Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Sat Feb 18 2023

Valid 12Z Sat Feb 18 2023 - 12Z Tue Feb 21 2023

...Pacific Northwest, Northern/Central Rockies, Northern High
Plains...
Days 1-3...

An amplifying trough centered near the Hudson Bay will expand
across the eastern and central CONUS, leaving increasingly moist
and confluent NW mid-level flow across the Pacific Northwest and
into the Northern/Central Rockies. Within this regime, repeated
shortwaves and associated vorticity impulses will rotate rapidly
southeastward, providing ascent through PVA and height falls,
while additionally driving modest surface waves /clippers/
southeastward into the Upper Midwest. Each of these impulses will
provide ascent, the lift will become increasingly intense
beginning late D2 into D3 (and beyond) as a NW to SE oriented
upper jet streak reaching 130+kts surges into the region leaving
the favorable LFQ diffluent portion overhead. This pacific jet
streak will additionally add moisture into the column, and PW
anomalies are progged to exceed +2 standard deviations above the
climo mean Monday and Tuesday.

This setup will produce waves of moderate snow associated with
each shortwave impulse, with the stronger impulse Monday driving a
clipper type low into the Upper Midwest/Great Lakes, followed by a
trailing southward sinking cold front. By D3, the setup becomes
quite impressive for heavy snow with the anomalous PWs being acted
upon by the deep synoptic lift through height falls, PVA, and LFQ
diffluence, with additional ascent developing along the cold front
through strengthening low and mid-level frontogenesis, as well as
increasing upslope flow behind the front. This is what will likely
drive the more widespread and heavy snow as far east as the
northern High Plains, and ECMWF EFI is already around 0.95 for
MT/WY D3-4. Snow levels through the period will waver between 2000
and 3000 ft, and may briefly rise D3 to 3500 ft with pre-frontal
WAA, but should begin to crash late in the forecast period. With
increasing omega driving heavier snow rates D3, accumulations are
likely to begin to spread out of the terrain and into lower
elevations.

WPC probabilities for more than 6 inches of snow on D1 are 50-80%
beneath the first shortwave, but confined to generally terrain
above 3000 ft in the Bitterroots, Northern Rockies, Absarokas, and
Tetons where locally 12+ inches is possible. For D2, the more
expansive snow begins to spread southeast from the WA Cascades
through much of the Northern Rockies and again into the NW WY
ranges where WPC probabilities for more than 6 inches reach as
high as 70% once again. However, the D3 WPC probabilities for 6+
inches become much more elevated and expansive, reaching 80+% in
the higher terrain, with 10-30% expanding even into the northern
High Plains of MT. More than 5 feet of snow is possible in some of
the higher terrain through D3, with more snowfall likely beyond
this period.

The heavy snow on D3 is likely the beginning of a long duration,
widespread, significant winter storm which will spread across much
of the northern tier of the country, and is described in the key
messages below.


...Upper Midwest...
Day 3...

A clipper type low pressure will strengthen beneath a sharp
shortwave trough digging out of Alberta and dropping southeast
into the Upper Midwest Monday into Tuesday. The guidance has
become more amplified and robust with this feature, and despite
its progressive nature it will likely feature impressive
downstream warm/moist advection, with a brief period of strong
280K moist isentropic upglide into the saturated DGZ. The warm
advection into this layer will drive a deepening DGZ supportive of
larger aggregates, which will help produce above climo SLR in what
is otherwise a cold atmospheric column. This will result in
moderate snow accumulations for which WPC probabilities for 4+
inches reach 20-40%, highest W/NW of Lake Superior where some
enhanced lake moisture and combined with upslope flow into the
Iron Ranges will support more impressive omega to wring out
available moisture as snow.

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

Weiss


...Key Messages for Major Winter Storm Next Week...

--Confidence is increasing that a major winter storm will spread a
large swath of heavy snow from the West Coast to the Upper Midwest
Monday through Thursday of next week.

--The combination of heavy snow and gusty winds will likely result
in widespread hazardous travel and may cause impacts to
infrastructure.

--Uncertainty remains with timing and storm track which will
determine where the axis of heaviest snowfall and most significant
impacts occur, but it is becoming more likely that this system
will be extremely disruptive.

--Very cold temperatures are likely along the West Coast and in
the Northern Plains behind this system, with record lows possible.

--Additionally, heavy mixed precipitation including snow, sleet,
and freezing rain may impact the Great Lakes and Northeast later
next week.