Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Sun Apr 16 2023

Valid 12Z Sun Apr 16 2023 - 12Z Wed Apr 19 2023


...Upper Midwest and Western Great Lakes...
Days 1-3...
A late-season winter storm will impact the region, with heavy snow
accumulations likely for portions of western Wisconsin, eastern
Minnesota, and Upper Michigan. 

A sharp upper trough will continue to amplify and assume a
negative-tilt as it moves from the Plains into the mid and upper
Mississippi Valley later this morning.  Model consensus shows a
closed low developing over the mid Mississippi Valley this
morning.  Surface observations are already reporting mixed
precipitation, including some light to moderate snow, across
portions of western Iowa and southern Minnesota.  As the low
continues to deepen and move northeast, precipitation will
continue to expand north along the upper Mississippi Valley into
the western Great Lakes region.  While precipitation across Iowa
and Minnesota is expected to be mostly snow, the high sun angle
and warm ground temperatures are expected to limit the potential
for heavy accumulations.  Farther east, a wintry mix is expected
and may result in some light ice accumulations over portions of
the Arrowhead Region, northwestern Wisconsin, and the western U.P.

Models continue to indicate that heavy snows are likely after
sunset and into the overnight hours, especially across portions of
western Wisconsin and the western U.P.  Guidance shows the system
continuing to deepen and slow as it moves from the Mississippi
Valley over Lake Michigan later today.  Strong upper forcing along
with low-to-mid level frontogenesis will help contribute to a band
of heavy snow developing on the west side of the system. 
Increasing snow ratios within this developing frontogenetic band
are expected to bolster the threat for heavy snow accumulations. 
HREF guidance shows a slow-moving band of heavy snow, with rates
of 1-2"/hr developing and remaining centered over southwestern
Wisconsin during the overnight hours.  Meanwhile, a second area is
expected to develop farther north over the western U.P. and
northwestern Wisconsin, where northerly winds are expected to
support lake-enhanced snows.  Snows are expected to continue into
the day on Monday, but gradually wane as the low begins to move
east, tracking into southern Ontario Monday night.  As the storm
moves east, some light snow accumulations are expected across the
eastern U.P. and Lower Michigan on Monday, and in the lee of the
lower Great Lakes Monday night into Tuesday.

WPC probabilities indicate that storm total amounts of 4 inches or
more are likely from southwestern Wisconsin to the Arrowhead
Region of Minnesota.  Although less certain, areas more likely to
see the heaviest storm totals are portions of southwestern
Wisconsin and along the shores of northwestern Wisconsin and the
western U.P., where WPC probabilities show a moderate to high
threat for accumulations of a foot or more.

...Pacific Northwest and Northern California to the Northern
Rockies...
Days 1-3...
A leading shortwave emanating from a broad circulation over the
northeastern Pacific and Gulf of Alaska will move along with its
associated frontal boundary across the Pacific Northwest later
today.  Snow levels, which will be high at the onset, will drop
tonight bringing at least some light accumulations down into the
Cascade passes by Monday morning.

While this leading wave is forecast to progressively lift to the
northeast, a more amplified system will quickly follow, bringing
additional organized precipitation into the Pacific Northwest late
Monday that will then spread farther south and east, impacting
both Northern California and portions of the northern Rockies on
Tuesday.  This will be followed by upstream energy and onshore
flow that are expected to support showers across the region into
early Wednesday.  Snow levels are forecast to fluctuate but remain
low enough to produce additional light accumulations in the
Cascade passes.  Meanwhile, heavy accumulations are expected
across the higher elevations of the Olympics, Cascades, the Oregon
and northwestern California coastal ranges, the Blue Mountains,
and the central Idaho and western Montana ranges.

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

Pereira