Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
507 PM EST Sat Dec 31 2022

Valid 00Z Sun Jan 01 2023 - 00Z Wed Jan 04 2023

...Western U.S....

Days 1-3...
A plume of deep moisture, directed by an amplifying shortwave, is
continuing to move inland across California this evening.  This
will support high elevation snow across the Sierra, and snow
levels are expected to steadily fall as the system moves inland. 
Snowfall totals on the order of several feet are likely here,
mainly above 8000 feet, before things abate considerably on
Sunday.  Heavy mountain snow will also make weather headlines
across central/northern Nevada, much of Utah, southeast Idaho, and
western Wyoming on Sunday as ample moisture over the Intermountain
West will interact with a lingering frontal zone, along with
favorable upper jet forcing as the California disturbance moves
inland.  This will also include the higher terrain of the Mogollon
Rim in Arizona and the San Juan Mountains in Colorado, especially
going into Sunday night and early Monday.  The potential exists
for 1-2 feet of accumulation for the mountain ridges, especially
where the flow is orthogonal to the terrain.

A weaker storm system from the Pacific is forecast to reach the
West Coast by Monday evening, with moderate snow reaching the
northern California mountains and then the Sierra by overnight
Monday and into Tuesday morning.  This low will likely take a more
southerly route compared to the ongoing event, with some
additional snow for the higher terrain of Arizona and western New
Mexico on Tuesday as the system continues to weaken.

...Central Plains and Upper Midwest...

Days 2-3...
The low crossing the Rockies is forecast to re-develop across
southeast Colorado Monday morning after the initial system
gradually dissipates over the north-central part of the state. 
This surface low is progged to track in a general east-northeast
direction across northern Kansas/southern Nebraska and then across
the Upper Midwest by Tuesday morning, with central pressures
generally in the 990s.  It should then reach the northern Great
Lakes by the end of the forecast period Tuesday evening before
exiting over Ontario.

A swath of moderate to heavy snow is expected from northeast
Colorado/southeast Wyoming to south-central Minnesota and northern
Wisconsin, with high probabilities for 4+ inch total accumulations
and moderate probabilities for 8+ inch totals.  The snow will
primarily be confined to the deformation zone to the northwest of
a surface low, where a trowal is also expected to be present. 
Some mesoscale banding of heavy snow is likely if conditional
symmetric instability develops, with snowfall rates possibly
exceeding one inch+ per hour, but future forecasts and model runs
will provide better clarity on the placement of these mesoscale
features.  Overall, there has been a slight increase in total
forecast snowfall within the primary band, but significant model
spread still exists for the Day 3 period on Tuesday.  For thermal
profiles, a blend of roughly 2/3 ECENS and 1/3 GEFS was used as a
baseline in the forecast process.  The SLR was slightly increased
within the expected heavier snow bands for Monday, and then used
default SLR for Tuesday over the Upper Midwest owing to higher
forecast uncertainty. 

Although this is currently not expected to be a major ice storm,
there is growing concern for a corridor of 1 to 2 tenths of ice
accretion from central Nebraska to northwestern Iowa and southern
Minnesota.  Recent model soundings are depicting a well defined
warm nose between 925 to 800 mb across much of this area with a
shallow subfreezing layer near the surface with cold air advection
from northeast winds.  The highest chances of a quarter inch of
ice are currently over northwestern Iowa, before a changeover to
light snow.

Hamrick/Weiss


...Key Messages for January 1-3 Winter Storm...

-- An area of low pressure will develop and bring snowfall to the
central High Plains beginning late Sunday. This low pressure will
intensify as it shifts northeast into the Great Lakes by Tuesday,
producing a swath of moderate to heavy snow, sleet, and freezing
rain.

-- Where precipitation remains all snow, periods of intense snow
rates exceeding 1â€/hr, along with some blowing snow, will create
snow covered roads and difficult travel.

-- An area of sleet and freezing rain is likely from eastern
Nebraska through parts of Wisconsin. The ice accretions due to
freezing rain may cause dangerous travel and isolated power
outages.

-- There remains considerable uncertainty in the storm track,
which will affect areas that receive the greatest impacts from
snow and freezing rain. However, confidence has increased that at
least minor to moderate impacts are likely due to this winter
storm.