Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
240 PM EST Tue Dec 27 2022

Valid 00Z Wed Dec 28 2022 - 00Z Sat Dec 31 2022


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

Two distinct atmospheric rivers (AR) will spread moisture onshore
this week leading to widespread precipitation across the western
CONUS. The first of these is progged to move onshore today with
probabilities for IVT exceeding 750 kg/m/s above 90% according to
CW3E. This AR will surge onshore within impressively confluent
mid-level flow south of a pronounced shortwave lifting into the
Pacific Northwest, and then spread southeast as far as the Central
Rockies into Wednesday as the shortwave opens but elongates into a
modest amplitude longwave trough. Aloft, a 150kt Pacific jet
streak will arc southeastward concurrently with the best moisture
moving eastward, providing additional lift, which when combine
with WAA ahead of a cold front moving eastward, will result in
widespread ascent and moisture to drive heavy precipitation across
a large portion of the West this evening through Wednesday night.
With robust WAA shifting onshore within the AR, 700-500mb temps
climb as high as +2 standard deviations according to the NAEFS
ensemble tables, which will drive snow levels to 7000-9000 ft
ahead of the cold front. While the cold front will race eastward
followed by rapidly dropping snow levels to around 3000 ft D2,
much of the heavy precipitation should occur before the front.
This indicates that while heavy snow accumulations are likely
across much of the terrain, significant pass impacts should be
confined to mountain passes above 6000 ft across the Sierra
Nevada, and above 3000 ft in the Cascades and Northern Rockies.
Still, pass level impacts to travel could be significant at many
of the area passes, including those across the Great Basin and
Four Corners. WPC probabilities for more than 8 inches of snow D1
are high in the Cascades of OR and WA, the Northern Rockies, the
Uintas and Wasatch of UT, and the CO Rockies/San Juans. Locally
snowfall pf 1-2 feet is likely in the higher terrain before
precipitation from this first wave winds down during D2.

After brief shortwave ridging blossoms across the Pacific Coast
behind the lead trough and accompanying AR, another shortwave will
quickly follow Thursday evening, with yet a third possibly
approaching Friday evening at the end of the forecast period. Both
of these appear weaker than the first wave, reflected both by
height anomalies and IVT forecasts. However, with snow levels
initially lower than the primary wave, despite WAA surging snow
levels to 4000-5000 ft during the most intense precipitation, it
is likely more waves of snowfall will spread across the West late
Thursday and again on Friday. The heaviest snowfall from these
other waves look to be displaced north from the Wednesday event,
and WPC probabilities for more than 8 inches of snow are highest
late D2 and D3 across the WA/OR Cascades, the Blue Mountains or
OR, the Sawtooth and Salmon River range of ID, and into the Uintas
and surrounding ranges of the Great Basin. Heavy snow will begin
to funnel back into Sierra Nevada on D3 as well.


...Central Plains & Midwest...
Days 2-3...

The broad longwave trough moving across the Intermountain West
Wednesday will leave downstream ascent through divergence and PVA
to help drive lee cyclogenesis in eastern CO Wednesday evening.
This low will then move quickly off to the northeast and weaken
through Thursday, but at least modest WAA will spread moisture
from the High Plains through the Upper Midwest. The guidance, with
the exception of the CMC which is much stronger with the
accompanying 700mb low, has trended weaker and drier today, but
there is still likely to be a band of mixed precip including snow,
sleet, and freezing rain from western KS into the Arrowhead of MN.
Significant accumulations are not expected, but some slippery
roads are possible due to freezing rain, especially in the Upper
Midwest, Wednesday night and Thursday where a few hundredths of an
inch of accretion are likely. The highest risk for any significant
snowfall is likely across NE CO from the Northern Palmer divide
and including Denver, where some PVA, weak deformation, and
upslope flow on northerly winds around the organizing low could
result in several inches of snowfall. WPC probabilities on D2 are
as high as 40% across this area.


Weiss