Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
208 PM EST Thu Jan 23 2025

Valid 00Z Fri Jan 24 2025 - 00Z Mon Jan 27 2025

...Great Lakes...
Days 1-3...

Cyclonic flow across the east and three separate shortwaves
crossing the Great Lakes will promote continued CAA as well as
upslope flow and pulses of LES throughout the region. The initial
shortwave and period of CAA will be somewhat short lived tonight as
a brief period of shortwave ridging follows in its wake, primarily
resulting in subtle WAA on Friday, before a second, but weaker and
displaced farther north, shortwave digs across the region driving
another cold front eastward. Then, a third system will quickly
trail and enter the Upper Great Lakes by the end of D3, but with
better forcing and lake-enhancement north of the U.S. - Canadian
border. In total, this will result in two rounds of CAA across the
cold
lakes (GLERL total ice coverage up to 24% with Lake Erie up to
80%), so despite steepening lapse rates the duration and intensity
of any subsequent lake effect snow (LES) will be modest.

This results in the heaviest snow likely occurring D2-D3 as
reflected by WPC probabilities for more than 6 inches reaching
70-90% east of Lake Ontario and across the Keweenaw Peninsula, but
D1 probabilities for 4+ inches are also high (70%) east of Lake
Ontario. 3-day total snowfall may eclipse 12 inches in the most
prolonged snow bands.


...The West...
Days 1-3...

An upper ridge dominating the flow across much of the West will
quickly be replaced by an amplifying trough beginning D1. This
will occur in response to a shortwave trough digging across British
Columbia/Alberta and connecting with secondary energy from the
Pacific moving into the Pacific Northwest. Together, these will
force a longwave trough to deepen, with height falls rapidly
beginning Friday across the Pacific and Interior Northwest as
secondary energy digs southward through the trough. With time, this
feature is progged to become even more impressive, taking on a
negative tilt near CA Saturday and then potentially closing off
into an amplified low as reflected by both ECMWF and GFS
deterministic 500mb fields, and supported by NAEFS 700-500mb height
anomalies falling to below the 10th percentile over CA and
portions of the Great Basin.

This synoptic evolution will help push a cold front southward
through the Central Rockies and Great Basin, while the placement of
the upper low results in downstream divergence and pronounced SW
flow atop the sinking front. The result of this will be increasing
isentropic ascent and expanding precipitation, generally in the
form of snow as the swath of precip pivots south from the Northern
Rockies through the Great Basin, accompanied by snow levels falling
from 1500-3000 ft ahead of the front to less than 500 ft behind
it. Most of the precipitation should be light to moderate as PW
anomalies are generally normal to below normal, but some heavier
snowfall is possible, especially D2 as a stripe of fgen develops
in the LFQ of a strengthening jet streak collocated with the
WAA/isentropic ascent from the Great Basin east to the Front Range
of CO. Some enhanced ascent will also occur in this area due to
increasing upslope flow on the NE flow around a high pressure to
the north.

As the closed upper low lazily sinks southward on D3 along the
central CA coastline, periods of light to moderate snow are also
likely to develop across the southern Sierra Nevada and southern
California ranges. Precipitation will be mostly driven off of upper
divergence due to a strengthening southwesterly jet over the
Southwest and most importantly steep lapse rates (>7.5C 700-500 mb
per the ECMWF). This will allow for snowfall levels to fall to
around 4000ft in southern CA, with levels a bit higher in the San
Jacinto range though Sunday.

This evolution will spread a swath of snowfall southward each day.
On D1, WPC probabilities for 4+ inches are moderate (50-70%) across
some of the higher terrain of central/western Montana and western
WY. By D2 the coverage of moderate probabilities spreads southward
into the central Great Basin and the CO Rockies, including the
Park Range, Medicine Bow Range, and immediate Front Range. WPC
probabilities for 4 inches or more are highest from the Front
Range to the Park Range of CO, with some lower probabilities as
far east as the Sierra. By D3 these moderate probabilities then
center over the southern Sierra and southern CA ranges. Days 1-3
snowfall could exceed 1 foot in parts of the Colorado Rockies.


For the days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss/Snell