Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Mon Mar 24 2025

Valid 00Z Tue Mar 25 2025 - 00Z Fri Mar 28 2025



...Great Lakes/Northeast...
Days 1-3...

An active later-season winter pattern will setup across the Great
Lakes and Northeast as troughing amplifies over the east and lobes
of vorticity drop southward out of Canada. This will produce
periodic cool and wintry weather across the region.

The period begins active as a wave of low pressure driven by a
strung out vorticity impulse lifts along the coast of Maine. This
system will move quickly into the Canadian Maritimes by 12Z
Tuesday, but a period of moderate to heavy snow is likely to
continue across Maine before this storm pulls away, and WPC
probabilities indicate a medium chance (30-50%) for an additional
2-4" in far northern Maine.

Behind this wave, cyclonic flow will intensify across the Great
Lakes as the resultant trough amplifies down towards the Mid-
Atlantic states. This will drive increasing CAA over the Great
Lakes, with the core of the coldest 850mb temperatures, falling to
as low as -15 to -18 Celsius, moving over the lakes Tuesday night
into Wednesday morning. This cold core combined with unidirectional
flow across the slowly warming lake waters will result in late-
season lake effect snow (LES), especially in the favored WNW snow
belt regions D1 and D2, with upslope snow into Northern New England
also likely D2. *WPC probabilities for this LES are moderate (50%)
for 4+ inches D1 in the eastern U.P., the far northwest L.P., and
across the Tug Hill Plateau. Probabilities for an additional 4+
inches D2 drop to just 10-30% and focus across the Tug Hill
Plateau.

Finally, an interesting development occurs late D2 into D3 as a
potent shortwave tracks over the Mid-Atlantic states, sharpening
the mid-level trough even more, and then lifts northeast off the
New England Coast. This occurs in tandem with a strengthening jet
streak lifting off into Canada leaving the favorable RRQ east of
New England, into which the amplifying trough will deposit the most
robust height falls. The guidance has trended less amplified with
this evolution, but it still supports a surface low developing and
strengthening into Wednesday morning. Depending on the exact track
of this low, some moderate snowfall may spread across New England
once again Wednesday night into Thursday, but at this time a more
suppressed solution is more likely, resulting in WPC probabilities
that are 10-30% for 4+ inches of snow from the Northeast Kingdom of
VT, across the White Mountains, and into much of northern and
central Maine.


...West Coast...
Day 3...

Impressive trough amplifying west of British Columbia will deepen
into a closed low as it pivots towards the Pacific Northwest coast
Thursday. The core of this low is progged to become quite
amplified, falling below the 0.5 percentile with respect to 700mb
heights (1st percentile with respect to 500mb heights). This is
indicative of a very strong low which will move eastward, but then
get pulled north within the amplified flow, reaching just off
Vancouver Island by the end of the forecast period.

This evolution will have a two-pronged effect on the West Coast.
First, confluent downstream flow will overlap with an intensifying
Pacific jet streak rotating around the base of this trough to
advect deeper moisture onshore, with both GEFS and ECENS IVT
probabilities reaching above 60% for a narrow corridor of 500
kg/m/s. This moisture will be acted upon by mid-level divergence
and upper diffluence to wring out this moisture through increasing
lift to result in widespread precipitation moving from central CA
northward through the Cascades and Olympics. Additionally, W/SW
mid-level flow will efficiently upslope into the terrain, leading
to locally more impressive lift and heavy precipitation across the
terrain.

Snow levels will initially be quite high, 7000-8000 ft, but will
drop steadily to as low as 3000 ft by the end of the forecast
period behind a cold front working to the east. With this event, a
lot of the precipitation occurs behind the cold front, but still
most of the heavy snow should remain above pass levels. WPC
probabilities are high (>70%) for 6+ inches from the Northern
Sierra, through the Shasta/Trinity/Siskiyou region, along the spine
of the Cascades, and into the higher Olympics. Locally 12+ inches
of snow is possible in the highest terrain,


For days 1-3, the probability of significant icing of at least
0.10" is less than 10 percent across the CONUS.


Weiss