Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Mon Dec 05 2022

Valid 00Z Tue Dec 06 2022 - 00Z Fri Dec 09 2022

...Great Basin through the Central Rockies...
Days 1-3...

A slow moving positively tilted trough will shift onshore
California Tuesday and then drift slowly eastward Wednesday while
amplifying across the Four Corners. This feature may deepen into a
closed mid-level low over the Central Plains by Friday. Downstream
of this wave, persistent divergence will drive ascent, while
downstream confluence helps to advect moisture west to east across
the region. At the same time, a subtropical jet streak of Pacific
origin will gradually amplify and arc to the northeast, placing at
least modest LFQ ascent into the Four Corners and Great Basin
while acting in tandem with the mid-level confluence to increase
moisture downstream with PW anomalies reaching +1 to +1.5 sigma on
Wednesday. While the highest PW anomalies may be just south of the
likely region of heaviest snow, there will still be sufficient
moisture to wring out as heavy snow, especially where the presence
of a stationary front wavering across CO/UT helps to enhance fgen,
resulting in heavier snow rates of 0.5" to 1" per hour. The
guidance has slowed the advance of the mid-level energy and
associated weak surface wave that may develop in the lee of the
Rockies, which has resulted in yet another increase in snowfall,
especially in the terrain of the CO Rockies. WPC probabilities
feature a high risk for more than 4 inches each of the 3 days
across the CO Rockies, and for Days 1-2 in the southern Wasatch,
Kaibab Plateau, and terrain across central NV. The heaviest snow
during the period is likely in CO, especially atop the higher
terrain of the Flat Tops, Sawatch, and San Juans, where 3-day
total snowfall will likely exceed 1 foot above 5000 ft, with snow
levels crashing D3 finally as the precip begins to wane and extend
to the east.


...Montana/northern Idaho...
Days 1-2...

Expansive mid-level low centered over northern Manitoba will spin
nearly in place through Wednesday before weakening and ejecting to
the east on Thursday. Around this feature, spokes of vorticity
will rotate cyclonically through pinched flow, driving periods of
PVA and pushing a cold front southward on Tuesday into MT. This
front will then likely waver nearly in place Wednesday while a
secondary lobe of vorticity swings around the upper gyre and the
Canadian surface high sinks southeast across Saskatchewan. A
northern stream jet streak progged to dive out of British Columbia
will place favorable LFQ ascent into MT Tuesday into Wednesday,
which will combine with at least modest low-level fgen and upslope
flow to enhance snowfall from the Northern Rockies through the
High Plains of MT, with lighter snows extending eastward into the
Northern Plains embedded within the jet streak aloft. Total
snowfall should be modest, generally less than 4 inches outside of
terrain features, but WPC probabilities for more than 4 inches are
above 80% in the Northern Rockies D1, with heavy snow likely
impacting Marias Pass, and 30-50% farther east into the area
around the Bears Paw Mountains and Little Belts/Big Snowy range.
For D2, lingering heavy snow accumulating more than 4 inches is
likely to be confined to the Northern Rocky Mountain front. More
than 2 inches of snow, with locally more than 4 inches possible,
is likely in a band from central MT through much of ND.


...Pacific Northwest...
Day 3...

A closed low exiting the Gulf of Alaska late Thursday will impinge
onto the Pacific Northwest coast producing height falls and PVA
into the area. Downstream of this low, confluent flow will
transport moisture eastward within a modest AR characterized by
GEFS IVT exceeding 250 kg/m/s. This will be accompanied by
increased WAA as mid-level flow backs to the SW ahead of the
trough axis, and at least subtle upper diffluence as a Pacific jet
streak surges towards the coast. The WAA will raise snow levels,
but only modestly to around 3000 ft, and generally west of the
Cascades, with snow levels east into the Columbia Basin and
Willamette Valley remaining less than 500-1000 ft. WPC
probabilities for more than 4 inches of snow are above 80% in the
WA Cascades and Olympics, and 40-60% as far south as the
Shasta/Trinity ranges in northern CA, with more than 10 inches of
snow likely in the higher terrain, especially of WA state. While
lowland snow is not currently progged to be significant D3,
moderate to heavy accumulations are likely at or below pass levels
in the Cascades. Additionally, more widespread and even lower
snowfall accumulations are likely just beyond this forecast period.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.

Weiss