Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 PM EST Tue Feb 27 2024

Valid 00Z Wed Feb 28 2024 - 00Z Sat Mar 02 2024

...Great Lakes & Northern/Central Appalachians...
Days 1-2...

A tightly clustered group of vorticity maxima embedded within a
deep, yet progressive, upper level trough will provide ample
positive vorticity advection over the Great Lakes tonight and into
Wednesday morning. At 250mb, a robust 175kt jet streak over the
Central Plains will place its divergent left-exit region over the
Great Lakes as well, thus providing no shortage of strong vertical
ascent atop the atmosphere. At lower levels, an exceptional 850mb
cold front will be quite intense as frigid Canadian air to the
north runs into an exceptionally warm airmass in the Nation's
Heartland. This will lead to a rapid changeover from rain to snow
as 700mb moisture flux overrunning the cold front leads to
anafrontal precipitation in wake of the cold frontal passage.
Heavy snow will ensue on the northwest flank of the 850mb low
tracking over Lake Michigan this evening with northern Wisconsin,
the U.P. of Michigan, and the northern-most portion of Michigan's
Mitten seeing the heaviest snowfall tonight and into early
Wednesday morning. While snow initially will be a byproduct of the
strong synoptic-scale forcing and anafrontal snowfall, snowfall
rates will increase on the mesoscale via strong 925-700mb CAA will
cause lake effect snow streamers to envelope the eastern half of
the Michigan U.P. and northern Michigan throughout the morning and
into the afternoon hours. Given the upper trough is quite
progressive, the northwesterly flow responsible for the lake
effect enhanced snow showers should taper off by Wednesday night.
WPC PWPF shows the northern half of the Michigan U.P. showing high
chances (>70%) for >4" of snow for this event, while chances are
more moderate (40-60%) in northwest Michigan. The Probabilistic
WSSI sports Moderate chances (40-60%) for Minor impacts in these
areas due to Snow Amount, but given the whipping winds on the
backside of the storm on Wednesday, Minor Impacts due to Blowing
Snow that result in reduced visibilities are also possible along
the Wisconsin coast and the tip of Michigan's thumb.

The aforementioned strong 925-700mb CAA will be responsible for
high elevation snowfall in the central and northern Appalachians
late afternoon today and into tonight. Cyclonic flow at 850mb with
40-50 kt winds directing a brief surge of 850mb moisture flux into
the central Appalachians is ideal for upslope enhanced snowfall in
the central Appalachians of eastern West Virginia and western
Maryland this evening. While most totals will be minor, WPC PWPF
shows low chances (10-20%) for snowfall totals >4" through
Thursday AM. The northern Appalachians have the greater chances
for heavier snowfall given their more extended period feeling the
effects of prolonged cyclonic flow supporting both upslope flow
into the the Tug Hill, the Adirondacks, Green, and White
Mountains. There is also likely to be single banded lake effect
streamers Wednesday night into Thursday that produce very narrow
and isolated swaths of heavy snow over northwest Pennsylvania and
northern New York. Lake effect snow will stick around up through
midday Thursday when the pressure gradient lessens and high
pressure builds in from the southwest. WPC PWPF shows moderate
chances (40-60%) for >4" of snowfall in the Tug Hill and portions
of the Adirondacks. It is worth noting that the Tug Hill sports
low chances (10-20%) for >8" through Thursday morning, which
speaks to the lingering potential for locally heavy snowfall
amounts in areas with both elevated terrain and beneath the single
banded lake effect bands. Meanwhile, areas such as northwest
Pennsylvania, the Southern Tier of New York, the Green Mountains
of northern Vermont, and both the White Mountains and Great North
Woods of New Hampshire sport low chances (10-30%) for >4" of snow.
Minor impacts as a result of these snowfall amounts late Wednesday
into Thursday morning are possible.

...West Coast & Northern Rockies...
Days 1-3...

**Significant multi-day winter storm to produce heavy snowfall
from the Cascades and Sierra Nevada to the Northern Rockies
through late week**

The big picture setup driving what will be a long-duration and
significant winter storm is an upper trough over the Gulf of
Alaska that will deepen and plunge south off the western coast of
Canada through Wednesday morning. The upper trough will continue
to dig south through Wednesday night becoming positioned west of
the Pacific Northwest coast initially, then west of northern
California by Thursday night. NAEFS is forecasting 200mb heights
starting late Wednesday and lasting through Friday that are
outside the CFSR climatology while 500mb heights are commonly <1st
climatological percentile. Meanwhile, the mean 250-500mb trough
will pump copious amounts of 850-500mb moisture into the Pacific
Northwest and Northern Rockies throughout the remainder of the
work-week. As the 250mb trough steadily pushes south, the
divergent left-exit region will make its way south as well,
fostering plenty of vertical ascent aloft. Simultaneously, mean
850-300mb southwesterly flow supports topographically-enhanced
snowfall rates along the Cascades, through the northern California
and southwest Oregon mountains, and into the Sierra Nevada. In
terms of time line, snow will pick up at elevations >3,000ft in
the Olympics and Cascades tonight and rise to as high as 4,000ft
throughout the day on Wednesday as steady 700mb WAA transpires. As
a cold front approaches Wednesday afternoon and 850mb winds out
ahead of the front strengthen, heavier snowfall rates will unfold
along the Washington Cascades and down into the Oregon Cascades by
Thursday. The plume of Pacific moisture will extend well inland
across the northern Rockies late Wednesday

By Thursday, as the cold front continues to dive south into
northern California, snow levels will plummet in its wake to as
low as 2,000ft but the heaviest snowfall will be confined from the
Mendocino, Trinity, and Shasta Nat'l Forests on east to the
northern Sierra Nevada. As the front advances south along the
spine of the Sierra Nevada, a steady diet of Pacific moisture will
be funneled favorably into the mountain range where both strong
synoptically-forced ascent and topographically-induced flow
orthogonally to the mountains will be maximized late Thursday into
Friday. NAEFS shows 500-700mb winds speeds Thursday afternoon and
through the day on Friday from northern California to the northern
Rockies and Great Basin that are above the 97.5 climatological
percentile, providing more than enough wind speed to support both
heavy snow and blowing snow. To add to this highly impactful
setup, a lobe of 500mb vorticity will pivot beneath the longwave
trough and approach northern California by Friday afternoon. This
added lift from strong 500mb PVA and a secondary cold front will
only further enhance the snowfall rates in the Sierra Nevada,
which is expected to stick around into the first half of the
upcoming weekend. The same stream of Pacific moisture, ample 500mb
PVA, and upslope enhancement will occur as far inland as the Bear
River Range of northern Utah, and both the Tetons and Absaroka of
western Wyoming and southern Montana.

The potential impacts of this event, particularly in the Sierra
Nevada, are leaning towards considerable to even extreme in many
cases. The WSSI-P between 00Z Fri - 00Z Sat shows moderate chances
(40-60%) for Extreme Impacts in the Sierra Nevada bordering I-80.
When rolling the 24-hr WSSI-P forward through Saturday morning,
Extreme Impact chances rise to an astonishingly high 60-80%. This
kind of signal out to late Day 3 - Day 4 is rare and indicative of
the kind of exceptional event that looks to unfold in the Sierra
Nevada. Snowfall will be measured in feet (>5 feet for storm
totals through Saturday) with hourly snowfall rates >3"/hr likely
Friday night into Saturday along the Sierra Nevada >5,000ft. In
addition, 700mb winds of 40-50 kts will mix down and be capable of
producing blizzard conditions along the Sierra Nevada Friday night
into Saturday. Areas that experience Extreme Impacts would witness
substantial disruptions to daily life which include extremely
dangerous or impossible travel, extensive and widespread closures
and disruptions, and possibly life-saving actions may be needed.
This event is likely to produce Major Impacts in the Olympics,
Washington and Oregon Cascades, the Siskiyou and Salmon Mountains,
and along the higher elevations of the southwest Oregon and
northern California Coastal Range. These areas sport high chances
(>70%) for snowfall totals >24" for the duration of the event with
some snowfall totals topping 5 feet in the tallest peaks of the
Cascades and Olympics. Snow will continue to fall in the Northern
Rockies into Saturday but through 00Z Saturday, WPC PWPF shows
high probabilities (>70%) for >18" of snow in parts of the
Bitterroots, Lewis Range, Boise, Sawtooth, and Tetons. The WSSI-P
sports more moderate probabilities (40-60%) for Moderate Impacts
in these ranges (minus the Lewis Range) starting late Wednesday
and lasting through Friday.

**Key Messages for the Major Western Winter Storm

--Winter Storm to Affect West Coast
A significant winter storm will affect the Northwest primarily
Wednesday and Thursday, before significantly impacting the
mountains of northern and central California Friday and Saturday.

--Heavy Mountain Snow Affecting Many Passes
Multiple feet of snow are likely (over 80% chance) for higher
elevations, especially above 5000 ft, including many of the
Cascade and Sierra passes. Extremely heavy snow rates surpassing 3
inches per hour are possible in these mountain ranges.

--Blizzard Conditions Expected in Sierra Nevada
Strong winds will cause significant blowing and drifting snow and
whiteout conditions, with travel becoming impossible in the Sierra
Nevada. There is a high chance (over 70%) of substantial,
long-lasting disruptions to daily life in the higher elevations of
the Sierra Nevada Friday through Saturday, where blizzard
conditions and over 5 feet of snow are expected.

--Cold Air Will Lower Snow Levels This Weekend
As the storm moves southward, snow levels will lower into some
northern CA and Sierra foothill communities. Much colder air is
forecast by the weekend with temperatures 10-20ĀF below normal.


Mullinax