Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Thu Apr 21 2022

Valid 12Z Thu Apr 21 2022 - 12Z Sun Apr 24 2022

...West Coast through the Northern Rockies...
Days 1-3...

A sprawling, occluded low currently centered well off the Pacific
Northwest coast will be sheared open by a reinforcing shortwave
trough that digs through southern California through tonight. The
cold front associated with the low will push east across the Great
Basin today with warm air advection snow to its east from central
Idaho southward to Nevada and Utah. Low level southwesterly flow
and the approaching upper trough will maximize upslope over the
Sierra Nevada which will see heavy snow spread down the length of
the High Sierras. Day 1 snow probabilities are high for over a
foot across this range with potential for 2 feet mainly over
central portions. Snow levels drop from around 6000ft to 5000ft
today, making for difficult travel across Sierra passes. Moderate
(>40%) to high (>70%) probabilities of at least 6 inches of snow
are forecast for the ranges of eastern NV, the Wasatch and Uintas,
Absarokas, Tetons, Big Horns, and northern CO ranges Days 1.5 to
2.5 Day 3 as the upper trough crosses the Intermountain West
through Friday night.

...Northern High Plains into Northern Plains...
Days 2/3...

The system crossing the Intermountain West will slow and deepen as
it becomes negatively-tilted Friday night. The upper jet will take
on an S-shape curve (with kissing jets maximizing ageostrophic
lift over eastern MT/western ND Friday night through Saturday)
before splitting as the mid-level system again closes off quite
vigorously by the time it reaches SD early Saturday. This will
promote strong surface cyclogenesis over the northern Plains with
surface  low pressure quickly deepens into the low 980s mb as it
transfers out of the central Rockies then lifts across 100W over
SD. Strong WAA will bring PW values of over 1 inch to around 100W
Friday night, just east of the developing rain/snow line and about
as high as it can get to still support snow. Temperatures will be
marginal to start but the rapid intensification will favor strong
winds developing to the north and northwest of the low as the
column cools enough to support heavy snow on the Plains, likely by
late Friday night over northeast WY and southwest MT. There is
potential for icing in the transition zone Friday night into
Saturday over far northeastern MT and northern ND as the low
levels cool quicker than the levels in the lower 1/3 of the column
as the system organizes and the WCB lifts into a TROWAL. WPC
probabilities of at least 0.10" ice remain low (<20%), though an
icing threat around the Turtle Mtns of ND are noted by WFO BIS.
Guidance remains well-clustered overall with the main wintry QPF
axis anticipated from northeast WY, across eastern MT, and western
ND. Snow looks to linger into Sunday as the system lifts slowly to
the northeast. Day 3 snow probabilities for over a foot are
moderate to moderately high over the swath   where banding is most
likely occur over far northeastern Wyoming, the Black Hills of SD,
eastern Montana, and western North Dakota.



...Key Messages for Northern Rockies and Northern Plains winter
storm Friday into Sunday...
--There is increasing confidence that a significant winter storm
will spread heavy wet snow and gusty winds from the Northern
Rockies through portions of the Northern Plains Friday into Sunday.

--The combination of heavy snow rates and gusty winds should
produce periods of lower visibility leading to difficult travel
conditions.

--Significant accumulations of heavy wet snow has the potential to
produce impacts to infrastructure including tree damage and power
outages.

--Prepare now for impactful winter weather and hazardous travel
beginning Friday and persisting into Sunday.



Jackson