Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EST Fri Dec 24 2021

Valid 00Z Sat Dec 25 2021 - 00Z Tue Dec 28 2021

...Northern Plains and Upper Midwest...
Days 1-3...

Two rounds of snowfall will bring moderate to significant
accumulations across the area through early next week.

The first wave will push from west to east across the Plains on
Saturday beneath a modest shortwave embedded within fast, nearly
zonal, flow. This shortwave will be accompanied by weak by
intensifying upper diffluence as the LFQ of a jet streak pivots
overhead, and the ageostrophic response to this development atop a
mid-level baroclinic gradient will produce a swath of light to
moderate snow with low-end banding potential. The column is cold
and the DGZ is deep, so SLRs will be high, leading to some higher
snowfall despite the fast motion and limited temporal duration of
QPF. WPC probabilities for 4 inches are as high as 40% along the
ND/SD border on D1.

More widespread and significant snowfall is likely to spread
across the region from SW to NE beginning late Sunday in the
Northern Plains and expanding into the Western Great Lakes by the
end of D3. This is in response to another shortwave ejecting out
of the Four Corners Sunday and taking on a negative tilt while
lifting into Canada on Monday. A deepening upstream longwave
trough across the Northwest will drive an intensifying and
poleward arcing jet streak into the Midwest, and the overlap of
mid-level height falls and upper diffluence will produce a surface
low which will strengthen as it lifts from the WY High Plains into
western MN. WAA ahead of this system will manifest as a modest
TROWAL surging higher theta-e into the area which will result in
heavy snow spreading from SD northeastward into the U.P. of MI.
WAA should lead to deepening of the DGZ which will support better
SLRs and heavier snow ahead of the low, with additional heavy
snowfall likely as the deformation pivots eastward behind the
surface low. This is likely to create a swath of heavy snowfall,
and WPC probabilities for 4 inches have increased to more than 50%
from eastern ND through the Arrowhead of MN, where local
enhancement is likely off of Lake Superior. Local maxima may
exceed 8 inches in places.


...Northeast...
Days 1-2...

A wave of low pressure will develop across the eastern Great Lakes
in response to overlapped height falls in the vicinity of a
shortwave and a modestly coupled jet structure with accompanying
upper diffluence. This low will move quickly from west to east as
it is embedded in generally fast/nearly zonal flow to be off the
New England coast Saturday evening. WAA ahead of this feature will
spread moisture northward, which when acted upon by the deep layer
ascent will drive an expanding area of precipitation northward
through New England. High pressure over Canada will maintain a
wedge down into the area to keep low-level cold air locked in
place, and as WAA drives a warm nose northward, a period of
freezing rain is likely from Upstate New York into Central and
Northern New England. Much of this area, outside of the St.
Lawrence valley and northern VT, will turn over to rain, but not
before moderate accretion occurs. WPC probabilities for freezing
rain on D1 are high for 0.1" from the Adirondacks into the
Berkshires and parts of the Greens and NH Monadnock region.
Locally more than 0.25" is possible. Further to the northeast, the
column will remain cold enough that the WAA will spread moderate
snowfall across NH and into ME Saturday morning, with snowfall
lingering as a trough extends NW from the departing low into
Sunday. WPC probabilities for 4+ inches of snow are 10-30%,
highest in the Whites of NH eastward into the Central Highlands of
ME.


...Western U.S...
Days 1-3...

An extremely active pattern will persist across the West through
the forecast period. A longwave trough will maintain its primary
axis just west of the Pacific Coast, while spokes of shortwave
energy shed and surge eastward onshore each day. Each of these
shortwaves will have the two-pronged effect of providing ascent
through height falls and PVA, while also pinching the mid-level
flow to enhance confluence and moisture advection into the region.
At the same time, prolonged and pronounced subtropical Pacific jet
energy of 130-150 kts will extend nearly across the CONUS through
Sunday, with a secondary jet streak digging down from the Gulf of
Alaska towards the coast on D3 /Monday/ to provide additional
ascent. Moisture within this jet streak and atop the mid-level
flow will drive impressive IVT and PWs as much as +2 to +4
standard deviations above the climo mean, highest across the
southern tier and on D1-2. However, even as moisture decreases
across the NW, waves of energy and surface lows will provide
enough ascent to persist snowfall. Additionally, snow levels which
will be high within the AR D1 of 5000-7000 ft, will drop as low as
sea level by Saturday night and Sunday across the NW, and to
1000-2000 ft across CA and the Great Basin Sun-Mon. This will
promote heavy snow in the mountains, and light accumulations even
into the valleys and lowlands.

Timing of the heaviest precipitation remains tricky due to
multiple waves of energy with model discrepancies, but it appears
the heaviest precipitation will gradually shift southward each day
as the longwave trough sharpens and spins southeastward. For D1,
WPC probabilities are high for 12 inches in the WA and OR
Cascades, the Olympics, the ranges of northern CA, parts of the
Northern and Central Rockies, and most impressively in the Sierra
where locally 3-4 ft is likely on D1. By D2, the highest WPC
probabilities for 12 inches shift out of WA, but increase in the
OR Cascades and northern CA ranges, while maintaining high
probabilities in the Sierra once again where snowfall rates of
2-3"/hr are likely. Probabilities for 12 inches also reach
moderate 50% in parts of the Wasatch and Uintas on D2. By D3, the
heaviest precipitation should be confined to CA where WPC
probabilities for 12 inches or more are again high for the
northern CA ranges and much of the Sierra. 3-day snowfall across
the Sierra will likely exceed 7 feet in much of the region above
7000 ft. Moisture spilling across the terrain will also be
impressive, and 3-day snowfall in the Intermountain West will also
exceed 1-2 feet in much of the terrain, including the CO Rockies,
the Sawtooth, the Uintas, and the ranges of NW WY.

In the lower terrain, models continue to suggest arctic air
pouring out of Canada to drive snow levels down to near sea level
as far south as the OR/CA border, and this will produce light to
moderate snowfall in the Coastal Ranges of OR/CA D1-2, with
several inches of snowfall becoming more likely in the lowlands
including Seattle, WA and Portland, OR. By D2-3, snow levels crash
into CA as well, and some light accumulations are possible into
the foothills and terrain outside of San Francisco, CA.


~~~Key Messages for Western U.S. Heavy Snow Through Early Next
Week~~~

Periods of heavy snow will continue across many of the mountain
ranges through Tuesday. Snowfall accumulations are likely to reach
several feet, and may exceed 7 feet in the Sierra.

Falling snow levels may allow for locally moderate snow
accumulations near sea level across the Northwest, especially late
Saturday through Monday. The foothills and elevated valleys of
California and the Great Basin will also see light to moderate
snowfall this weekend and next week.

Travel will remain dangerous and is discouraged, especially along
mountain passes where long duration closures are likely. Whiteout
conditions, downed trees, and power outages are all possible.
Dangerous avalanches are likely in the Sierra, Washington
Cascades, Northern Rockies,and Wasatch.

Extremely cold temperatures will overspread the Northwest early
next week, with some record cold possible especially over the
Pacific Northwest. Low temperatures below 0F will be common east
of the Cascades through the Northern Plains.


Weiss