Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
421 PM EDT Tue Oct 24 2023

Valid 00Z Wed Oct 25 2023 - 00Z Sat Oct 28 2023

...Pacific Northwest through the Northern Plains...
Days 1-3...

A multi-phase winter storm is expected to bring periods of heavy
snow from portions of the Pacific Northwest to the northern Plains
into the latter part of the week.

A deep mid-to-upper level trough centered over British Columbia
will continue to dig south through the evening, with a closed low
forecast to develop near the Washington coast early Wednesday. 
The system is forecast to settle farther southeast across
Washington and Oregon on Wednesday before transitioning to an open
wave and ejecting east across the northern Intermountain West and
the northern Rockies on Thursday, and then the northern Plains
Thursday night into early Friday.

At least two rounds of heavy snow are expected to translate east
across the region.  Mid level energy ejecting east of the low,
along with favorable upper jet forcing, will promote ascent and
multiple waves moving east along a low level baroclinic zone
sagging south across the region.  This initial round is expected
to impact the Cascades to the northern High Plains through
tomorrow, with widespread coverage of snow accumulations greater
than 4 inches covering the northern Cascades, and from the
northern Rockies to northwest North Dakota.  Snow levels in the
Cascades are expected to drop below 2000ft in the Washington
Cascades and 3000ft in Oregon, with accumulating snow likely for
the passes beginning tonight and continuing into Wednesday. 

This initial round will continue to move east across North Dakota
into northwest Minnesota as a second organized round develops
farther west.  Strong forcing associated with the ejecting upper
low, interacting with a modest atmospheric river spreading inland
will support heavy snow developing across portions of the northern
to central Rockies late Wednesday into Thursday.  This next round
is forecast to develop farther south, with greater impacts over
the southern Montana and the western to central Wyoming ranges. 
Upslope flow, enhanced by an area of low pressure deepening over
western Wyoming, is expected to raise the threat for heavy
accumulations across this region. 

As the upper wave ejects east, snow will once again spread into
the northern Plains on Thursday before diminishing from west to
east across the Rockies Thursday night and the northern Plains by
early Friday.

In additional to the higher peaks of the northern Cascades, WPC
probabilities indicate that multi-day totals of 8 inches of snow
or more are likely (greater than 80 percent) across portions of
the northern Rockies from west-central Montana to northwest
Wyoming, and the northern Plains from east-central Montana to
north-central North Dakota. 


***Key Messages for the Pacific Northwest through the Northern
Plains Winter Storm***

-- A long duration winter storm starting today across the Pacific
Northwest and Northern Rockies will continue through the end of
the week across portions of the Northern Plains.

--There is high chance (80%+) for more than 8 inches of snow, with
locally 1-2 feet likely, in the higher terrain of the Cascades and
portions of the Northern Rockies. This will cause hazardous travel
at many of the mountain passes.

--Multiple waves of heavy snow will stretch across Montana and
into North Dakota tonight through Friday morning. Bands of heavy
snow with rates of 1"/hr have a high chance (70%+) of producing
more than 8 inches of snowfall in this region, with a low chance
(10-30%) of locally 18 inches.

--Travel may become difficult due to snow-covered or icy roads and
poor visibility. Temperatures will fall into the teens over much
of the region by Thursday night.


Pereira