Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EST Fri Feb 17 2023

Valid 00Z Sat Feb 18 2023 - 00Z Tue Feb 21 2023

...Threat increases for Major Winter Storm over much of the
Western and Northern U.S. next week...

...Pacific Northwest, Northern Rockies into the Central Rockies...
Days 1-3...

An active period of winter weather has begun with initial
shortwave troughs and impulses rounding an eastern Pacific ridge
that extends into the Gulf of Alaska that are before a powerful
trough which approaches the PacNW on Monday and sets up a pattern
change that makes for particularly active weather across much of
the CONUS through next week. 

A notable precursor shortwave trough dives south-southeast from
British Columbia to the Northern Rockies on Saturday on the
forefront of a north-northwesterly jet. Limited amplitude, a
progressive nature, and subtle upper diffluence into a region of
normal to slightly below normal PWs produces generally moderate
snowfall over the Northern Rockies where Day 1 snow probabilities
are moderately high for 6 or more inches across ranges near the
northern ID/MT border and around Glacier NP. Though the trough
axis continues down the WY Rockies through Sunday, rates decrease
in the continental air and Day 2 snow probabilities for 6 or more
inches are low over ridges just south of Glacier NP (Mission Ridge
being the main one) as well as the Absarokas, Tetons to the
northern Wasatch and the Medicine Bow/Park Range along the WY/CO
border.

More intense shortwave trough energy/vorticity (it's a low
amplitude trough) rides the increasingly strong and now
northwesterly jet southeastward down the Canadian Rockies Sunday
night, spreading over the northern High Plains Monday. Impressive
deep layer ascent, aided by intense jet level diffluence along the
left exit of the jet streak, along with an increase in moisture as
700-500mb flow becomes impressively confluent in the persistent NW
flow from the northern Pacific. The overlap of this mid-level
confluence with the aforementioned jet streak will drive PW
anomalies to as high as +2 sigma by Monday, which in a region of
continued synoptic level ascent will result in quickly increasing
snowfall coverage and intensity to dive south of the border and
across the Cascades through the Northern Rockies late Sunday, then
diving over the north-Central Rockies Monday. Snow levels are
1500-2000 ft and Day 3 WPC probabilities for six or more inches
are high over the WA Cascades, northern ID/MT border ranges, the
Absarokas and Tetons.

This is all ahead of the powerful trough which is noted in the Key
Messages below.


Significant icing is not forecast across the CONS Days 1-3.

Jackson


Key Messages for Major Winter Storm Next Week...

--Confidence is increasing that a large swath of heavy snow will
impact areas from the West Coast to the Upper Midwest next week.

--Heavy snow is forecast begin across the Pacific Northwest and
northern Rockies on Monday, expand into the Sierra Nevada and
central Rockies on Tuesday, then into the Midwest by mid-week.

--A combination of heavy snow and gusty winds would result in
particularly hazardous travel conditions in impacted areas.

--Uncertainty remains with timing and storm track over the
northern Plains and the Midwest, which will determine where the
axis of heaviest snowfall and related impacts.

--In addition, record cold temperatures are possible along the
West Coast and the northern Plains by mid-week.