Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
451 PM EDT Fri Apr 09 2021

Valid 00Z Sat Apr 10 2021 - 00Z Tue Apr 13 2021


...Pacific Northwest to the Northern Plains...
Days 1-3...
A well-defined shortwave trough and its associated frontal band
are forecast to drop from western Canada into the Pacific
Northwest, bringing another round of mountain snow beginning late
Friday and continuing into Saturday.  Probabilities for
significant accumulations have increased with the overnight model
runs, with the latest WPC PWPF indicating storm total
accumulations of 4-inches or more likely down into the northern
Cascade passes, with accumulations of 8-inches or more likely
across the higher elevations of the northern Cascades and
Olympics.  The system will quickly shift east, bringing snow into
the northern Rockies on Saturday, with some potential for
significant accumulations across the northern Idaho, western
Montana, and northern Wyoming ranges.

As the system continues east, models continue to show a binary
interaction by day 3 (Sun night and Mon) with this feature and an
upper low slowly retrograding over the Upper Midwest. This will
likely lead to a west-northwest elongation of the TROWAL over the
northern Plains, with rain changing to snow across ND and
northwest MN. Probabilities for widespread heavy accumulations
remain low at this time (24hr probs <50% of 4 inches for day 3),
however some of ensemble members continue to support banding
(deeper UVVs into the DGZ) within the enhanced 850-700 mb
deformation/frontogenetic zone, coinciding with weakening 700-500
mb theta-e lapse rates and symmetric stability.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Hurley/Pereira