923
FOUS11 KWBC 200705
QPFHSD

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

Valid 12Z Sun Apr 20 2025 - 12Z Wed Apr 23 2025


...Washington Cascades & Northern Rockies...
Days 1-2...

A shortwave embedded within broad troughing across the western
CONUS will eject south along the coast of British Columbia and dive
across WA/OR this evening into early Monday. This feature will
amplify modestly as it crosses the Northern Rockies Monday morning,
stringing out into the Northern Plains on Tuesday. This
progression will drive a surface cold front southeast across the
region Monday, with synoptic ascent through overlapping height
falls and weak jet diffluence atop the baroclinic zone leading to
weak surface low development as well. Together, this will spread
moderate precipitation across the region, with intensity and
coverage peaking the end of D1 into D2. Snow levels during this
time will fall steadily from around 4500-5000 ft to 2500-3000 ft,
leading to at least some pass- level snow accumulations, although
the general modest forcing and transient nature of the feature will
keep snowfall moderate. WPC probabilities D1-D2 for more than 6
inches of snow are moderate (40-70%) across the higher WA Cascades
and for the Northern Rockies near Glacier NP as well as parts of
the Absarokas and Wind River Range.


...Western Great Lakes..
Days 1-2...

A strengthening low pressure system moving out of the Southern
Plains this afternoon will lift progressively northeast, reaching
the Upper Peninsula of Michigan by Monday afternoon. As this
occurs, precipitation will expand downstream in response to
increasing moist advection along the 295-300K surface north from
the Gulf Coast. While the column will generally be too warm for
wintry precipitation, increasing ascent through a developing axis
of deformation NW of the surface low will lead to local dynamic
cooling effects, which will be most pronounced in the higher
elevations of northern WI, the western MI U.P. and Arrowhead of
MN. Here, there remains quite a bit of spread among the various
deterministic and CAMs with respect to snowfall intensity and
location, but northerly flow combined with the enhanced mesoscale
lift across the higher elevations will result in precipitation
changing to snow. Snow could come down heavy for a brief period at
onset, with rates of 1"/hr possible beginning around 06Z-12Z
Monday. While heavy accumulations may be limited, as reflected by
WPC probabilities that peak around 30% for at least 4 inches of
snow in the Keweenaw Peninsula, Porcupine/Huron Mountains, and
along the Gogebic Range into northern WI, at least minor snow
impacts are probable Monday in these areas due to briefly heavy
snow rates.


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


Snell/Weiss





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