FOUS11 KWBC 210819
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Thu Mar 21 2024

Valid 12Z Thu Mar 21 2024 - 12Z Sun Mar 24 2024

...Pacific Mountain Ranges... Days 2-3...

A large, barreling upper low off the West Coast on Friday will
begin directing a healthy fetch of 850-700mb moisture at the
Pacific Northwest and northern California. NAEFS is indicating an
IVT of 300-400 kg/m/s (>90th climatological percentile) will
steadily work its way into California on Friday. This IVT will be
the primary driver in producing heavy snowfall in the
Salmon/Siskiyou Mountains of northern California, as well as along
the spine of the Sierra Nevada. These mountain ranges also reside
beneath the diffluent left-exit region of 180kt 250mb jet streak
located off the Southern California coast Friday night. NAEFS is
showing 200mb winds Friday afternoon and into Saturday that are
above the observed 200mb wind speeds within the CFSR climatology
database for this time of year. This shows that along with the
usual strong upslope ascent, that impressive synoptically-forced
ascent will be present as well.

Snow levels will initially start out between 4,000-5,000ft Friday
night but drop to as low as 3,000ft by Saturday as the upper
trough moves into the Northwest. However, snow totals that likely
surpass 12" will be at elevations >5,000ft Saturday and through
Sunday morning. Farther north, while the moisture transport is not
as prevalent, the Pacific Northwest lies directly ahead of the
approaching upper low, which will still support prolonged
synoptically-forced ascent aloft and lowering snow levels. Snow
levels will be a near carbon-copy of the scenario farther south in
the California Ranges, but the Cascades Range and Olympics

5,000ft will have the best odds of seeing cumulative snowfall

totals top 12 inches for this event. As the upper low moves ashore
and over the Northwest on Saturday, the axis of Pacific moisture
will be directed farther inland as well, leading to periods of snow
for elevations >6,000ft in the Great Basin, the Blue Mountains of
eastern Oregon, the Boise/Sawtooth of Idaho, the Tetons and
Absaroka, and event as far inland as the Wasatch and Colorado
Rockies by Saturday night. WPC PWPF has identified the Sierra
Nevada as the only mountain range sporting high chances (>70%) for
snowfall totals >12". Once the snowfall threshold dips to 6",
however, just about every aforementioned mountain range above
sports moderate-to-high chances (50-80%) for >6" of snowfall l;ate
Friday night and through Saturday night as well.

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

Even in wake of the storm system that tracked into the Upper
Midwest Thursday night, the large dome of high pressure over the
Canadian Prairies and lower pressure in the West continues to
support a prolonged low-level easterly flow regime into the western
Montana Rockies, as well as far south as the Absaroka and Big
Horns of northern Wyoming. Another 700mb FGEN band will strengthen
throughout the day Friday and into Friday night as 700mb WAA
increase over the northern High Plains. In addition, much of
Montana lies beneath the divergent right-entrance region of a 100kt
250mb jet streak. This is a recipe for periods of snow on Friday,
however snowfall rates will not come down heavily enough during
the day on friday to do much more than Minor Impacts. Snow will be
able to accumulate better Friday night once the sun sets.

By Saturday morning, the approaching upper low positioned over the
Pacific Northwest will spawn a new surface low somewhere over
eastern Idaho Saturday morning that then tracks into northern
Wyoming by Saturday evening. This new surface low grows to become
rather anomalous, eventually dropping below the 1st climatological
percentile according to NAEFS over central Wyoming Saturday
evening. To the north of this surface low, easterly 850mb winds
will increase in strength from central Montana to the Dakotas to
the point where 850mb zonal wind anomalies also dip below the 1st climatological percentile. These 850mb winds will also direct a
surge of 850mb moisture flux that originates from a LLJ that
originates as far south as the Lower Rio Grande Valley. With a
strengthening low pressure system emerging into the northern High
Plains Saturday night and cold Canadian High Pressure anchored to
the north, all the ingredients are coming together for a potential
significant winter storm from the northern Rockies to the Northern
Plains this weekend. This far out, however, there remains
uncertainty in storm track, duration, precipitation type, and of
course, snowfall totals.

Between 12Z Friday - 12Z Saturday, WPC PWPF shows the Lewis Range
as having low chances (10-30%) for >6" of snow with elevations

6,000ft most favored. By 12Z Saturday - 12Z Sunday, the low-to-

moderate chance swath (30-50%) for >6" of snowfall grows
substantially to include most of the northern Rockies in elevations

6,000ft, through central and eastern Montana, and into western

North Dakota. Of course, the storm system will continue to produce
heavy snow and blustery winds into Sunday, so those potentially in
the path of this impending winter storm will want to check in with
their local WFO and our WPC medium range suite of products of the
latest regarding this winter storm for Sunday and into next Monday.

Mullinax


***Key Messages for Northern Plains Winter Storm March 23 - March 26***

-- Winter storm becoming likely:
Confidence continues to increase that a large storm system will
produce a swath of heavy snow, freezing rain, and sleet across
portions of the Northern Plains and Upper Midwest into early next
week.

-- Widespread heavy snow:
Heavy snow is expected to overspread eastern Montana late
Saturday,
then expand into the Northern Plains and Upper Midwest by Sunday
night, producing significant accumulations. Snow should exit the
area during Tuesday.

-- Significant impacts due to snow, ice, and wind:
A combination of heavy snow and gusty winds, as well as freezing
rain south of the heavy snow, will likely produce areas of
hazardous travel and impacts to infrastructure.

-- Forecast changes anticipated:
Uncertainty remains with the timing and location of the storm
track, which will affect where the most significant impacts will
occur. Keep up to date with the latest forecast as this storm
evolves.

$$
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FOUS11 KWBC 220807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Fri Mar 22 2024

Valid 12Z Fri Mar 22 2024 - 12Z Mon Mar 25 2024

...Northern Rockies & High Plains, Central Rockies, Upper Midwest...
Days 2-3...

**Significant late season winter storm becoming increasingly likely
from the Northern Rockies and Northern High Plains to as far east
as the Upper Midwest this weekend

In wake of the storm system that tracked through the Upper Midwest
Thursday night, the large dome of high pressure over the Canadian
Prairies working in tandem with lower pressure in the West supports
a prolonged low-level easterly easterly flow regime into the
western Montana Rockies as well as central Montana and as far south
as the Absaroka and Big Horns. Another 700mb FGEN band will
strengthen throughout the day today and into Friday night as 700mb
WAA increases over the northern High Plains. In addition, much of
Montana lies beneath the divergent right-entrance region of a 100kt
250mb jet streak. This is a recipe for periods of snow today,
however snowfall rates will not come down heavily enough during the
day to do much more than Minor Impacts. Snow will accumulate more
effectively Friday night.

By Saturday morning, the approaching upper low in the Pacific
Northwest will spawn a new surface low somewhere over eastern Idaho
Saturday morning that will track into northern Wyoming Saturday
evening. The surface low will continue its trek southeast into
eastern Colorado, where NAEFS suggests it will eventually reach
MSLP values that are <0.5 climatological percentile. Meanwhile, the
850mb LLJ will accelerate over the Great Plains with winds topping
the 97.5 climatological percentile over the southern Great Plains
around 06-12Z Sunday. The LLJ will advect anomalous 850-700mb
moisture northward and wrap around the northern flank of the
strengthening surface low, and give rise to a blossoming shield of
heavy snow early Sunday morning from western Montana and northern
Wyoming to the Dakotas. Strong 850-700mb WAA and copious amounts of
Gulf of Mexico moisture will make its way into the Upper Midwest
where heavy snow will then ensue over southern Minnesota. By 00Z
Monday, a significant winter storm will be well underway, causing
strong winds and heavy snow from the Central Rockies on east
through the Black Hills, the Dakotas, and into the Upper
Mississippi Valley. NAEFS by 00Z Monday shows MSLP values in parts
of southwest Kansas and the TX/OK Panhandles that fall outside the
observed MSLP minimum for this time of year (~980mb), showcasing
the remarkable strength this storm will reach Sunday evening. By
early Monday morning, the storm will track northeast towards the
Missouri River with a pronounced TROWAL over the eastern Dakotas
and western Minnesota being the focus for the heaviest snowfall.

There remains some uncertainty in regards to the position and track
of the storm, but most ensemble members are on board with this
becoming an impressive late March winter storm. Latest WSSI-P shows moderate-to-high chances (50-80%) for Moderate Impacts from
eastern Montana and along the ND/SD border to south-central
Minnesota through early Monday morning. There are similar
probabilities for Moderate Impacts that stretch as far south as the
Sand Hills of northern Nebraska. There are even some low-to-
moderate chances (30-50%) for Major impacts in parts of eastern
South Dakota, southwest Minnesota, and throughout the Twin Cities
metro area. The primary drivers in the WSSI-P algorithm are Snow
Amount and Blowing Snow, the latter of which, shows >60% chances
for Minor Impacts as far south as the Palmer Divide and into
central Nebraska as a powerful cold front passes through these
areas on Sunday. There are more details on the event in the Key
Messages below, but residents in these areas are encouraged to
closely monitor the latest forecasts this weekend as this storm
will likely continue to cause hazardous impacts for parts of the
Northern Plains and Upper Midwest into the start of next week.


Mullinax


***Key Messages for Northern Plains Winter Storm***

--Significant winter storm likely

Confidence continues to increase in a large storm system that will
produce an area of heavy snow over the Northern Plains and into the
Upper Midwest, along with some areas of mixed precipitation this
weekend into early next week.

--Widespread heavy snow expected

Heavy snow is forecast to overspread eastern Montana late Saturday,
then expand into the Northern Plains and Upper Midwest by Sunday
afternoon. There is a high chance (>70%) of at least eight inches
of snow from the North Dakota/South Dakota border eastward into
Minnesota and northern Wisconsin.

--Hazardous impacts due to snow, snow load, and wind

A combination of heavy snow and gusty winds will likely produce
areas of blowing and drifting snow, as well as possible blizzard
conditions Sunday into Monday. Hazardous travel, along with some
disruptions and closures are expected late Saturday into Monday.
The combination of strong winds and heavy, wet snow on trees and
power lines may result in tree damage and power outages.

--Additional forecast changes anticipated

Uncertainty remains with precipitation type for southern-most areas
and a possible sharp northern snowfall gradient in northern-most
areas. Closely monitor the latest forecasts as the storm evolved
over the next 24-48 hours.

$$
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