Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Sat Apr 15 2023

Valid 12Z Sat Apr 15 2023 - 12Z Tue Apr 18 2023


...Upper Midwest and Western Great Lakes...
Days 1-3...
While differences in the details remain, the signal for a
significant late-season winter storm continues to become clearer
for portions of Minnesota, Wisconsin, and Upper Michigan.  An
upper trough moving across the central Rockies this morning will
continue to amplify as a well-defined shortwave moves through the
base of the broader scale trough.  Models have been consistent in
showing the trough assuming a negative-tilt as it moves across the
central Plains on Saturday, before a closed low develops while the
trough lifts across the mid Mississippi Valley early Sunday.  This
will likely support rain changing to wet snow on the backside of
the system, especially from Iowa northward into Minnesota.  While
warm ground temperatures and the high April sun angle are expected
to limit the potential for widespread significant amounts, some
light accumulations do appear likely during the day, especially
across portions of northern Iowa into southern Minnesota. 
However, by Sunday night as the system continues to amplify while
it tracks northeast, the threat for heavy snow is likely to
markedly increase.  Deep southerly flow ahead of the low is
forecast to draw anomalous moisture up from the Gulf of Mexico
that will be directed into an area of enhanced lift, supported in
part by strong upper jet forcing and low-to-mid level
frontogenesis, on the northwest side of the low.  Banded heavy
snow is expected to develop, with heavy accumulations becoming
more likely Sunday night into early Monday across portions of
western Wisconsin, the western U.P. of Michigan, and the Arrowhead
Region of Minnesota.  As the guidance continues to move into
better agreement, WPC probabilities for accumulations of 4 inches
or more have increased from southwestern Wisconsin to the
Arrowhead, with indications that embedded heavier totals are
likely, especially over northern Wisconsin and the western U.P. of
Michigan.  Northwesterly flow on the backside of the system as it
tracks from Wisconsin into Lower Michigan is expected to support
some lake-enhanced heavier totals, with WPC guidance showing
significant probabilities for storm totals of a foot or more
centered over the region.  While snow showers are expected to
persist in the lee of the lakes through Monday, the threat for
additional heavy snow is forecast to wane by Monday night as the
low moves into eastern Canada.

...Pacific Northwest and Northern California...
Days 2-3...
A series of shortwaves embedded within a broad circulation
currently centered over the Gulf of Alaska and the northeastern
Pacific are expected bring the return of wet weather into region
during the second half of the weekend.  Shortwave ridging
currently centered over the area is forecast to give way to a
leading shortwave and its associated frontal band on Sunday.  With
snow levels expected to rise during the day, the Cascade passes
are likely to see rain initially before transitioning to snow as
the cold front passes Sunday night.  This leading wave is forecast
to lift quickly northeast; however, a trailing low is expected to
bring additional precipitation into the Northwest that is also
likely to spread farther south and east into Northern California
and the northern Intermountain West.  With snow levels expected to
fall as this system moves onshore, this could bring significant
accumulations to the higher elevations of the Olympics and
Cascades, the southwestern Oregon and northwestern California
coastal ranges, as well as the northern Sierra Nevada.

The probability for icing over 0.1" is less than 5 percent Days
1-3.

Pereira