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

Valid 00Z Mon Dec 25 2023 - 00Z Thu Dec 28 2023

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

A major winter storm will bring heavy snow with blizzard
conditions, and significant ice accretions to portions of the
Plains and Upper Midwest through early this week.

Phasing shortwaves over the Central Plains will manifest as an
intense closed 500mb low centered over Kansas by Monday night.
NAEFS ensemble tables indicate height anomalies within this closed
low will reach -2 sigma at 500 and as low as -3 sigma at 700mb,
reflecting the intensity of this evolution. This low will continue
to deepen through Tuesday, driving intense ascent downstream
through mid-level divergence and spokes of PVA as impulses rotate
around the main gyre. Height falls may be rather minimal due to
the slow progression (and even retrograde motion) of this low, but
synoptic ascent will remain intense within the LFQ of a robust
170kt subtropical jet streak arcing across the Gulf Coast.
Moisture advection downstream of the center of this low will also
be impressive, with NAEFS ensemble tables indicating PW anomalies
reaching as high as +4 sigma into the Upper Midwest. This moisture
will be efficiently drawn northwest into the system through strong
285K-290K isentropic upglide with mixing ratios reaching 4-5 g/kg,
which will lift within this WCB into a strong TROWAL. The overlap
of this moisture and ascent, within a slow moving system, will
result in copious precipitation falling as heavy snow in the
Central Plains and, interestingly, heavy freezing rain farther
north as the storm becomes so wrapped up and occluded through D2.

The guidance has continued a subtle NW shift today, and this has
caused a translation westward of the heaviest snow axis. While
some uncertainty in the exact placement continues, where heavy
snow falls, snowfall rates of 1-2"/hr are likely as reflected by
the WPC prototype snow band tool. The intense isentropic upglide
and at least the potential for CSI through weak theta-e lapse
rates support these snowfall rates. Additionally, the setup
appears to support an intense deformation axis which will pivot
W/SW around the low, causing a prolonged duration of heavy
snowfall in some areas, and enhancing snowfall accumulations.
Despite SLRs that may generally be a bit below climo for late
December, this long duration event will support accumulations that
have a greater than 70% chance of exceeding 6 inches in parts of
SD/NE on D1, with a similar footprint on D2 extending westward.
Where this band pivots most efficiently, locally 18-24 inches of
snow is possible. There is likely to be a broad footprint of more
than 4 inches of snow as well, extending from north-central KS,
through the High Plains of CO, and northward to the ND/SD border.
Major impacts to travel are likely as gusty winds reaching 50 mph
cause blizzard conditions.

Farther north, the impressive isentropic upglide will surge a warm
nose to the west around this low and beneath the primary TROWAL.
This will result in heavy precipitation rates, at least until the
dry slot, which could be significant, lifts northwest. This
precipitation will initially fall into a column that is below
freezing, but this warm nose will quickly overwhelm the column
around 850mb, resulting in a changeover to sleet, and primarily
freezing rain. North of this dry slot, which is expected to reach
far southwest MN and into eastern SD, a rare, prolonged, freezing
rain event is possible. Despite heavy precipitation rates and a
lack of dry advection to offset the latent heat release
self-limiting to freezing rain, there is a high chance (50-80%) of
more than 0.25" of ice for eastern ND and the northern Coteau des
Prairies. Still moderate probabilities for more than 0.25" exist
along the Buffalo Ridge, eastern SD, and portions of north-central
MN. While there remains uncertainty into this exact evolution, it
is becoming more likely that some areas will experience
significant impacts due to freezing rain.


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

A potent shortwave digging across the Pacific will approach the
Pacific Northwest Tuesday, driving enhanced ascent through height
falls and PVA, aided by increasing upper diffluence within the LFQ
of an approaching 130kt jet streak. Together, these will produce
increasing synoptic lift into an environment that becomes more
saturated thanks to IVT reaching as high as 500-750 kg/ms, nearing
+2 sigma according to NAEFS, through confluent mid-level flow
downstream of the primary trough axis. This overlap of moisture
and ascent will wring out precipitation ahead of a warm front
beginning Monday morning, and persisting through Tuesday aftn when
the subsequent cold front crosses eastward drying out the column.
Snow levels will rise with the passage of the warm front from
around 3000 ft early Monday to over 5000 ft by Tuesday. This will
limit snowfall impacts to primarily above pass levels, but WPC
probabilities for more than 4 inches of snow D1 reach 40-60% in
the higher terrain of the northern Cascades, and 60-80% on D2
before precip wanes. Total snowfall may exceed 2 feet in the
highest peaks, but at pass level should be a few inches at most.

However, freezing rain accretions could be more significant than
snow for populated areas within this region. As the WAA overruns a
stubborn Canadian high pressure to the east, easterly winds will
trap the cold air at the surface resulting in an overrunning
precipitation scenario supporting rounds of freezing rain. There
is some uncertainty as to the depth of the cold air, but even some
of the Cascade passes such as Snoqualmie and Stevens could see
light icing before a changeover. The most significant ice is
likely into the Cascade foothills, Columbia basin, Columbia Gorge,
and Hood River valley, however, where WPC probabilities for more
than 0.1" reach 30-50% D1 into D2, with locally more than 0.25"
possible near the Columbia Gorge.


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

--Impactful winter storm develops tonight
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. Snowfall rates at times of 1-2"/hr will
likely yield 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.

--Freezing rain for parts of the Upper Midwest
A wintry mix of sleet and freezing rain will likely begin Monday
afternoon. There is a high chance (70+%) chance for 0.1" or more
of ice from southwest MN through the eastern Dakotas and eastward
towards the Arrowhead of MN. This will likely lead to slippery
roads and sidewalks, with isolated power outages also possible.


Weiss