Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Fri Apr 22 2022

Valid 00Z Sat Apr 23 2022 - 00Z Tue Apr 26 2022

...Central Rockies through the Northern Plains...
Days 1-3...

A rapidly strengthening mid-level trough will evolve into a closed
low over the High Plains Saturday morning, and then lift slowly
northeast into Ontario, Canada by Monday aftn. This closed low
will become quite anomalous, noted by NAEFS ensemble predictions
for 700mb heights reaching -4 standard deviations below the climo
mean. As this low pinwheels across the region, intense upper level
diffluence within a strongly coupled jet structure will work in
tandem with the height falls to drive surface cyclogenesis in the
lee of the Rockies. This low will then deepen as it lifts into
Minnesota by Saturday evening. Downstream of this cyclone,
impressive warm and moist advection will surge out of the Gulf of
Mexico driving PWs to more than +3 standard deviations, with
intense theta-e ridging wrapping into a well developed TROWAL into
the Northern Plains Saturday into Sunday. At the same time, 295K
isentropic ascent within the WCB will drive mixing ratios above
4g/kg into the system, leading to an expansive precipitation
shield with prolonged wintry precip likely NW of the surface low.
The track of this low is pretty well agreed upon by the guidance,
and despite SLRs which are likely to be quite low due to the
intense WAA and late April sun angle, heavy snowfall is nearly
certain from parts of the Rockies northward through eastern MT and
western SD/ND. For the High plains of ND/SD/MT, the guidance seems
to match the conceptual model for a pivoting deformation band
through which snowfall rates will likely exceed 2"/hr as shown by
the WPC snowband prototype tool. This band may linger in this
area, the result of which has been an increase in total
accumulation this aftn.

There will likely be two distinct regimes of heavy snowfall from
this system. The first is primarily on D1 in the terrain from the
Absarokas of MT southward through the Uintas of UT and into the CO
Rockies, NW WY ranges, and Big Horns. Here, steep lapse rates
beneath the upper low combined with copious moisture noted by high
PW anomalies and strong winds leading to upslope in some places
should produce heavy snowfall exceed 6" in many of the ranges. The
heaviest snow in the mountains D1 is likely in the Big Horns and
Absarokas where WPC probabilities for more than 12 inches are
60-90%, and locally 18" is possible.

North and east of the mountains, into the High Plains of
WY/MT/SD/ND and including the Black Hills, a prolonged heavy
snowfall event is likely beginning Friday night and persisting
through early Sunday. Here, intense moist isentropic upglide will
be nearly continuous as the TROWAL arcs, and where the deformation
band pivots, many hours of 2+"/hr snowfall is likely. WPC
probabilities for 12+ inches on D1 are above 50% from the Black
Hills northward through much of eastern MT and along the borders
of MT/SD/ND. This band will pivot northward into D1.5 and D2 where
WPC probabilities for 12 inches are above 80% as far north as the
ND/Canada border. With snowfall rates of 2"/hr likely, some areas
may exceed 2 feet of snow as reflected by the 90th percentile in
both the NBM and WSE. Most of the snowfall should end before D3,
but continued blowing and drifting could linger impacts into
Monday, with some light snow accumulating into eastern ND and MN
before the storm ejects into Canada.

With strong WAA driving much of the precip, there is likely to be
a transition zone with some light freezing rain, primarily across
parts of northern and central ND. WPC probabilities for 0.1" of
accretion are as high as 10% in some pockets.

A trailing impulse crossing CO Sunday night into Monday will
interact with residual RRQ diffluence from the departing jet
streak and a brief uptick once again in column moisture. This
system may bring additional moderate snow to the CO Rockies, for
which WPC probabilities indicate a 10-30% chance for 6 inches
extending as far south as the Sangre de Cristos.

For days 1-3, the probability of significant icing greater than
0.25 inches is less than 10 percent.


...Key Messages for Northern Rockies to Northern Plains Blizzard
later today into Sunday...

--A significant winter storm will produce heavy wet snow and
strong winds from the Rockies into portions of the northern Plains.

--A combination of heavy snow and strong winds will produce
blizzard conditions across portions of the northern Plains.

--Travel may become very difficult to impossible. Power outages
and tree damage are expected. Significant impacts to livestock are
also possible.

--Prepare now for impactful winter weather and delayed travel.


Weiss