Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Fri Dec 22 2023

Valid 00Z Sat Dec 23 2023 - 00Z Tue Dec 26 2023

...Four Corners...
Day 1...

An amplified closed low over Baja California this evening will
advect northeast into the Four Corners region Saturday evening as
it fills and becomes absorbed into a larger trough driven by a
northern stream impulses digging out of the Pacific Northwest.
Despite the weakening of this feature, impressive ascent will
occur into the area through downstream divergence, height falls,
and a wave of upper level diffluence within the LFQ of a poleward
arcing, but modest, jet streak. At the same time, S/SW 700mb flow
will drive enhanced moisture northward through the Desert
Southwest and into the Four Corners, to surge PWs to as high as +3
sigma according to NAEFS, with 700mb flow favorably upsloping into
the terrain to enhance ascent and wring out available moisture.
Snow levels will begin around 8000 ft, but will fall to around
6000 ft by the end of D1 as a cold front shifts eastward, but most
of the heavy snow will be confined to the higher terrain because
of this. WPC probabilities D1 for more than 6 inches of snow reach
50-70% in the White Mountains of AZ, and above 90% in the San
Juans, with 1-2 feet of snow likely in the higher terrain of the
latter.


...Pacific Northwest to the Northern Rockies...
Day 1...

A sharp mid-level shortwave dropping south off the coast of
British Columbia will race eastward tonight across the Pacific
Northwest and then shift into the Northern Rockies Saturday while
splitting. This will drive a surface cold front eastward beneath
it, resulting in an increase in ascent through low-level
convergence, height falls, PVA, and waves of upslope. A pocket of
enhanced mid-level RH will move across the region during this
shortwave passage as well, with some lingering moisture likely in
the wake of the front due to briefly confluent mid-level flow, but
overall the precipitation D1 should be transient across the
Pacific Northwest and Northern Rockies, limiting precipitation
amounts despite what should be widespread coverage. Snow levels
ahead of the front D1 will be generally 4000-5000 ft, but fall
quickly to less than 2000 ft after fropa, and reach below 500 ft
by the end of D1 but with limiting precipitation. WPC
probabilities for more than 4 inches of snow exceed 50% in the
higher terrain of the northern Cascades, as well as eastward
across parts of the Northern Rockies near Glacier NP and across
the Absarokas and NW WY ranges. Locally 6-12" of snow is likely in
the higher terrain. Impactful snow is likely at some of the higher
passes as well, including Bozeman, Targhee, and Raynolds Passes.


...Central Rockies into the Central Plains...
Days 2-3...

Complex evolution of the mid-level pattern will result in two
waves of low pressure and multiple areas of moderate to heavy snow
for the holiday weekend. The pattern begins with a northern stream
impulse digging out of Alberta with a southern stream impulse
opening across the Four Corners. The interaction of these two
features over the central High Plains by Sunday morning will
result in the shearing out of the southern stream impulse, but an
amplification of the longwave trough from the north. The guidance
has trended faster and weaker with the setup, which has resulted
in a lowering of the maximum snowfall across the region. However,
there is still likely to be an axis of moderate to at times heavy
snow from the Sangre de Cristos and CO Rockies through portions of
NE/SD on D2. Much of this snow will be along and north of a fast
moving surface low which will strengthen in the lee of the Rockies
on Saturday before weakening as it moves northeast. Despite the
modest intensity of this low, pronounced synoptic ascent and ample
moisture characterized by PW anomalies of +2 to +3 sigma according
to NAEFS will result in an axis of heavy precipitation. DGZ depths
are modest throughout the region so intense snowfall rates should
be the exception and not the rule, but fast moving multi-bands
with convective rates are possible in areas of enhanced fgen near
the surface low, especially in eastern CO and into western NE.
Despite that potential, total accumulations are likely to be
moderate, reflected by WPC probabilities for more than 4 inches
peaking at 50-60% in the Sangre de Cristos and the Front Range,
and remaining below 30% elsewhere. Local amounts above 6" are
likely in the higher terrain on D2.

During D3, this first low and associated weakening shortwave will
eject to the northeast, but immediately in its wake a secondary
shortwave will dig into the amplified trough resulting in a closed
mid-level low across the Central Plains. This will result in
impressive downstream moisture advection with robust 285K
isentropic upglide driving mixing ratios exceed 4g/kg northward,
potentially lifting cyclonically into a TROWAL within this WCB.
The closed nature of the mid-level low will likely yield a
strengthening but slow moving, potentially retrograding, surface
cyclone, above which increasing diffluence within the RRQ of a
poleward arcing jet streak will help intensify ascent. The
guidance still features a lot of uncertainty with respect to
intensity and placement, but it is possible a pivoting deformation
axis will develop NW of the surface low, driving intense ascent
into this moist environment to produce heavy snow across portions
of the eastern Central Plains. Current WPC probabilities for more
than 4 inches of snow on D3 are 10-30% from northeast NE into
southeast SD, but locally higher snowfall is probable. East of
this snow, some light icing is likely where the warm air overruns
colder surface temperatures, resulting in WPC probabilities for
more than 0.1" of ice reaching as high as 10%, highest in the
Coteau des Prairies.

Weiss