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

Valid 00Z Fri Apr 22 2022 - 00Z Mon Apr 25 2022

...West Coast through the Northern Rockies...
Days 1-2...

Upper low west of Oregon will move eastward overnight as a
separate southern vort max on its southern periphery races inland,
ultimately taking over as the dominant feature as the wave
reorganizes into a sharp trough by Friday morning. The cold front
associated with the leading edge of the thermal gradient will
continue to move through the Great Basin today with warm air
advection snow across Nevada and then Utah (Wasatch and Uintas).
Low level southwesterly flow and the approaching upper trough will
maximize upslope over the Sierra Nevada overnight Day 1 where
probabilities are moderate (>40%) for over an additional foot of
snow. Snow levels will drop from around 6000ft to 5000ft making
for difficult travel across Sierra passes. On Day 2, system will
start to tilt negatively as the jet distorts and upper divergence
increases, promoting additional snow over the Absarokas, Tetons,
Big Horns, and northern CO ranges. WPC probabilities for at least
6 inches of snow are moderate to high (40-70% and higher),
especially above 7000 ft.


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

The system crossing the Intermountain West will slow and deepen as
the upper jet takes 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 deepening 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 (+2 to +3 sigma) to around 100W Friday night from the
southeast, 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 (blizzard conditions) 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. This may limit accumulations as such strong winds
can rip apart any aggregating snow but it could be offset by
intense banding that the global models may be underestimating.
Next couple cycles of CAM guidance will shed some light on banding
intensity. Guidance remains well-clustered overall with the main
wintry QPF axis anticipated from northeast WY, across eastern MT,
and into western ND. The Black Hills in SD may also see local
enhancement with upslope into the area. WPC probabilities for at
least 8 inches of snow are high (>70%) over this area. 

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
above the surface as the system organizes and the WCB lifts into a
TROWAL. WPC probabilities of at least 0.10" ice remain low
(10-30%), though an icing threat around the Turtle Mtns of ND is
noted by WFO BIS.

By Day 3, snow will linger on the southwest side of the departing
system as it lifts slowly to the northeast. Day 3 snow
probabilities for over 4 inches are generally up to about 50% over
central North Dakota.

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 winter
storm Friday into Sunday...

--A significant winter storm will likely produce heavy wet snow,
gusty winds, and freezing rain from the Rockies into portions of
the northern Plains Friday into Sunday.

--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 young
livestock are also possible.

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


Fracasso