Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 PM EST Thu Feb 29 2024

Valid 00Z Fri Mar 01 2024 - 00Z Mon Mar 04 2024

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

**Significant multi-day winter storm to produce very heavy
snowfall from the Cascades and northern Rockies into the Sierra
Nevada and central Rockies late week**

A prolonged winter storm that will blanket much of the mountainous
terrain in the Pacific Northwest, California, and northern/central
Rockies in very heavy snow through the upcoming weekend is in its
infancy stage as heavy snow unfolds over the Olympics, Cascades,
and the mountains of northern California. The primary atmospheric
feature driving this highly impactful winter system is an
exceptionally deep 250-500mb trough diving south from the
northeast Pacific that will direct a steady diet of 700-850mb
moisture into the western U.S.. NAEFS shows 200-500mb heights that
are so low that they fall outside the CFSR climatology (1979-2009)
for this time of year starting this afternoon and persisting (at
200mb) all the way through Sunday afternoon over the Northwest.
This impressive upper trough will also foster roaring winds at
mid-upper levels throughout the duration of the event. NAEFS shows
500-700mb winds tonight will top the 99th climatological
percentile and this will be a common place for the duration of the
event. Precipitation will continue to work its way south this
afternoon through northern California and into the Sierra Nevada
tonight, while persistent periods of heavy snow continue in the
Cascades and Olympics. In the northern Rockies a cold front and
the divergent left-exit region of a 130kt jet streak supplies the
vertical ascent aloft and sufficiently cold temperatures
throughout the depth of the atmosphere to support periods of heavy
snow from the Bitterroots and Sawtooth mountains to the Tetons and
the Wind River Range

By Friday, the upper trough will continue to dig south just off
the West Coast while an embedded 500mb shortwave trough spawns a
compact surface low just off the Oregon Coast later that
afternoon. This disturbance will accompany a powerful 120kt 500mb
jet streak whose divergent left exit region will become ideally
placed over north-central California by Friday night. Combined
with the intense PVA ahead of the 500mb vort max, the impressive
500mb jet streak dynamics, and the barrage of rich 850-700mb
moisture, strong vertical ascent aloft and along
topographically-favored will generate prolific snowfall rates
along the Sierra Nevada above 4,000ft late Friday afternoon and
through Friday night. The 12Z HREF shows high probabilities (>70%)
for 3"/hr snowfall rates after 00Z Sat that appear to continue
into Saturday morning. During this 12-hr span (00-12Z Sat) there
are at least 6-9 hr windows where >3"/hr snowfall rates are
occurring somewhere along the spine of the Sierra Nevada. Add in
the >80 kt 500mb wind speeds over the Sierra Nevada and blizzard
conditions are anticipated Friday night into Saturday. Meanwhile,
farther inland, the left-exit region of a 150kt 250mb jet streak
will become placed over southern Idaho and into western Wyoming.
The steady stream of Pacific moisture will continue to support
heavy snow through Friday night and into Saturday.

By Saturday, the axis of highest 850-700mb moisture flux will be
directed from central and southern California into the heart of
the Intermountain West. Strong jet streak dynamics aloft and
impressive upslope flow will keep heavy snow in the forecast at
elevations >2,000ft in the Cascades and Olympics, >3,000ft in the
Sierra Nevada, and >6,000ft in the Sawtooth, Tetons, Wind River,
and Bear River Ranges. The impressive 700-500mb wind field will
also be responsible for strong wind gusts that cause
blowing/drifting snow and whiteout conditions. The cold front will
become stationary from the Sierra Nevada and Wasatch to the
Colorado Rockies which will remain in place through Sunday. This
front will act as train tracks for the ongoing slug of Pacific
moisture into these mountain ranges. By Sunday evening, snow will
have concluded in the Cascades with light snow lingering in the
Northern Rockies, but heavy snow will remain in place from the
central and southern Sierra Nevada to the Wasatch, the Tetons/Wind
River Range, and northern Colorado Rockies.

In summary, this winter storm is likely to be one of the most
impactful of the winter season in the Sierra Nevada with no
shortage of heavy snow throughout most mountain ranges in the
Northwest and both the northern and central Rockies. The WSSI
shows Extreme Impacts forecast along the Sierra Nevada, in the
mountains of northern California and along the Coastal Range of
northern California and southwest Oregon above 3,000ft, the
tallest peaks of the southern Oregon Cascades, the Bear River
Range in northern Utah, and the peaks of the Olympics. The Sierra
Nevada are likely to witness the most extreme impacts with as much
as 5-12ft of snow, blizzard conditions that result in significant
blowing/drifting snow and whiteout conditions, and wind gusts
topping 60 mph (locally as high as 100 mph in the higher terrain)
that will cause tree damage and power outages. Compounding the
damaging wind threat is Snow Load, which the WSSI also depicts
Extreme Impact potential at elevations >6,000ft. Elsewhere, Major
Impacts (considerable disruptions to daily life) are depicted
along the northern Oregon/southern Washington Cascades, the Blue
Mountains of northeast Oregon, the Sawtooth of southern Idaho, the
Tetons, and Wind River Range. Snowfall in these areas will also be
measured in feet (2-4ft with localized amounts topping 5ft
expected).

...Northern Plains...
Day 3...

As the 250mb jet streak and strong 500mb PVA works its way into
the northern Rockies and High Plains, a surface low will deepen
over east-central Montana while easterly 850mb winds on the
northern flank of the low wrap in 850mb moisture flux. This will
prolong the periods of heavy snow in parts of the western Montana
Rockies into the day on Saturday. Meanwhile, the deformation axis
will primarily be located over the southern Canadian Prairies, but
a warm front draped west to east from North Dakota to northern
Minnesota will also be a focus for periods of snow and even a
wintry mix in these areas Saturday afternoon and evening. Strong
300K isentropic glide via southwesterly flow will be the primary
supplier for periods of snow in these areas. As the warm front
lift north, the bulk of the wintry precipitation should lift north
into southern Canada, but there is still the possibility for this
front to stick around longer near the International Border. Should
guidance trend in that direction, heavier snow could stick around
longer along the northern border of North Dakota and Minnesota. By
Sunday morning the storm will be racing towards the Red River of
the North with heavy snow on the northwest flank of the low
beneath the TROWAL. Uncertainty remains in the placement of the
TROWAL with the GFS farther north and the ECMWF farther south. The
latest WPC PWPF sports low-to-moderate chances (30-60%) in
northeast Montana and northwest North Dakota with the higher end
of those listed probabilities closest to the US/Canada border.


The probability of icing of at least 0.1" is less than 10 percent.


***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 the Sierra Nevada
passes (such as I-80). Extremely heavy snow rates surpassing 3
inches per hour are likely.

--Blizzard Conditions Expected in Sierra Nevada
Very 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-Sunday, where blizzard conditions and
5-12 ft of snow (and locally higher) 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 will likely down
trees and power lines, resulting in widespread power outages. 

--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 10-20 degrees
below normal.


Mullinax