Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Fri Apr 26 2019

Valid 00Z Sat Apr 27 2019 - 00Z Tue Apr 30 2019


Days 1-2...

...Northern Plains through the Great Lakes...

Apart from an additional southern shift shown by some of the
daytime runs, models have remained consistent - signaling the
likely potential for a late season storm with moderate to locally
heavy snow accumulations from South Dakota eastward to southern
Michigan.  

A shortwave trough currently moving across the northern Rockies is
expected to amplify as it moves into the High Plains, with a
surface low deepening as it moves east across Nebraska Friday
evening.  GFS remains warmer than the general model consensus,
with most models show rain changing to snow on the northwest side
of the system.  Favorable upper jet forcing along with strong low
level convergence along an inverted surface front/low level front
should enhance precipitation rates and the changeover from rain to
snow.  That said, suspect the GFS is underdone with snow
accumulations across northern South Dakota to southwest Minnesota
late Friday into early Saturday. 

Farther east, morning soundings showed plenty of low to mid level
dry air remaining in the wake of a downstream trough.  This along
with strong dynamics is expected to support snow or rain changing
to snow farther east across southern Minnesota, northern Iowa into
southern Wisconsin and far northern Illinois Saturday morning. 
With several models shifting the axis of heavier amounts south and
given the potential for an additional adjustment, forecast
confidence in the details is limited.  However, models continue to
show the potential for a narrow band of moderate to heavy snowfall
within this region - supported by low to mid level frontogenesis
and favorable upper jet forcing.    Latest WPC probabilities
showed a Moderate Risk for 8-inches and a Slight Risk for
12-inches or more from the eastern Minnesota-Iowa border into
southern Wisconsin.  However, given the recent model trends, these
areas may continue to shift farther south with the overnight runs.

This system is forecast to move progressively to the east Saturday
night into Sunday, producing a narrow swath of light to moderate
snow amounts from southern Michigan to Upstate New York before
moving offshore around midday.

Day 2-3...

...Northern Rockies to the Upper Midwest...

Models show another shortwave dropping southeast from western
Canada into the northwestern U.S. Saturday night.  Strong upslope
flow west of a low dropping south from Alberta into eastern
Montana may support some heavy accumulations along the ranges of
western Montana, with WPC guidance showing some moderate to high
probabilities for amounts of 8-inches or more along portions of
the Lewis Range and Little Belt Mountains.  As the system turns
east, with a closed low developing over eastern Montana, some
heavy amounts are possible across portions of northeastern Montana
into northwestern North Dakota.  By late Sunday, models show the
system beginning to shear out ahead of a digging trough over
central Canada.  Greatest potential for heavy snow Sunday evening
into Monday will center near the U.S.-Canada border from eastern
North Dakota to the Arrowhead of Minnesota.

Pereira