Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Sun Dec 24 2023

Valid 12Z Sun Dec 24 2023 - 12Z Wed Dec 27 2023

...Central Rockies through the Northern Plains and Upper Midwest...
Days 1-3...

***Also see WPC Key Messages for this event***

A complex and vigorous winter storm will unfold over the next
couple of days across the Plains into the Upper Midwest as
multiple mid-level vorticity maxes congeal and coalesce into a
deep (Z5 anomalies -1 to -2 sigma) upper low and with a wrapped-up
surface reflection and occluded frontal boundary beneath an
increasingly S-shaped upper jet and north of a screaming
subtropical jet over the Lower Mississippi Valley. Significant
snow is likely on the northwest side of the low with freezing rain
to the north. A lead system today with a southern stream shortwave
will help spread generally light to modest snowfall from the
western High Plains across central NE into south central SD behind
a frontal boundary extending into Canada. CAA will change rain to
snow this morning as the wave continues northeastward. WPC
probabilities for at least 4 inches of snow are near and below 50%
over portions of western NE into SD, and generally below 40% in
portions of the Front Range as the last piece of energy rotates
through.

By late this evening/overnight, the players in the formation of
the commanding upper low will start to come together as a surface
low lifts through the Ozarks. Southern extension of the mid-level
trough will help continue to tap central/eastern Gulf moisture (PW
anomalies are already +3 to +5 sigma in the warm sector) as strong
isentropic upglide feeds northwestward into the developing low
(albeit, only deepening to around 1000mb at best). As the upper
low dives beneath (south) and eventually atop the surface low,
finally becoming stacked by late on the 26th, TROWAL signature
will support heavy snow in an arcing area from central southern SD
southward as CAA wraps around underneath the low. Such overly
wrapped systems are difficult for the models to simulate, but the
guidance has come into better agreement with the evolution with a
shift westward in the track and subsequent warm air intrusion both
aloft in the warm nose and at the surface ahead of the cold front.
Heaviest snowfall will be into the cold air just west of the mixed
precip zone where strong ascent through the DGZ may yield over a
foot of snow. Despite the modest pressure of the surface low, the
1028mb high to the northwest will sustain a strong pressure
gradient with blowing snow and blizzard conditions likely over an
expanse of the Plains. Into day 3 (Tue/26th), the then stacked
system will lose its dynamical forcing and precipitation will
slowly subside as the system wobbles back eastward after
completing its cyclonic loop across the Corn Belt. WPC
probabilities for at least 4 inches on Christmas into the 26th are
>50% over a large area from northern central KS northward to
central SD. Within this footprint, probabilities for at least 12
inches are moderate (around 40%) near the central SD/NE border
southeast of Pierre.


Farther north and northeast, there is greater uncertainty in the
precipitation type and timing of warm air intrusion, which will
likely yield some light to moderate freezing rain accumulation.
Northern stream front will bring in colder air to the region
(eastern ND/SD into northern MN) today/tonight, but as the cyclone
develops/deepens to the south, milder air will spread northward
both aloft and at the surface. There is likely to be a zone of
freezing rain that may advance northwestward as the low loops
around Iowa, though this still coincides with an area of highest
uncertainty in ptype (though generally not a lot of support for
sleet). WPC probabilities for at least 0.10" of icing are moderate
(40-70%) from the SD/MN border northward then northeastward into
the Arrowhead. The ensemble spread is large, with some members
showing more than 0.50" icing, but others show barely any at all
(function of QPF and thermal timing). Nevertheless, the potential
does exist for higher amounts as shown in the NBM and WPC
probabilities of at least 0.25" (still low at about 10 to 30%).

...Pacific Northwest...
Days 2-3...

A sharp mid-level trough beneath a skinny upper jet over the
Northeast Pacific Christmas afternoon will bring a warm front to
the coast and a surge of moisture into the Pac NW on Monday, with
a trailing system on its heels into Tuesday/Wednesday. Modest
influx of moisture transport (IVT and PW > 90th percentile) will
support moderate snowfall for the mountains as snow levels rise
from around 2500-3500ft early Monday to over 5000ft overnight.
Some of the lower passes (e.g., Snoqualmie) may see some
accumulation before a changeover to rain, higher passes (e.g.,
Washington Pass) will take much longer and may see more
significant accumulation. WPC probabilities for at least 4 inches
of snow are moderate or higher (>40%) above about 4000ft. Farther
south, there is a risk of freezing rain along the sheltered
valleys through the Cascades into Oregon, where cold air on day 1
will be reluctant to dislodge as precipitation overrides the
region. High pressure over Idaho may keep enough easterly flow to
sustain several hours of freezing rain especially just east of the
Cascades, including through the Columbia Gorge into the Columbia
Basin. There, WPC probabilities for at least 0.10" icing are
moderate (40-60%).


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

-- A significant winter storm will impact parts of the
northern/central Plains and Upper Midwest with heavy snow,
freezing rain, and strong winds during the Christmas holiday
period (Sunday through Tuesday). Widespread travel impacts are
likely.

-- Snow developing early Monday will increase in coverage and
intensity through the day. Snowfall rates at times near 1"/hr will
likely yield significant snow accumulations. There is a 40-70%
chance of exceeding 12 inches of snow from south central SD into
central NE with lower amounts elsewhere.

-- The combination of heavy snow and strong winds (30-50 mph)
could produce near-blizzard conditions. This will result in
dangerous travel due to near-zero visibility. Gusty winds could
bring down tree branches.

-- A wintry mix of sleet and freezing rain will likely overspread
parts of the Upper Midwest beginning Monday afternoon. There is at
least a 50% chance for 0.10" or more of ice in parts of the area.
This will likely lead to slippery roads and sidewalks, with
isolated power outages also possible.


Fracasso