Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Mon Apr 17 2023

Valid 00Z Tue Apr 18 2023 - 00Z Fri Apr 21 2023

..Upper Midwest and Western Great Lakes...
Days 1-2...

Sprawling low pressure centered over northern MI this afternoon
will continue lifting northeast into Ontario through this evening.
Wrap around snow continues to transition to lake enhanced snow off
Lakes Superior and Michigan. Day 1 snow probabilities for >4" are
moderate in the Keweenaw Peninsula and Huron Mtns of the U.P. and
in northwest L.P. east of Grand Traverse Bay.

Warm air advection spreads like to moderate precip over the Upper
Midwest Wednesday morning with a risk for light icing over
northeast MN and northern WI into the U.P. of MI where there are
pockets of low Day 2 probability for >0.1" ice.


...Pacific Northwest through Northern California to the Northern
Rockies and onto the Northern Plains...
Days 1-3...

Upper low pressure will linger off British Columbia through
midweek with a lobe currently swinging toward the WA/OR coast,
extending down to far northern CA that shifts inland over the NW
through Tuesday and redevelops over the northern Plains (near the
MT/ND border) Tuesday night that stalls/persists through at least
Friday.

Snow levels decrease below 2000ft tonight under height falls as
the upper trough moves in over the PacNW, reaching the northern
Rockies Tuesday. Ample Pacific moisture in the onshore flow makes
for moderate rates with moderate to high Day 1 snow probs >6" over
the Cascades, higher Coast Ranges, and the northern Sierra Nevada
as well as the Salmon River and Clearwater Ranges in ID, into
western MT and even the northern Wasatch and Wind River Range in
WY. Continued onshore flow into the PacNW from the low off BC
allows further Cascade snowfall Tuesday night/Wednesday where Day
2 snow probs for >6" are moderate (as well as the Olympics and
Klamath Range).

The redevelopment of the low over the western Dakotas into eastern
MT brings a Tuesday night/Wednesday focus to snow there with
potential banding which would overcome marginal thermals over the
Plains. Day 2 snow probs for >4" are low to moderate mainly for
elevation and along the northeast MT/northwest ND border. More
areas may fill in over eastern MT as more CAMs with better
mesoscale depiction get into the probability suite.

The system occludes over the Northern Plains Wednesday focusing
precip farther east over the Upper Midwest Wednesday
night/Thursday as the colder precip  back over the western side of
the northern Plains getting strung out a bit with rates
decreasing. However, there may be sufficient cold air along the
Canadian border to allow snow with low Day 3 probs for >4" over
northern ND and northern MN.


Jackson