Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 PM EDT Sat Apr 19 2025

Valid 00Z Sun Apr 20 2025 - 00Z Wed Apr 23 2025


...Southern Rockies...
Day 1...

The significant winter storm that has been plaguing the Central and
Southern Rockies since Thursday will wane quickly tonight, with
lingering moderate to heavy snow persisting primarily across the
Sangre de Cristos through Sunday morning. The weakening and waning
of this system is due to the opening of the parent closed mid-level
low as it moves across NM tonight, leaving the strongest ascent
through overlapping height falls, divergence, and jet level
diffluence displaced to the east across the Southern Plains and
into a much warmer column. By 12Z Sunday, all remaining
precipitation across NM and into the Southern Plains is expected to
be rain. Until that happens, WPC probabilities indicate a low risk,
10-30% chance, for an additional 4 inches of snow across the
southern Sangre de Cristos, generally above 6000 ft.


...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 Sunday evening. 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 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 for more than 4 inches of snow are moderate
(30-50%) across the higher WA Cascades, and low (10-30%) for the
Northern Rockies near Glacier NP as well as parts of the Absarokas.
During D2, elevated probabilities above 50% for an additional 4+
inches continue in the WA Cascades, but expand and become higher
(70-90%) across the Absarokas and into the Wind River Range.


...Western Great Lakes..
Days 2-3...

A strengthening low pressure system moving out of the Southern
Plains Sunday 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 the U.P. and Arrowhead of MN. Here, there remains
quite a bit of spread among the various deterministic models, but
northerly flow combined with the enhanced mesoscale lift across the
higher elevations will result in precipitation changing to snow,
with briefly heavy snow rates of 1"/hr possible. While total
accumulations will still likely be limited,as reflected by WPC
probabilities that peak around 30% for just 2" of snow in the
Keweenaw Peninsula and the Huron Mountains, 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%.


Weiss