Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sat Apr 23 2022

Valid 12Z Sat Apr 23 2022 - 12Z Tue Apr 26 2022

...North-Central Rockies through the Northern Plains...
Days 1-2...

A rapidly deepening upper low currently over WY with the surface
low moving into SD will lift slowly northeast, crossing northern
MN Sunday. Ahead of this cyclone, impressive warm and moist
advection surging from the Gulf of Mexico will wrap around this
system into a well developed TROWAL over northeast WY/eastern
MT/western ND/SD through tonight. Despite SLRs generally below
10:1 due to the intense WAA and late April sun angle, heavy
snowfall will occur in TROWAL bands. Guidance matches the
conceptual model for a pivoting deformation band through which
snowfall rates of 2"/hr are expected per the WPC snowband
prototype tool and the HREF 1-hr snow probabilities. These bands
will be oriented in the direction of motion which will maximize
snowfall with local totals reaching 2ft possible in this fairly
narrow stripe that includes the Black Hills. Day 1 snow
probabilities are at least moderate for over 6" from the
west-central CO ranges through a swath centered on the MT/ND
border with high probabilities for over a foot through the
previously mentioned corridor on the northern Plains.

With strong WAA driving much of the precip, there will be a
transition zone with sleet and freezing rain over western ND and
far northwestern SD today, lingering longest over northern ND with
Day 1 WPC probabilities for over 0.1" ice accretion 20 to 30% over
the Turtle mountains of northern ND.

This band will begin to shift east late tonight and with
progressive motion, will progressively cut down on snow totals to
the east, keeping the heavy snow generally west of 100W. Day 2
snow probabilities for over 4" are below 40% and limited to
northeast ND and far northwest MN.  Heavy snow will taper off
Sunday, but a reinforcing impulse from the Canadian Prairies keeps
at least light snow continuing over northern MN into or through
Monday.


...Colorado...
Day 2...
A trailing impulse maintains a positively tilted trough over the
central Rockies CO Sunday into Monday. The eastward shift of this
trough over CO will wring out residual moisture and result in
moderate snow with moderate Day 2 probabilities for over 6" snow
in terrain of central CO down through the CO side of 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 the Northern Plains Blizzard...

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

--Heavy snow rates which may exceed 2"/hr will combine with strong
and gusty winds to produce blizzard conditions across portions of
the northern Plains through tonight.  

--The blizzard conditions will create extremely limited visibility
at times. Travel will be very difficult to impossible, with
significant travel delays likely.

--The snow will be heavy and wet, and may accumulate up to 2 feet
in some areas. This may create isolated power outages and tree
damage. Significant impacts to livestock are also possible.


Jackson