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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 1951Z Mar 01, 2024)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Fri Mar 01 2024

Valid 00Z Sat Mar 02 2024 - 00Z Tue Mar 05 2024

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

**Significant multi-day winter storm to produce extremely heavy
snowfall for the Sierra Nevada with widespread snow eastward to
the Rockies**

Today, the highly anomalous upper trough (200-500mb heights that
are outside the CFSR 1979-2009 climatology) 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 this
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 this evening. 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 terrain will generate prolific snowfall
rates along the Sierra Nevada above 4,000ft late Friday afternoon
and through Friday night. The 00Z HREF shows high probabilities
(>70%) for 3"/hr snowfall rates after 00Z Saturday that persist
into Saturday  Any time between 00Z Sat - 00Z Sun, there are at
least 9-12 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 evening 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 both
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 help funnel a relentless slug of Pacific moisture into
these mountain ranges through the remainder of the weekend. By
Sunday evening, snow will have concluded in the Cascades with
light snow lingering in the Northern Rockies, but heavy snow will
stick around from the central and southern Sierra Nevada to the
Wasatch, the Tetons/Wind River Range, and northern Colorado
Rockies. Monday is a near carbon-copy setup with westerly mean
850-300mb flow directing additional Pacific moisture, and
resulting moderate-to-heavy snow across the northern Sierra
Nevada, Oregon Cascades, and as far inland as the Wasatch,
Sawtooth, and Tetons.

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, and the
Bear River Range in northern Utah. 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 tallest peaks) that can 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
southern Oregon Cascades, the Olympics, the Sawtooth of southern
Idaho, the higher terrain of the central Great Basin in Nevada,
the Tetons, the Wasatch, and Wind River Range. Snowfall in these
areas will also be measured in feet (2-4ft with localized amounts
topping 5ft expected) in most of these areas.

...Northern High Plains...
Days 2-3...

As the 250mb jet streak and strong 500mb PVA works its way into
the northern Rockies and High Plains on Saturday, a surface low
will deepen over east-central Montana while easterly 850mb winds
on the northern flank of the low wraps 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 an icy wintry mix in these areas Saturday afternoon
and evening. Strong 300K isentropic glide via southwesterly flow
will be the primary supplier for an icy wintry mix in these areas.
Latest WPC PWPF sports low chances (10-20%) for >0.1" of ice
accumulations in parts of north-central North Dakota Friday night
into Saturday morning. 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. 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. Most global
guidance suggests the TROWAL pivots along the US/Canada border to
produce >6" snowfall totals in northeast Montana and northern
North Dakota. However, some CAMs suggest the bulk of the snow
could remain north in southern Canada with more minor
accumulations still on the table. The latest WPC PWPF sports
moderate-to-high chances (50-70%) for >6" snowfall totals in far
northeast Montana and northwest North Dakota with the higher end
of those listed probabilities closest to the US/Canada border.
Northwest North Dakota has low chances (10-30%) for snowfall
totals >12". Northwest North Dakota is also where the WSSI-P
highlights moderate chances (40-50%) for Moderate Impacts between
Sunday morning and Monday morning.


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

--Heavy Mountain Snow Affecting Many Passes
Multiple feet of snow are expected (90+% chance) for higher
elevations, especially above 5000 ft, including the Sierra Nevada
passes (such as I-80) where 5-12 ft of snow is forecast. Extremely
heavy snow rates exceeding 3 inches per hour are likely.

--Blizzard Conditions in the Sierra Nevada
Strong winds gusting 60-80 mph, locally over 100 mph at the
highest peaks, will cause significant blowing and drifting snow
and whiteout conditions, making travel impossible in the Sierra
Nevada. Substantial, long-lasting disruptions to daily life are
expected in the higher elevations through Sunday.

--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 cause difficult
travel, and 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 to bring
accumulating snow down into many valleys. Much colder air is
forecast by the weekend, with temperatures 10-20 degrees below
normal.

Mullinax