Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Mon Dec 25 2023

Valid 12Z Mon Dec 25 2023 - 12Z Thu Dec 28 2023

...Central Rockies through the Northern Plains and Upper Midwest...
Days 1-2...

A major winter storm will bring heavy snow with blizzard
conditions, and significant ice accretions to portions of the
Plains and Upper Midwest over the next two days.

The amalgamation of multiple mid-level vorticity centers is
imminent this morning, with the coalescence promoting an
increasingly occluded/wrapped surface front/low today into
tomorrow. Despite a modest central surface pressure down to around
1000mb, the nearly 1030mb high to the N/NW/W of the low will help
maintain a pathway for surface cold air out of southern Canada
around then underneath the low across the Plains along with strong
and gusty winds. Lead-in temperatures have been mild, but northern
stream cold front draped to the north of the developing low has
brought in sub-freezing air to the western side of the Plains into
the northern Upper Midwest where it will slow its progression in
deference to the southern low. Moisture transport has been linked
to the Gulf for the past couple of days and will continue to rush
northward today into an already well above normal column (PW
anomalies +3 to +5 sigma just east of the cold front in the warm
sector, and +1 to +3 into the cold air) on SE flow from Georgia
northwestward to Iowa(!). Strong upglide in the WCB into a
developing TROWAL will promote heavy snow on the northwest side of
the low as it performs a cyclonic loop across the Corn Belt back
through eastern NE and then eastward across MO. Snowfall rates may
exceed 1-1.5"/hr with increasing wind across portions of the
Plains as a deformation band parallels the motion of the surface
low, resulting in blizzard conditions (whiteouts) and drifting
snow. With the strong overturning of the system as it occludes
today, a warm nose above the deck with stream northward and
northwestward through MN into the eastern Dakotas, resulting in a
changeover from any snow to sleet and then freezing rain. Closer
to the northwest side of the precipitation shield, enough of a
low-level northeasterly flow may allow for a longer duration of
freezing rain, especially over eastern North Dakota, with
significant impacts.

WPC probabilities for at least 4 inches of snow cover a large area
on day 1 -- most of SD and NE -- which slips westward and
southwestward on day 2 as the low rotates farthest westward in its
loop, reaching near the WY border and into northeastern CO and
northwestern KS. Within this large footprint, two-day
probabilities of at least 8 inches are are high (>70%) from
central SD into central NE. Storm totals may exceed 18 inches in
some locations, depending on mesoscale banding that may become
more evident in the near term.

The freezing rain aspect of this storm has remained more uncertain
as a function of the westward extent of the warm nose (often
handled poorly in the models up until the very end). However,
there does seem to be convergence toward eastern ND and far
northwestern MN southward to the northern Coteau des Prairies and
into the Buffalo Ridge. There, WPC probabilities for at least
0.10" of ice are high (>70%), and probabilities for at least 0.25"
are moderate (40-70%).


...Pacific Northwest...
Days 1-2...

A sharp mid-level trough will swing into western WA later tonight,
with a lead warm front pushing to the coast today followed by a
weakening cold frontal passage tomorrow. Broad lift via an
incoming jet streak arcing across British Columbia will promote a
fairly steady round of wintry precipitation for the Cascades
(snow) and foothills (snow/freezing rain) down into the valleys
within and east of the Cascades (freezing rain). Relatively low
snow levels around 3000ft this morning will rise to over 5000ft by
tomorrow, generally above most passes. However, several inches are
possible at the higher passes (e.g., Stevens) before a changeover.

The colder air mass in place this morning, thanks to surface high
pressure over Idaho, will be reluctant to mix out or be scoured
out by the incoming warm front, especially sheltered valleys
within and east of the Cascades. Though the surface high will
slowly sink southeastward today into early Tuesday, incoming
overrunning precipitation will likely fall as rain as milder air
streams in on SW flow but falls into the shallow cold air mass at
the surface, favoring freezing rain accumulations in the Cascade
foothills, Columbia basin, Columbia Gorge, and Hood River Valley.
Even farther north through the WA Cascades, strength of the warm
front may not be enough to dislodge the colder air across some
passes and east of the crest for several hours, and icing is
likely there today as well. WPC probabilities of at least 0.10"
icing are low (10-40%) in and near the WA Cascades but high (>70%)
in the Columbia Gorge/basin into the Hood River Valley.
Significant icing of at least 0.25" is possible there as well with
probabilities around 40%.


...Northern California...
Day 3...

An upper low well west of WA/OR will drive a front into the West
Coast. With a track northward of the upper low, height falls will
be brief into day 3 and a narrow moisture surge will weaken
through the day. However, upslope enhancement in the
Shasta-Siskiyous and northern Sierra will wring out several inches
of snow above about 6000ft and perhaps close to a foot at the
highest peaks.


...Northern Maine...
Day 3...

Combination of a northern stream front draped along the Canadian
border, approaching/unfolding upper low out of the Midwest, and
forming coastal low just off the Mid-Atlantic headed northeastward
will combine to increase precipitation over the Northeast Wed into
early Thu. Temperatures are mild over most of the region but the
Canadian front will bring in marginally colder air to at least
northern Maine as precipitation overrides the region. This may
result in an area of light freezing rain over central interior
Maine with snow to the north in the deeper colder air, where WPC
probabilities for at least 4 inches of snow are low (around 10%).


...Key Messages for the Christmas Winter Storm...

--Impactful winter storm gets underway today
A significant winter storm will impact parts of the
northern/central Plains and Upper Midwest with heavy snow,
freezing rain, and strong winds through Tuesday. Widespread travel
impacts are likely.

--Heavy snow for parts of South Dakota & Nebraska
Snow developing early Monday will increase in coverage and
intensity through the day today. Snowfall rates at times of
1-2"/hr will lead to significant snow accumulations. There is a
high chance (70+%) of 12" or more of snow from south-central SD
into central NE. More than 4" of snow is expected for most of NE,
SD, and into northern KS.

--Dangerous travel due to blizzard conditions
The combination of heavy snow and strong winds (gusts up to 55
mph) will produce blizzard conditions. Travel will be difficult to
near impossible due to snow-covered roads and whiteout conditions.
Gusty winds could bring down tree branches and power lines.

--Significant icing for parts of the Upper Midwest
A wintry mix of sleet and freezing rain will begin today over
northwestern IA into southwestern MN and expand northward. Over
0.25" icing is forecast over the eastern Dakotas into
western/southwestern MN. Power outages and tree damage are likely
in areas that receive significant icing. Any icing may lead to
slippery roads and sidewalks.


Fracasso