Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 AM EDT Mon Apr 21 2025

Valid 12Z Mon Apr 21 2025 - 12Z Thu Apr 24 2025


...Western Great Lakes...
Day 1...

By the start of the forecast period (12Z Mon), a deep low pressure
system tracking across central WI and accompanying deformation
zone in northern WI/western MI U.P. is expected to dynamically cool
the column enough for potentially heavy snowfall rates. This
transition from a cold rain to wet snow should be well underway by
12Z this morning (as early as 08Z per several CAMs) and lead to at
least a few hours with snowfall rates up to 1"/hr from the MN
Arrowhead through the western Lake Shores of Superior into inland
portions of northern WI through early this afternoon. However,
given current wet- bulb temperatures in the mid-to-upper 30s along
with moderate rainfall, snowfall accumulations will likely be
limited. Still, at least a few inches of snow are possible and
could lead to some travel impacts where higher rates are realized
and visibility is lowered. WPC probabilities for at least 2 inches
are moderate (30-60%) from far northern WI through the western MI
U.P., including the Keweenaw Peninsula, Porcupine Mts and Bayfield
Peninsula.


...Northern Rockies...
Day 1...

A shortwave crossing the northern Rockies today is already
producing a broken precipitation shield across the region early
this morning, which includes mountain snow showers. This system
will slide eastward with modest amplification through tonight. The
accompanying trough axis is then progged to lift northeast into
Saskatchewan and the Dakotas just after 00Z Tuesday, with a subtle
negative tilt indicated in model progs suggesting downstream ascent
will be maximized across the northern Rockies as this evolution
occurs. The progression of this shortwave combined with at least
weak upper diffluence will help drive pressure falls, and a wave of
low pressure is expected to track east through the region,
dragging a cold front in its wake. While this low will be weak and
contribute minimally to ascent, the trailing cold front will help
both to lower snow levels below 5,000 ft and provide the impetus
for upslope enhancement on the post- frontal flow. The overall
modest ascent and transient nature of this system will limit snow
totals, but WPC probabilities indicate a moderate risk (50-70%) for
6+ inches throughout portions of the northern Rockies in the
vicinity of Glacier NP as well as the northern Absarokas.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.



Snell