Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EDT Sun Apr 6 2025

Valid 00Z Mon Apr 07 2025 - 00Z Thu Apr 10 2025


...Northern Great Lakes...
Day 1...

A potent shortwave digging out of Manitoba, Canada will cross the
Northern Great Lakes tonight and Monday with brief but robust
height falls and PVA. Downstream of shortwave, brief but impressive
LFQ diffluence will overlap the strongest height falls as a jet
streak pivots northward, leading to developing surface
cyclogenesis. This low, and its attendant cold front, will cross
the Great Lakes as well, leading to modestly enhanced ascent and
post-frontal CAA. The combination of these features will cause a
stripe of moderate to locally heavy snow (rates 0.5 to 1 inch per
hour), with the limiting factor to accumulations being the
transient nature of this wave. Where synoptic snow associated with
this low pressure and post-frontal modest lake effect snow combine,
WPC probabilities are high (>70%) for 6+ inches, generally across
the eastern U.P. and into the Huron Mountains. Otherwise, light
snow of 2-4" is possible for the rest of the U.P. and far northern
L.P. of MI.


...Interior Northeast...
Days 1-3...

Split flow over the middle of the CONUS will merge over the
Northeast as both southern stream and northern stream impulses will
interact across the area. The southern stream impulse will remain
south of the area, with the shortwave tracking across the
Carolinas, but a lobe of vorticity strung out to the northeast will
lift northeast into New England as it becomes embedded within
greater confluence of the mid-level pattern. Enhancing this
confluence is a more potent shortwave, potentially closing off
across Ontario as it exits the Great Lakes, moving into northern
New England during Tuesday. This more potent impulse will bring
much cooler temperatures to the region, while moisture is fed
northward within the primary plume from the south.

Together, this will result in two rounds of precipitation: the
first as rain changing to snow, the second as all snow. For the
first round, increased moisture on IVT above the 99th percentile
according to NAEFS lifts into New England Monday morning. While the
core of this IVT will remain south of the area, PW anomalies may
reach the 90th percentile as moisture streams into the area.
Initially, this precip will be rain, but cooler air to the north
will support a changeover from rain to snow across Upstate NY and
central New England. This will generally result in a long duration
(12-24 hours) of just light snow, but WPC probabilities indicate a
low change (10-30%) for 4+ inches of snow in the higher terrain of
the Catskills, Berkshires, southern Greens, and Worcester Hills.

Then late D1 into D2, the secondary more potent shortwave and its
accompanying low pressure and cold front track across New England.
This will bring a secondary area of precipitation, falling as all
snow, from Upstate NY through northern New England. This system
remains progressive, but a few inches of snow are possible,
especially in the higher terrain, where WPC probabilities indicate
a 50-90% chance of at least 4 inches of snow from the White
Mountains of NH into much of northern and central ME.
Additionally, along this cold front, convective snow showers and
isolated snow squalls are possible, which could create dangerous
travel due to gusty winds and heavy snow rates causing low
visibility, but with limited additional snowfall accumulations.


...Pacific and Interior Northwest...
Days 2-3...

Short wavelength but high amplitude trough-ridge setup to start he
period will gradually evolve into a more zonal flow as the Pacific
trough sheds spokes of vorticity eastward. During this time,
pinched SW flow between the two features will drive higher moisture
onshore and IVT which in two rounds may exceed 250 kg/m/s according
to ECENS and GEFS probabilities. As the trough pivots northeast
into WA and British Columbia during Tuesday, this will enhance flow
onshore leading to expanding precipitation from northern CA through
the Northern Rockies, generally in two waves (1 on Monday, another
on Tuesday) before flow shifts to a more zonal pattern, and
eventually broad ridging by the end of the forecast period bringing
an end to the precipitation.

Despite the rounds of precipitation and the modest trough onshore,
snow levels will fall only gradually, from about 6000-7000 ft
early, to as low as 3500 ft late Tuesday. Despite this, the
heaviest precipitation may not temporally overlap with the lower
snow levels, suggesting the heaviest snow accumulations will remain
above most pass levels. Still, WPC probabilities are high (>70) D2
for 4+ inches of snow across the higher terrain of the Olympics,
along the spine of the Cascades, and moderate (30-50%) into parts
of the Northern Rockies. During D3 these probabilities wane
rapidly, but some light additional accumulations are possible
especially in the WA Cascades. While pass level snow is likely to
be modest, a few inches of snow is progged by WPC probabilities at
both Stevens and Snoqualmie Passes.


WPC probabilities for ice accumulations above 0.1" throughout the
CONUS are below 10%.

Weiss