FOUS11 KWBC 010856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Fri Mar 01 2024

Valid 12Z Fri Mar 01 2024 - 12Z Mon Mar 04 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**

Deep upper low west of British Columbia this morning will continue
to advance southeastward into the weekend as a strong upper jet
punches into NorCal, favoring broad divergence aloft with
southwesterly flow bringing in moisture out of the Pacific. Cold
front has pushed into the northern Rockies/Great Basin and into
the northern Sierra where it will slow, stall, then dissipate as
another wave moves into the region overnight. Snow levels will
continue to fall into Saturday and early Sunday. By then the
entire longwave pattern will start to relax and weaken as
precipitation starts to wane. Over the next two days, however,
several feet of snow are likely over the Sierra, driven by the
sustained moisture flux into the terrain at a nearly orthogonal
angle with impressive lift into/through the DGZ which will support
extremely high snowfall rates of 3-5"/hr along with strong winds
over 50-80+ kts. East of the Sierra, the broad SW flow will carry
moisture across the Great Basin to the central ID ranges and into
western WY along the lead cold front today, promoting heavy
snowfall into the mountains beneath the extended jet. By Saturday
into Sunday, increased westerly flow into the central Rockies will
favor a southward push of the snow across the NV ranges, the
Wasatch, and into the CO Rockies as the 130kt jet streak passes
through from west to east. By Sunday, another wave, albeit weaker,
will move into NorCal as heights rise across the west with its
passage, bringing in another round of snow.

Total snowfall will be extreme over the high Sierra, with max
totals over 10ft possible for the entire event. Even just on D1,
2-4ft+ is quite probable above 5000ft, and especially above
6500ft. WPC probabilities for at least 4 inches of snow are at
least 50% at elevations as low as about 3000ft or so.
Probabilities for at least a foot of snow are high (>70%) across
central ID and especially into the Tetons and Wind River Range in
WY. Into D2, the moisture plume will still lie across the Sierra
on a more westerly flow as the trough axis starts to come ashore,
nudging the axis of heaviest snow (still 2-4ft) over the central
to southern Sierra eastward across UT where WPC probabilities for
at least a foot of snow are highest in the Wasatch down into
southwestern UT. By D3, the highest probabilities for at least
another 6 inches of snow over the OR Cascades southward to the
northern/central Sierra, as well as across the CO Rockies.

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

Exiting shortwave out of the Rockies associated with the second
wave from the larger upper trough will lift through eastern MT
early Sunday and deepen as it rotates northward into southern
Canada. Surface low pressure is expected to deepen across/east of
100W with a favored area for snow on its NW side along the
US/Canadian border. Models have struggled with the evolution of
this system and its QPF placement/amounts, but the potential rapid
deepening of the system in the mid-levels as the upper jet swings
into the Upper Midwest suggests at least some potential of heavy
snow in the northern High Plains/northern Plains Sunday along a
surface trough axis, though mostly this will be across southern
Canada per the consensus. WPC probabilities for at least 4 inches
of snow are >10% along/north of I-94, and as high as about 70%
along the Canadian 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 through 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.


Fracasso

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FOUS11 KWBC 020850
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Sat Mar 02 2024

Valid 12Z Sat Mar 02 2024 - 12Z Tue Mar 05 2024

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

**Significant multi-day winter storm producing extremely heavy
snowfall in the Sierra Nevada**

Deep trough and embedded upper low will move into the West Coast
this morning, continuing an onslaught of extremely heavy snow over
the Sierra Nevada during the first part of the D1 period. Nose of
the 130kt jet streak into the northern Sierra will lie atop
impressive lift through the column into the DGZ, maximized by
well-aligned upslope flow, to continue to yield snowfall rates of
3-6"/hr this morning in the Sierra, slowly sinking southward into
the afternoon and easing in intensity. However, another wave of
heavy snow will move through the northern to central Sierra this
evening with rates of 1-4"/hr. WPC probabilities for another 12
inches of snow today through 12Z Sun are around 100% in the high
Sierra (where 2-3ft+ will be common), and >50% above about 3000ft.
To the north and east of the Sierra, incoming troughing will
promote widespread snowfall over much of the Northwest/Great Basin
into the northern/central Rockies as snow levels lower regionwide.
WPC probabilities for at least 8 inches of snow are highest in the
NorCal ranges, the higher Cascades in OR into southern WA, across
the NV ranges, into the Wasatch northward to the Tetons/Wind River
Range, and as far east as the CO Rockies.

Into D2, the leading front and height falls will move into the
Plains while a trailing stream of vorticity (all the way back to
southern Alaska) will continue to feed into NorCal and associated
with a more elongated jet streak. Snowfall rates will be much
lower than with the current system, on the order of 1-2"/hr,
supporting more isolated totals in excess of 8 inches or so. WPC
probabilities of at least 8 inches are highest (>50%) above about
4000ft as snow levels bottom out with the passage of the cold core
aloft.

By D3, another system within the very northern side of the
relentless troughing will dive into the Pacific Northwest,
focusing snowfall primarily over the OR Cascades into the NorCal
ranges (Klamath, Shasta/Siskiyous). WPC probabilities for at least
6 inches of snow on D3 are highest (>50%) from the southern WA
Cascades southward to the northern Sierra as snow levels slowly
rise. Strong westerly flow will also favor the Blue Mountains into
the central ID ranges with spillover precip.


...Northern High Plains...
Days 1-2...

Leading height falls associated with the western system will swing
into the Black Hills overnight tonight into early Sunday, deepen
in response to upper jet buckling over southern Canada, and take
on a negative tilt as it forms a closed low over ND Sunday
evening. A surface low will move out of northeastern WY across SD
and lift northward through the Red River Valley of the North in
Canada by late Sunday afternoon. Temperatures are fairly marginal
across the High Plains though dew points are in the teens closer
to the Canadian border. Frontal boundary south of the ND/SD border
will lift northward as a warm front as the surface low approaches,
promoting an area of mixed precip and some light icing over
central/eastern ND into northern MN. WPC probabilities for at
least 0.10" icing are around 10-25%. Northwest of the surface low
and closer to the mid-level center will be an area of light to
moderate snow, enhanced by a surface trough/deformation axis
extending into the Canadian Prairies. Wrapped up evolution of the
system will promote at least a brief extension of a TROWAL into
the region, though the heaviest snow will be over Canada (roughly
200 miles north of the border) where the pivot point will have the
longest residence time. WPC probabilities for at least 4 inches of
snow are highest (near 90%) along the Canadian border in far northern/northwestern ND, and trail off from north to south to
about 10-20% near I-94 in western ND and also into far
northwestern MN.


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

--Heavy Mountain Snow Continues
Extremely heavy snow rates of 2-6 inches per hour are ongoing in
the Sierra. Total snow accumulations of 5-12ft, especially above
5000 ft, are forecast through late Sunday. Many passes across the
Sierra are closed as of early Saturday. Avalanches are
increasingly 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.

--Colder Temperatures and Lower Snow Levels
As the storm moves south, snow levels will lower to bring
accumulating snow down into many valleys. Much colder air is
forecast this weekend, with temperatures 10-20 degrees below
normal.

Mullinax

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