Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 PM EDT Sun Apr 20 2025

Valid 00Z Mon Apr 21 2025 - 00Z Thu Apr 24 2025


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

A shortwave will be moving onshore the WA/OR coast at the start of
the forecast period /00Z tonight/ and then move progressively
eastward with modest amplification through Monday. The accompanying
trough axis is progged to lift northeast into Saskatchewan and the
Dakotas just after 00Z Tuesday, and the subtle negative tilt
indicated in model progs suggest 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 (from 6000-7000 ft early to 3000-5000 ft late)
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 for portions of the Northern Rockies in
the vicinity of Glacier NP, as well as the Absarokas and Wind
Rivers.


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

The guidance has continued to trend stronger with a low pressure
system moving across the Upper Midwest this evening and into the
Western Great Lakes Monday morning. Additionally, the trend has
been for a slightly farther east track in response to the 500mb low
closing off a bit later over Iowa before lifting into the eastern
U.P. of Michigan, collocated with the LFQ of a poleward arcing jet
streak, which will strengthen modestly as it pivots around the
closed low.

As this entire feature shifts northeast, increasing moisture on
295K-300K isentropic ascent will wrap into the system, leading to
an expansion of precipitation, some of which will become heavy in
response to strong fgen along the leading warm front. This will
remain in the form of rain, but to the NW as the upper low deepens,
an axis of deformation is still progged to rotate southward across
western Lake Superior, and the accompanying ascent should
dynamically cool the column to change rain to periods of heavy
snow. While accumulating snow will be confined to periods when the
snowfall rates are the heaviest (and both HREF and WPC snowband
tool probabilities indicate a 10-30% chance of 1+"/hr snowfall
rates), the potential for significant accumulations has increased,
especially in the higher terrain of the MN Arrowhead, Keweenaw
Peninsula, Huron Mountains, and Porcupine Mountains. Here, WPC
probabilities are as high as 50-70% for 2+ inches, and as high as
10-30% for 4+ inches. As this will be a heavy and wet snow, the
potential for moderate impacts and disruptive driving due to snow
load and snow rates have also increased.


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


Weiss