Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
220 PM EST Wed Feb 28 2024

Valid 00Z Thu Feb 29 2024 - 00Z Sun Mar 03 2024

...Great Lakes...
Day 1...

Cyclonic flow at 850mb packing 30-40kt winds will direct lake
effect snow bands over the eastern Michigan U.P. and across
portions of the interior Northeast tonight and into Thursday
morning. The latest HREF depicts low chances (10-30%) for 1"/hr
snowfall rates over the eastern Michigan U.P., while there are
low-to-moderate chances (30-50%) for a single lake effect snow
band oriented WNW-ENE to the southwest of Oswego, NY to produce
1"/hr snowfall rates Thursday morning. Sampled soundings from CAMs
within this single lake effect snow band sport impressive vertical
velocities within the dendritic growth zone and sufficient
instability to potential lead to thundersnow early Thursday
morning. WPC PWPF shows moderate-to-high chances (40-70%) for >8"
of snowfall just south of the Tug Hill for the single lake effect
band over central New York. Moderate probabilities (40-60%) for
>4" of snow are depicted over parts of the eastern Michigan.
Meanwhile, areas such as the Catskills, Berkshires, Green
Mountains, and both the White Mountains and Great North Woods of
New Hampshire sport moderate-to-high chances (50-70%) for >4" of
snow. This is due to a combination of anafrontal snowfall in wake
of the cold frontal passage and this evening and a brief window
for modest upslope flow into these aforementioned ranges. Minor
impacts as a result of these snowfall amounts late Wednesday into
Thursday morning are possible with Moderate Impacts within the
single lake effect snow band in central New York.

...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 in the Gulf of Alaska
that digs south and becomes positioned west of the Pacific
Northwest coast tonight, then west of northern California by late
Thursday. NAEFS is forecasting 200mb and 500mb heights, starting
Thursday and lasting through Saturday, that are outside the CFSR
climatology while 700mb heights are commonly <1st climatological
percentile. Meanwhile, the mean 250-700mb trough will pump copious
amounts of 850-500mb moisture into the Pacific Northwest and
Northern Rockies throughout the remainder of the work-week and
into the upcoming weekend. 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 tonight and into Thursday. As a cold
front approaches this 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 on Thursday.

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
through Friday afternoon. By Friday night, the left-exit region of
a roaring 125kt 500mb jet streak will be directly over the
northern and central Sierra Nevada, which will not only increase
synoptic scale forcing aloft but further enhance snowfall rates
and aid in causing blizzard conditions into the day on Saturday.
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 Wind River Range of western
Wyoming late Friday and into Saturday.

The expected impacts of this event, particularly in the Sierra
Nevada, look to feature a rather large swath of Extreme Impacts
Friday and into Saturday. The WSSI-P now shows an high chances
(70-90%) for Extreme Impacts along the spine of the Sierra Nevada,
including along the I-80 pass. This kind of signal out to Day 3 is
rare and indicative of the kind of exceptional event that looks to
unfold in the Sierra Nevada starting as early as Thursday
afternoon in the northern Sierra Nevada, but peaking in intensity
Friday night and into Saturday. 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, the robust wind field within the
700-500mb layer will mix down and be capable of producing blizzard
conditions along the Sierra Nevada Friday night into Saturday. Not
only will visibilities be at whiteout levels at times, but wind
gusts around 65 mph could result in downed trees and power lines.
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 life-saving actions may be needed. The latest
WSSI is identifying Major Impacts (considerable disruptions and
hazardous travel) 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.
Several feet of snow is also expected in these mountain ranges
along with reduced visibilities and drifting from blowing snow.
Snowfall totals in the Sawtooth and Boise Mountains look to range
between 2 to 4 feet through Saturday afternoon, with similar
amounts in the Tetons, Wind River and Bear River Ranges through
Saturday afternoon but snow will likely still be piling up through
Saturday night.


***Key Messages for the Major Western Winter Storm***

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

--Blizzard Conditions Expected in Sierra Nevada
Strong winds will cause significant blowing/drifting snow and
whiteout conditions, making travel 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-Saturday, where blizzard conditions and
5+ ft of snow are expected.

--Widespread Damaging Wind in the Western U.S.
Wind gusts of 55+ mph are forecast across much of the West,
particularly across higher elevations and the Intermountain West
where 75+ mph gusts are possible. These winds would likely down
trees and power lines, resulting in power outages across affected
areas. 

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

Mullinax