Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu Apr 08 2021

Valid 12Z Thu Apr 08 2021 - 12Z Sun Apr 11 2021


...Pacific Northwest and Northern Rockies...
Days 1-3...

A pair of amplified shortwaves -- the first Thursday morning, and
the second Saturday morning -- will generate periods of heavy
snowfall across the Pacific Northwest and northern Rockies into
the weekend.

The initial shortwave -- an amplified wave digging into the
Pacific Northwest this morning -- will continue to move east,
reaching the northern Rockies later today.  Strong dynamics,
supported in part by the left-exit region of the upper jet, will
likely encourage brief periods of heavy snow.  However, the
progressive nature of the system, along with modest moisture, is
expected to hamper the potential for widespread heavy
accumulations through early Friday.  WPC Day 1 PWPF (ending 12Z
Fri), shows high probabilities for accumulations of 6-inches or
more largely confined to the southern portions of Montana's
Yellowstone Country.

A shortwave ridge sandwiched between the two systems is expected
to bring dry conditions to much of the region during the day on
Friday.  However, wet weather is forecast to return to the
Northwest by late Friday as the next system digs along the coast
of British Columbia into the region.  This will bring another
round of moderate to heavy snow into the Olympics and northern
Cascades late Friday into early Saturday, with the WPC Day 2 PWPF
(ending 12Z Saturday) showing high probabilities for additional
accumulations of 6-inches or more.  Like the leading system, this
second wave is expected to move progressively to the east,
generating mainly light snows as it moves into the northern
Rockies on Day 3.  An upstream ridge building into the region will
support drier conditions across the Northwest by early Sunday.

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

Pereira