Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 PM EDT Sun Mar 19 2023

Valid 00Z Mon Mar 20 2023 - 00Z Thu Mar 23 2023

...The West...
Days 1-3...

An active and robust Pacific jet stream will be responsible for a
busy first half of the week in the West. A jet streak out ahead of
a gradually deepening upper trough off the West Coast the next
couple days will place its diffluent left exit region over
California, the Great Basin, and the Central Rockies, fostering a
favorable atmospheric environment for prolonged vertical ascent.
Given the time of year and no significant cold temperatures
available, snowfall through the short range will largely be
confined to the higher elevations while the valleys and coastal
areas stay primarily rain. That said, heavy snowfall will still be
common throughout the higher elevations of the West. The first
storm system today will have brought an influx of 850-700mb
moisture from the West Coast to the Four Corners region. WPC PWPF
tonight and through Monday shows 60-80% probabilities for >8" of
snowfall from the Sierra Nevada and tallest peaks of the OR
Cascades to the Wasatch, San Juans, and the Elk mountains of
west-central CO. The San Juans most notably have the highest
probabilities (80%) for >12" of snowfall. Farther north, snowfall
totals will be lower but are still likely to range between 6-12"
in some rages such as the Sawtooth and into the Absaroka. Periods
of mountain snow will linger over the northern and central Rockies
into Monday night and gradually diminish by Tuesday morning.

By Tuesday morning, a more potent Pacific storm system will have
its sights set on California with an even great influx of Pacific
moisture and colder temperatures at mid-levels. This will help to
bring snow levels down to as low as 4,000-5,000 feet, but still
keep the heaviest snowfall totals well above 5,000 feet. Snow will
likely be measured in feet across the southern Sierra Nevada by
Wednesday morning with localized amounts surpassing 3 feet
expected. Even the southern California ranges such as the San
Gabriel, San Bernadinos, and Sierra Madre are forecast to pick up
1-2 feet of snow in elevation above 5,000 feet. The WSSI does
depict Extreme impacts in these ranges and the southern Sierra
Nevada, largely due to the combination of both Snow Amount and
Snow Load. Farther east, portions of south-central NV, southwest
UT, northern AZ, and into southwest CO can expect heavy snow in
their higher terrain >6,000 feet. The mountains encompassing Zion
National Park in southwest UT and the San Juans are the best bet
to see snowfall totals of 1-2 feet with locally higher amounts
possible. These areas, as well as the highest peaks of northern
AZ, are forecast by the Day 3 WSSI to see Major impacts and even
some isolated Extreme impacts due to the combination of both
snowfall totals and the heavy/wet consistency of the snowfall
throughout the region. This will make for difficult to even
impossible travel conditions in these mountain ranges into
mid-week.

...Northern Plains through the Upper Midwest...
Day 3...

A pair of upper level disturbances; one ejecting out of the
Rockies and one diving south from the Canadian Prairies will work
in tandem to produce a wave of low pressure in the Central Plains
Monday night. Southeasterly 850mb flow advecting moisture poleward
into the northern High Plains, where temperatures remain below
freezing, provides a favorable setup for snow on the northern
flank of the developing 850mb low in South Dakota Tuesday morning.
This 850mb low lies beneath the diffluent left-exit region of a
140 knot jet streak that will only further help to deepen this
storm system into Tuesday evening. Guidance agrees that as the
850mb low deepens, the fetch of 850-700mb moisture flux only
increases, providing plenty of moisture over the Dakotas and
northern MN. How strong this low can become and where the
deformation zone on the northwest flank of the 850mb sets up will
determine which areas from the Dakotas to northern Minnesota could
receive up to 6" of snow. Latest WPC PWPF does show a swath of
10-30% probabilities for >6" of snowfall over eastern ND and
across northern MN. The coast of the MN Arrowhead feature the
highest probabilities (40-50%) for >6" between Tuesday evening and
Wednesday evening. These same areas are depicted on the
experimental PWSSI to have 50-60% odds of producing Minor impacts
during the same time frame. It is worth noting that some
deterministic guidance shows the potential for totals >8", but
given the lingering spread in guidance when it comes to storm
track and where the deformation axis sets up, confidence in
where/if totals surpass 8" remains low confidence at this time.


The probability of receiving at least 0.1" accumulation of ice is
less than 10%.


Mullinax