Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Tue Nov 4 2025

Valid 00Z Wed Nov 05 2025 - 00Z Sat Nov 08 2025

...Pacific Northwest through the Central Rockies...
Days 1-3...

An active pattern across the Pacific will continue through the week
as a closed low lifts slowly into British Columbia, leaving
persistently confluent W/SW flow across the Pacific and channeled
into the West Coast. This will result in an extend period of
impressive moisture advection and IVT surging onshore, and
probabilities for a strong atmospheric river (AR) moving onto the
coast exceed 80% from both the GEFS and ECENS ensemble systems on
D1. The intensity of this moist advection is additionally reflected
by NAEFS IVT percentiles that exceed the 99.5 percentile across CA
Wednesday afternoon.

This suggests an impressive precipitation plume will push from
CA/OR northeast Wednesday aftn through Thursday aftn, with
precipitation spillover reaching as far as the Northern and Central
Rockies. However, most of this precipitation will fall as rain as
the accompanying WAA surges snow levels to 7000-8000 ft, keeping
any snow well above pass levels through Thursday. A secondary surge
of precipitation associated with another impulses embedded within
the confluent onshore flow will lower snow levels at least subtly,
to around 5000 ft, with additional heavy precipitation. At this
time, impactful snow will likely occur at some of the higher
passes, especially Washington Pass in the WA Cascades before Friday
morning, before this final impulse shifts east causing precip to
wind down across the Cascades but continue, in lighter fashion
across the Northern and Central Rockies.

WPC probabilities D1 are moderate (50-70%) for more than 6 inches
of snow in the higher Sierra and northern WA Cascades, before
expanding and shifting east into Salmon River/Sawtooth ranges, the
Tetons, and Northern Rockies, while at the same time continuing
across the northern WA Cascades. On D3 the heaviest snow is
expected across the WA Cascades where WPC probabilities reach above
70% for 8+ inches above 5000 ft, with moderate probabilities
(30-50%) for 6+ inches continuing across portions of the Northern
Rockies and higher elevations of ID and NW WY.


...Northern New England and the Adirondacks...
Day 2...

A rapidly intensifying clipper-type low pressure will move
progressively E/SE from Ontario, Canada, across New England, and
then out to sea south of the Canadian Maritimes by 00Z/Friday. This
low will spread significant moisture eastward on robust low-level
fgen leading to modest TROWAL development pivoting across Northern
New England. While the accompanying WAA will help spread
significant moisture into the area, it will also drive snow levels
to 4000-5000 ft, suggesting that except for the highest peaks of
northwest Maine, the pre-low precipitation will be all in the form
of rain. However, as the low kicks east, rapid cold advection in
its wake, aided by impressive isallobaric flow into the deepening
system, will cool the column dramatically, reflected by snow levels
falling to 500-1000 ft by 12Z Thursday. This cooling column
combined with increasing upslope flow into the terrain on the NW
flow will result in a period of moderate to heavy snowfall in the
higher terrain, with light snow gradually spreading into the lower
elevations of VT/NH/ME and Upstate NY before the DGZ dries and
precipitation wanes Thursday aftn.

The heaviest snow is likely above 2000 ft in the Adirondacks and
Whites, although some moderate accumulations are also likely in the
highest Greens. This is reflected by WPC probabilities which
indicate a moderate risk (50-70% chance) for more than 4 inches of
snow, with locally as much 8 inches in the highest Presidential
Range peaks. Light accumulations of a dusting or more are likely
elsewhere from Lake Ontario through central Maine, except across
the Champlain Valley, resulting in the first notable snow of the
year in some of the lower elevations of northern New England.


The probability of significant ice across the CONUS is less than
10 percent.


Weiss