FOUS11 KWBC 050922
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EST Wed Feb 5 2025

Valid 12Z Wed Feb 05 2025 - 12Z Sat Feb 08 2025

...Pacific Coast through the Northern Rockies and Northern Plains... Days 1-3

...Particularly active winter period across the west ends with a
potent low that tracks across California Thursday night, the
northern Rockies Friday, and the northern Plains Friday night...

Cold core low off the WA coast this morning shears into a
positively tilted trough through today from the zonal 150kt jet
streak to its south. Reinforcing shortwave energy takes the base
of the trough and pushes it through northern CA Thursday night as
another zonal jet streak intensifies sending the trough into the
northern Plains Friday night (before tracking through New England Sunday).

Day 1: The low heights and SEly low level flow provide moderate
snow to the Cascades and PacNW Coastal Ranges through this
evening. Terrain enhancements allow 1"/hr rates there and snow
levels will be at or near sea level. Day 1 snow probs for >12" are
40-70% above about 2000ft in the OR Cascades and Klamath Mtns.
While Seattle and Portland have been generally spared impactful
snow through this cold phase so far, there are now 30-50% Day 1
snow probs for >2" in the Lowlands. Heavy snow continues this
morning over the Northern Rockies where probs for >8" additional
after 12Z are 40-70%, particularly in the Absarokas and Wind River.

Day 2: An atmospheric river with PW of 1" reaches the central CA
coast on Thursday south of the wave that consolidates and ejects
east. Another swath of heavy precipitation crosses CA with snow
levels over the Sierra Nevada rising to 6500ft by Thursday evening.
Day 2 snow probs for >18" are 50-80% for the Sierra Nevada and the Shasta/Siskiyou.

Another surface low track is then traceable through northern NV
early Friday which means more moderate to locally heavy snow for
interior southern Oregon and the Sawtooth/Salmon River and the
western WY ranges Thursday night through Friday where Day 2.5 snow
probs for >8" are 30-80%.

Day 3: The still positively-tilted trough shifts over the northern
High Plains Friday night with strong lee-side cyclogenesis tracking
over southern KS. Gulf moisture streams north up the Great Plains
ahead of this low and, combined with increasing upper divergence
including kissing jets over SD Friday evening, allows snow bands
to flourish over the Dakotas Friday night. Despite the progression
from the intensifying zonal jet to the south of trough, sufficient
cyclonic flow allows west-east bands to move with their
orientation and maximize snowfall over a somewhat narrow swath. The
GFS remains north of the Canadian with this axis depiction and the
EC is a bit more diffuse with the intensity. However, Day 3 probs
for 20-50% for >6" over northern SD are reasonable at this time.

...Upper Midwest/Great Lakes... Days 1-2...

Lead shortwave downstream of the closed upper circulation off the
Pacific Northwest will eject downstream through the Northern Plains
later this afternoon with increased mid-level ascent focused
within the difluent axis ahead of the mean trough. A progressive
forward motion of the disturbance will limit the upper bounds of
the potential snowfall across the Upper Midwest into the Great
Lakes, however sufficient forcing and modest omega within a DGZ
depth of 50mb will generate snowfall rates capable of reaching
0.5-1"/hr during the height of the event. Snowfall will likely
break out after 18z with increased areal coverage as we move
through the evening today, ending the tail end of the D1 period.
Models have been insistent on the greatest potential over Northern
MN near the International border over into the Arrowhead, Bayfield
Peninsula of WI, and the U.P. of MI where synoptic snow persists
into Thursday morning. Latest WPC probabilities for >4" are running
between 40-70% with some small pockets of ~80% located over the MN
Arrowhead near the Canadian border and along the north shores of
Lake Superior. Lower probs exist for >6" of snowfall (10-40%)
within the same areas, but the progressive nature of the storm will
likely limit widespread heavier totals with more isolated pockets
of those higher totals in areas where banding persists the longest.
Very cold air with subterranean DGZ returns Thursday across Lake
Superior. LES continues into Friday for the U.P., but for now Day 2
snow probs for >4" are 30-60% for the Keweenaw Peninsula and the
eastern U.P.

...Midwest to Northeast including the Central Appalachians... Days 1-2...

Progressive shortwave trough over southern CO early this morning
will rapidly advance eastward across the Great Plains today and
reach the Mid-Atlantic late tonight. Smaller shortwave
perturbations will also advect over the aforementioned regions with
lighter precip development over the Mid-Atlantic today ahead of
the main slug of moisture associated with the primary shortwave
trough. The separation of the deeper layer moisture can be inferred
from the higher IVT signatures advecting eastward into the Mid-
Atlantic, a solid +3 deviations according to the latest NAEFS
anomalies with values approaching 600-700 kg/ms on the 00z
GFS/ECMWF output. Moisture advection regime will be prominent
within the 925- 650mb layer as noted within relevant bufr soundings
within the zones of expected impact. A strengthening v-vector wind
component across the Southern Ohio Valley and points east into the
Central Mid- Atlantic indicate a prolific meridional response to
the amplifying 5H pattern as the trough approaches. 850mb LLJ being
forecast within the latest deterministic maintain posture of a
fairly robust 50-60kt jet core, a significant contributor to not
only the advection of moisture, but also of a prominent WAA regime
that will allow for a general mixed ptype pattern as we move
through the event this afternoon through Thursday.

Across the Ohio Valley and Mid-Atlantic, antecedent airmass under
the influence of a 1034mb surface high over Quebec will lead to
sufficient cooling within the lower confines of the boundary layer
with drier air focused within the 925-700mb layer, a setup that will
be come relevant with regards to anticipated wet bulb temperatures
as moisture from the west and southwest approaches. There is a
general consensus on the timing of the advancement of the precip
core for each region with the afternoon and early evening time frame
the initial stages for precip onset across the Midwest and Ohio
Valley as the main axis of difluence downstream of the approaching
mid-level shortwave provides focused ascent in-of the above areas.
Wintry precip is expected to breakout in the form of ZR over IL/IN
before shifting eastward into OH with an IP/ZR mix transitioning to
all ZR before precip field decays the very end of the D1 period.
Further east into the highlands of Western PA and WV, increasing
warm air advection pattern will transpire ahead of the trough with a
tongue of warmer air flooding a significant portion of the primary
thermal layers leading to primary ptype beginning as a liquid
hydrometeor. The issue becomes the cold air positioned within the
lowest confines of the PBL as the surface ridge to the north allows
for a filtering of a very shallow polar airmass across much of the
Mid-Atlantic and Northeast before shifting eastward as blocking
remains diffuse across the Northwest Atlantic. Anticipate
temperatures in the Laurel Highlands down through Western MD and the
High Country of WV to settle into the upper 20s prior to initial
precip with wet bulb temperatures within the range of 24-27F, a
textbook signature for prolonged ZR within the terrain leading to
significant ice accretion >0.25".

Across he central Mid-Atlantic east of the Blue Ridge,
temperatures will exhibit similar evolution with cold air filtering
east of the Appalachian front with a fresh cold/dry air injection
into the PBL leading to colder temperatures leading with widespread
upper 20s to low 30s readings likely as precip encroaches from the
west. Despite the high shifting off the coast, limited mixing
within the lower PBL will thwart a rapid warming of the primary
thermal layer located from the surface to 850mb, however the layers
above will be warm quickly with a sharp nose of WAA being progged
within the 850-650mb layer leading to a mixed ptype after perhaps a
very short window of light snow prior to the main moisture plume
reaching east of the mountains. IP/ZR combo will be present across
the Central Mid- Atlantic within the Piedmont with the majority of
precip shifting to all ZR prior to sunrise for all locations. The
question becomes the depth of the cold air and the process of cold
air damming within the confines of the northern Shenandoah to
places west of the Chesapeake over VA/MD/PA. The trends are for a
sharp delineation of prolonged freezing rain and rain along the
fall line northwest of I- 95 with greater potential for ice
accretion further northwest away from that marker. This will allow
for a sharp gradient in the ice accumulation forecast with <0.1"
likely east of the fall line, including the DC/Balt/Philadelphia
metros with >0.1" likely northwest with upwards of 0.2 plausible in
the topographically favored areas of the northern neck of VA, WV
Panhandle, Blue Ridge, Catoctins, and Parrs Ridge across NoVA up
through Central MD and far Southern PA.

Farther north and northeast across the Northern Mid-Atlantic,
initial burst of snow is anticipated before shifting to more IP and
ZR with IP being a more dominant ptype south of I-80 with snow
more common north and northeast of the interstate leading to WWD
probabilities for >4" to run between 40-60% for the higher
elevations of Upstate NY and Northern New England that include
areas like the Tug Hill, Adirondacks, Green and White Mountains of
VT/NH. Light ice accumulations are anticipated for a large area
encompassing much of the Northern Ohio Valley through the Mid-
Atlantic and Northeast. WPC probs for >0.1" of ice accretion is
between 40-70% across Northern Ohio Valley and the Piedmont east of
the Blue Ridge to the fall line northwest of I-95. Areas west of
the Blue Ridge to the Laurel Highlands of PA and Highlands of MD/WV
are upwards of 80-90% for similar totals with probs greater than
40% for >0.25" leading to Ice Storm Warning issuance this past
forecast cycle. Latest WSSI continues to predict a large area of at
least moderate impacts within those zones expecting greater than
0.2" of ice accretion due to impending disruptions in travel with
impacts that could include closures and delays that impact daily
life. Despite lighter accums further east into the Megalopolis,
light ice accumulations will still provide some hazardous travel
conditions for many during the Thursday AM commute. These impacts
will span through the Shenandoah Valley up through much of Central
and Eastern PA, highlighted by widespread Minor WSSI depictions
all across the affected areas.

Given the relatively large geographic coverage of at least 0.10" ice
potential and the increasing chances for over 0.25" of ice accretion
in parts of WV, MD, and PA, Key Messages continue for this event
which are linked below.

Jackson/Kleebauer

...Winter Storm Key Messages are in effect. Please see current
Key Messages in the link below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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