FOUS11 KWBC 030807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Mon Feb 3 2025

Valid 12Z Mon Feb 03 2025 - 12Z Thu Feb 06 2025

...Western U.S. across the Northern Rockies... Days 1-3...

...Atmospheric River (AR) and upper low maintain prolonged heavy
snow from Oregon/California border and Sierra Nevada to the
Northern Rockies through Wednesday...

Current WV satellite imagery pinpoints our two distinct features
that have been controlling the ambient weather pattern across the
Western half of the CONUS. The first is an analyzed ULL centered
over Vancouver island that has provided significantly lower heights
across the PAC Northwest. The steady onshore component of the mean
flow has provided waves of small mid-level perturbations to meander
onshore with light precip in-of the Cascades of WA/OR. The main
player in the ULL positioning is the aid it has provided for our
second "player" within atmospheric schema; a progression of
shortwave troughs moving ashore within a west to east aligned
stationary boundary bisecting NorCal through the Northern Sierra,
extending inland to as far east as the Absaroka and Wind River
Ranges in western WY. Each shortwave pulse has aided in increased
IVT advection with a highly anomalous (+5 to +8 deviations via
NAEFS) transport of PAC moisture into the confines of the above
locations and everywhere in-between (Northern NV thru ID). This
direct moisture source has provided a steady ground for moderate to
heavy snowfall within the higher terrain confined over the above
locations with the upslope component within the Shasta/Siskiyou
areas aiding in heavy snow totals >12" above 6500ft.

This pattern will maintain general continuity given the prolonged
structure of the AR wave train with some adjustment in the aligned
flow likely to occur later this afternoon. A more amplified
shortwave trough coupled with a southwestern retrograde of the ULL
off the BC coast will provide a slightly more meridional component
to the flow with a greater southwest-northeast alignment shifting heavier
precip further south compared to the past 24-36 hrs. This will
generate a better signal for heavy snow within the Northern Sierra
with areas along I-80 getting into the greater potential for
significant snowfall Monday night and beyond. The pattern evolution
will manifest itself for a period of 24-36 hrs before the primary
shortwave trough currently centered over the Pacific finally shunts
eastward with a landfall timing centered around Tuesday night,
kicking out of the area later Wednesday morning. This will lead to
significant snowfall across the Northern and Central Sierra on
Tuesday through that Wednesday morning time frame totals >12"
highly likely within the latest snowfall probability fields for D1
and D2 (50-80%).

Further inland, the pattern progression will provide some changes
in the overall time frames of impact as the realignment of the mean
flow will allow for the heavier snowfall encompassing portions of
ID through western WY to scale back in intensity with more
lingering light to moderate snowfall anticipated late Monday night
through much of Tuesday. Current snowfall totals across the
Absaroka and Wind River Ranges have been pretty significant since
the beginning of the pattern evolution with the inland extension
of the AR so any additional accumulation will still provide
impacts, but will at least curb the heaviest precip threat to
points further northwest. Conditions will deteriorate over Central
ID into the Northwest Rockies in MT thanks to the flow alignment
and shortwave perturbations advecting overhead. Probs for >8" over
those areas will rise to 70-90% during the D2 time frame, a
considerable adjustment compared to the previous periods.

The aforementioned shortwave over the Pacific will move inland of
CA with sights downstream over the Northern Rockies during the D3
time frame leading to a secondary pulse of heavier snowfall back
across the interior with an emphasis on the Absaroka and Wind River
Ranges once again. Associated upper-level speed max with the
disturbance will nose inland with the Northern Rockies situated
within the LER of a 125kt jet streak providing ample support for a
better defined QPF maxima that will situate itself over the above
areas. High probs are forecast for >8" (40-70%) on D3 likely
bringing totals over the course of 5-days to 3 to 6' thanks to the
onslaught of heavier precip and broad upslope flow within the
interior ranges of the west.

...Great Lakes, Upper Midwest, & Northeast... Day 1...

Persistent zonal flow regime will continue through D1 with an
analyzed speed max over the Northern Plains this morning pushing
eastward within the west to east aligned flow. Nose of the 25H jet
will push through the Upper Midwest, eventually nosing into the
northern Lower Peninsula of MI with a swath of snowfall breaking
out just before the beginning of the period. As the jet core moves
overhead, increasing ascent under the influence of the jet will
generate a period of moderate to heavy snowfall as regional omega
improves significantly within the 850-600mb layer as progged by
recent bufr soundings in the zone of interest. A solid 3500' of
favorable DGZ depth provides a sufficient layer for better crystal
growth that will promote a better defined dendrite scheme during
peak snowfall later this morning and afternoon before the setup
wanes. The progressive nature of the event will limit the max
potential below double-digits, however west-east aligned banding
signatures within recent CAMs signal areas that could benefit from
prolonged longitudinal banding with the storm motion also focused
on a similar west to east alignment. Areas that can situate within
persistent banding will likely experience rates ~1"/hr for several
hours before the event fades as primary forcing shifts eastward
into Ontario and eventually the Northeast U.S. Snowfall probs for
>4" are forecast between 40-70% across the Door Peninsula of WI,
extending east through the northern half of the Lower MI peninsula
with a maxima focused downwind of Grand Traverse Bay, including the
town of Gaylord.

Further downstream over Upstate New York and New England, the same
jet presence upstream over the Great Lakes will shift focus into
the interior Northeast with increasing ascent generally aligned
over areas downwind of Lake Ontario within the Tug Hill and
Adirondacks, eventually encompassing Northern New England with the
greatest impacts in-of the Green and White Mountains of VT/NH. The
strongest signal for heavy snow is within the Adirondacks and
adjacent North Country with probs for >4" upwards of 70-90% within
those zones with 40-70% extending back towards the Tug Hill and
points west near Pulaski and north along I-81. Snow levels across
New England will be sufficient for snowfall above 1000', however
greater impacts will be mainly relegated to zones above 2000' with
a strong gradient in the snow probability fields for >4" with <20%
in those lower elevations with a solid 50-80% within the highest
elevations of the Green and White Mountains.

Light ice accumulations will be found across areas of Northern PA
and Western NY within a shallow moist layer present south of the
main axis of lift, especially in the post-frontal regime that will
transpire as the disturbance exits through Northern New England.
Totals will be solidly <0.1" with a majority of the ice accretion
likely relegated to a glaze to perhaps 0.01-0.05" at max.

Day 3...

Secondary disturbance(s) originating upstream across the Northern
and Central Rockies will eject eastward within a very progressive
upper flow evolution. Northern wave over WY/MT will progress east-
northeast across the Northern Plains with snowfall breaking out
downstream within a increasingly diffluent mid-level pattern.
Light to moderate snow will transpire over Dakotas with the
greatest impactful snowfall likely to occur over the Upper Midwest
in MN, especially as the disturbance moves into the Arrowhead by
the end of D3. Modest probabilities of >4" exist within northern MN
and the Arrowhead with a swath of 40-60% located over Duluth and
points northeast, as well as the northern most reaches of MN with
the highest probs long the Canadian border.

...Midwest to Mid-Atlantic... Day 3...

Second disturbance referenced in the section above will motion more
west to east across the Plains into the Ohio Valley and Mid
Atlantic by the second half of D3. High pressure over the Great
Lakes will shift eastward into the Northeast U.S by the beginning
of D3 with cold air funneling south into the Mid Atlantic with
guidance signaling a classic CAD wedge pattern initiating east of
the Appalachian front. Limited blocking downstream over the North
Atlantic will preclude any chance for the high pressure to hang on
across the region with the continued eastward progression of the
surface ridge eventually shifting off the Atlantic seaboard.
Despite its positioning, time frame of interest for precipitation
across the Ohio Valley and Northeast will be occurring after peak
diurnal heating with wet bulb temperatures into the upper 20s and
low 30s by the time precipitation breaks out across areas of IN/OH
over into the Central Mid-Atlantic. Primary concern with the
pattern is the low to mid-level WAA regime as the surface
reflection encroaches from the west leading to a mixed ptype likely
correlating to more icy (IP/ZR) hydrometeors compared to plain
snow that we will see for points well to the north. Recent trends
within deterministic have been for colder wedge to hang on more
thoroughly for places east of the Apps with the highest ice probs
located in- of the Laurel Highlands due to elevation factors
leaning colder at precip onset, as well as a secondary ice maxima
over the northern Ohio Valley where cold air will be able to hang
on through much of the event given stronger WAA pattern displaced
further downstream into the Mid-Atlantic comparatively.

Ice probs of >0.1" are very high (80+%) across west-central PA
with the focus in-of the Laurel Highlands where even some modest
20-40% probs for >0.25" of ice accretion are forecast as of the
latest WWD progs. Northern Ohio Valley doesn't carry the
probabilistic maxima for >0.25" like the mountains to the east,
however >0.1" ice accretion is sufficiently within the 40-70%
range, very much a non-trivial depiction that will have to be
monitored closely. Lower probs exist within the Central Mid
Atlantic east of the Blue Ridge, however a similar prob of 40-70%
for >0.1" of ice exists for locations northwest of the fall line
across MD extending into southern PA with lower, non-zero probs
further east into the I-95 corridor and adjacent lowlands. Setup is
delicate with multiple variables involved including depth of cold
air, strength of LLJ providing WAA pattern, as well as timing of
precip onset. These factors will determine the magnitude of ice
potential in-of the Mid-Atlantic and Ohio Valley with guidance
starting to hone in on specifics as we move closer to the event.

Kleebauer
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