FOUS11 KWBC 230718
QPFHSD

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

Valid 12Z Thu Jan 23 2025 - 12Z Sun Jan 26 2025

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

Cyclonic flow across the east will be amplified by a shortwave
moving through the flow which will push a cold front eastward
across the Great Lakes late Thursday into Friday, followed by
renewed CAA. This CAA will be somewhat short lived as a brief
period of shortwave ridging follows in its wake, primarily
resulting in subtle WAA D2, before a second, but weaker and
displaced farther north, shortwave digs across the region driving 
another cold front eastward. This will result in two rounds of CAA
across the now cold lakes (GLERL total ice coverage up to 24%), 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 D3 as reflected
by WPC probabilities for more than 6 inches reaching 30-50% east of
Lake Ontario and across the Keweenaw Peninsula, but D1 
probabilities for 4+ inches are also high (70%) in the eastern 
U.P., along the eastern shore of Lake Michigan, and 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 the latter
half of 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 below 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 D3 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.

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 Montana. By D2 the coverage
of moderate probabilities increases and spread across the
Absarokas, NW WY ranges, and into the CO Rockies including the Park
Range. By D3, WPC probabilities for 4 inches or more are highest
across the Front Range and Park Range of CO, with some lower
probabilities as far east as the Sierra. Days 2-3 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


...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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