Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Wed Apr 20 2022

Valid 00Z Thu Apr 21 2022 - 00Z Sun Apr 24 2022

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

Multi-vorticity center upper low will wobble its way inland into
the Pac NW by early Friday as it sharpens but temporarily opens
up. 130kt jet this evening will sink southeastward and weaken a
bit but still drive broad scale upper divergence as a moisture
plume eases out of Oregon and into the Great Basin. Cold front
associated with the upper low will slowly march eastward with warm
air advection snow to its east from central Idaho southward to
Nevada and Utah. Southwesterly flow will maximize upslope over the
Sierra where the highest amounts are forecast due to impressive
IVT values (+3 to +4 sigma). WPC probabilities for at least 6
inches of snow are highest in areas above 5000ft across the
northern Sierra Day 1 and and 1-3 ft is likely over the higher
peaks over the 72-hr period. This will make for difficult travel
across some of the higher passes through the region. Moderate
(>40%) to high (>70%) probabilities of at least 4 inches of snow
are forecast for the Uintas, Absarokas, Tetons, Big Horns, and
northern CO ranges into Day 3 as the system evolves into its next
phase.

...Northern High Plains into Northern Plains...
Day 3...

System exiting the interior West will slow and deepen as it
becomes negatively-tilted by Day 3 (late Fri into Sat). Upper jet
will take on an S-shape curve before splitting as the mid-level
system again closes off quite vigorously over South Dakota on
Saturday. This will promote strong cyclogenesis over the western
central Plains as low pressure quickly deepens into the 980s mb as
it transfers out of the central Rockies then lifts across 100W.
Strong WAA will bring PW values near 1 inch to around 100W, just
east of the 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. There may be a transition from rain to
freezing rain to snow 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 are low (<20%) at this time. Guidance remains well-clustered
overall with expected axes differences. Snow will continue beyond
the forecast period as the system lifts steadily to the northeast.
Total snow accumulations may exceed one foot in some areas where
banding may occur. WPC probabilities of at least 4 inches of snow
are moderate (>40%) to high (>70%) over much of northeastern
Wyoming, western SD (especially over the Black Hills), eastern
Montana, and western North Dakota.

The probability of significant icing greater than 0.25" is less
than 10 percent.

Fracasso