Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EDT Thu Mar 27 2025

Valid 12Z Thu Mar 27 2025 - 12Z Sun Mar 30 2025


...West Coast through Northern/Central Rockies...
Days 1-3...

An anomalously deep, cold-core low off OR near 135W will further
occlude and curl up toward Vancouver Island through Thursday night
with the occluded front crossing the West Coast this morning. Snow
levels continue to rapidly drop to around 4000-5000ft today across
the Cascades into the northern CA ranges, then to around 3500ft
Friday with the passage of the upper trough axis. Accumulation
will eventually reach to some of the passes in the Cascades by
Friday, but the rates will drop around that time as well. WPC
probabilities for at least 8" of snow are high (>70%) on Days 1-2
across the high elevations of the Cascades and into the northern CA
ranges/northern Sierra.

Moisture will continue inland through the northern Rockies Friday
through Saturday with periodic heavy snow above 5000ft snow levels
on Friday and 4000ft on Saturday. Snow also begins to enter the
central Rockies by Saturday as a trailing shortwave crosses the
region and drops snow levels below 7000-8000 ft. Snowfall
probabilities for at least 8" remain greatest across western MT and
towards the Little Belts and Bighorns of WY, where 50-80% values
are found.


...Upper Midwest/Northern Great Lakes/Northeast...
Days 2-3...

Flattening upper ridging exiting the High Plains Thursday combined
with lee-side surface cyclogenesis centered over South Dakota
Thursday night will favor broad WAA-driven precipitation along and
north of a surface boundary draped over the Upper Midwest into the
Great Lakes. With the fast zonal/progressive flow, this will
quickly expand eastward from southern Canada and the Northeast
through the daytime on Saturday. Temperatures are marginal for
most areas, confining snow to areas closer to the MN- Canadian
border through Friday and northern NY/VT/NH into ME on Saturday.
However, this band of snow could be quite potent as the region
falls within the right entrance region of a 130 kt jet and IVT over
the 90th climatological percentile over the Great Lakes per the
NAEFS on Friday night. The narrow, but intense WAA combined with
favorable synoptic ascent and moisture could lead to snowfall rates
of 1-2"/hr, particularly across parts of New England by the end of
D2.

Meanwhile, a sub- freezing surface layer will be overrun from the
S/SW with warmer air aloft, promoting a long, but narrow swath of
sleet/freezing rain from the northern Plains eastward across the
U.P. of Michigan and northern WI through Ontario, the Adirondacks,
and central New England. Ptype from the models (and NBM) show a
strong signal for freezing rain (impressive for late March), but
this may be initially modulated by Friday afternoon insolation
before focusing over the U.P. of MI Friday evening. Day 2 ice
probabilities for >0.25" have risen to 30-60% over much of the U.P.
and the far northern L.P. Day 2-3 ice probabilities for >0.25"
across the Northeast are 20-40% and highest across the northern
Adirondacks as precipitation lingers through the end of Day 3 while
warmer air begins eventually lifting back north. Day 2 snow probs
for >4" have a very sharp gradient along the MN- Canadian border
with 50-70% values at the top of the Arrowhead of MN. For the
Northeast and New England on Days 2-3, probabilities for >6" have
increased to 30-50% for northern VT/NH and into central ME, with
higher values (60-80%) in the Whites of northern VT and neighboring
areas in ME.


...Northern Plains/Upper Midwest/Northern Great Lakes...
Day 3...

Quickly following the northern Great Lakes system on Day 2 a
shortwave
trough axis that crosses the PacNW Friday reaches the northern
Rockies on Saturday along with a southern piece of energy crossing
the central Rockies and promotes Colorado lee- side low
development on Saturday. Broad convergence should allow some
moderate to locally heavy snow bands to form around the Black Hills
of SD (maybe far northern NE) eastward into MN by Saturday
afternoon. The 12Z GFS/EC have come into better agreement, though
the EC remains a bit farther north with the axis of heavier snow.
WPC probabilities for at least 4" of snow are 30-50% from northern
WI to the western U.P., with lower probabilities (10-30%) extending
from the Black Hills of SD through central MN. These probabilities
are likely to increase over the next few forecast cycles once
better agreement in the northern extent of QPF comes into focus.
With
high pressure remaining locked in over the southern Hudson Bay,
there remains a strong signal for mixed ptype from MN through WI
and into the U.P. and northern L.P. of MI. Day 3 freezing rain
probabilities for >0.25" are currently low (10-30%) from central WI
through the northern L.P. of MI.



Snell