Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Tue Apr 05 2022

Valid 00Z Wed Apr 06 2022 - 00Z Sat Apr 09 2022

...Pacific Northwest...
Day 3...

A potent shortwave digging into the Pacific Northwest Friday
morning will be preceded by confluent mid-level flow and
pronounced WAA to surge moisture into OR/WA on D3. PW anomalies
Friday morning reach above +1.5 standard deviations above the
climo mean, into which a surface front, height falls, and modest
LFQ diffluence will provide ascent to expand a swath of
precipitation beginning Friday morning. Snow levels within the WAA
rise as high as 6000-7000ft, but will drop though Friday night to
as low as 3000 ft causing rain to change to snow even at some of
the passes. WPC probabilities are moderate for more than 4 inches
in the WA Cascades D3, with some light accumulations possible at
the passes. More snow is likely as snow levels continue to drop
into D4.


...Northern Plains/Upper Midwest/Western Great Lakes...
Days 1-3...

Anomalously deep closed low developing across the Upper Midwest
and deepening over the Great Lakes is forecast to drop to as low
as -3 standard deviations compared to climatology for 500mb
heights. This low will become a sprawling gyre centered over the
Great Lakes by Friday morning, and the extremely amplified pattern
will drive very slow movement of features through the period.

A surface low beneath this strengthening upper low will move
across MN D1, then WI on D2, and finally fill and eject out of MI
on D3. This low will initially deepen, but as it occludes and
becomes vertically stacked it will remain generally of the same
intensity as it drifts eastward through late week. Despite a lack
of additional forcing, deep warm and moist advection surging out
of the Gulf of Mexico on a LLJ reaching 40+ kts will drive
anomalous moisture into the system, with the WCB driving the
theta-e ridge northward into a TROWAL to support heavy
precipitation. The combination of continued moisture and periodic
vorticity impulses will persist ascent and snowfall across the
Northern Plains/Upper Midwest through the period, with some
enhancement possible as a deformation axis attempts to stretch
somewhere near the MN/ND border Wed into Thu. Additionally,
enhancement in snowfall is likely due to upslope and cooler
thermal profiles in the higher terrain of the Iron Ranges of the
MN Arrowhead, the Coteau des Prairies, Pembina Escarpment, the
Black Hills, and due to LES on the south short of Lake Superior
near the Bayfields Peninsula.

On D1, WPC probabilities for more than 4 inches are high in the MN
Arrowhead where locally more than 12 inches is possible in the
higher terrain due to upslope and lake enhancement. Additional
high probabilities for more than 4 inches exist in the Black Hills
and eastern SD on D1. Late D1 into D2, the deformation axis may
become most intense and spin across western MN leading to a
moderate risk of more than 4 inches of snowfall on D1.5 according
to WPC probabilities. As the low begins to pivot eastward, strong
CAA will cause LES south of Lake Superior from the Bayfields
Peninsula of WI eastward along the U.P. of MI. WPC probabilities
on D2 and D3 exceed 50%, but the total areal coverage of snowfall
should wane each day.


For Days 1-3, the probability of significant icing of at least
0.25" is less than 10 percent.

Weiss