.
Scattered snow showers and periodic restrictions are expected.
Improvement is expected later on Thursday as high pressure
builds in, but another round of restrictions from snow is
possible starting on Friday.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until noon EST Thursday for PAZ008-009-
     074-076-078.
OH...None.
WV...Winter Weather Advisory until noon EST Thursday for WVZ510>513.
     Winter Storm Warning until noon EST Thursday for WVZ514.

&&

$$

SYNOPSIS...MLB/Milcarek
NEAR TERM...MLB/Milcarek
SHORT TERM...MLB/Milcarek
LONG TERM...MLB/Milcarek
AVIATION...CL