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

Valid 12Z Tue Oct 24 2023 - 12Z Fri Oct 27 2023

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

A significant early-season winter storm will begin tonight in the
Northern Rockies and Cascades and then expand into the High Plains
and Northern Plains through Friday.

A 500mb trough will dig along the Pacific coast and into the
Pacific Northwest by Wednesday morning and then amplify into the
Northern Great Basin by Thursday before slowly opening and then
advecting northeast into the Dakotas by the end of the forecast
period. This evolution will drive periodic PVA as vorticity lobes
eject from the primary trough, with downstream height falls and
divergence additionally providing ascent. At the same time,
complex upper level jet evolution will produce enhanced diffluence
through coupled jet streaks early, with more impressive diffluence
aloft developing as a primary jet strengthens over Canada Wed-Thu
leaving its impressive RRQ draped from the Northern Great Basin
through the Northern Plains. This synoptic level lift will act
upon a low-level baroclinic gradient as a surface cold front drops
southward from Canada, resulting in multiple waves of low pressure
moving E/NE through the period. This will lead to at least two
rounds of heavy snow moving across the region, and although
confidence in the exact timing and placement remains modest, there
is a high likelihood for heavy snowfall each day during the period.

The first wave of heavy snowfall is expected to develop tonight
across the Cascades and Northern Rockies as impressive divergence
and increasing Pacific moisture overlap to produce precipitation.
Snow levels will drop rapidly from around 4000-5000 ft early, to
2000 ft in the Cascades, and down to the surface in the High
Plains. Moisture spreading across the area will fall as rounds of
heavy snow, especially in the Cascades and Northern Rockies, but
eventually expand into the High Plains. Although the accompanying
surface wave for this first part of the event appears modest, the
synoptic setup will support the potential for a band of heavy snow
translating northeast across MT and into ND. This will occur in
tandem with a cooling column and lowering DGZ, with impressive
850-600mb fgen driving pronounced ascent into this snow growth
zone. This could support snowfall rates in excess of 1"/hr despite
modest (but increasing) SLR. The fluffiest snow is likely across
the High Plains, and this could accumulate rapidly noted by WPC
probabilities D1-D2 as high as 60% for 6+ inches during this first
band. The nature of this band, which may move along its long axis,
could drive impressive snowfall gradients with some areas
receiving both much more, and much less, than this. Farther west
into the terrain, SLR will likely be more modest, but impressive
synoptic lift and upslope flow could still produce heavy snow
exceeding 12 inches in the Cascades, as well as portions of the
Absarokas and Crazy Mountains on D1. Additionally, with the snow
levels lowering to around 2000 ft, there are moderate to high
probabilities for more than 4 inches of snow at many of the
Cascade passes, including Snoqualmie, Stevens, and Santiam Passes.

This first wave will likely move progressively into ND during D2
with continued heavy snow possible within the band NW of the
surface wave. However, only a brief break may occur before even
more impressive moisture shifts eastward along a second wave of
low pressure. Isentropic ascent on the 295-305K surface beneath
the intensifying Pacific jet streak will drive plentiful moisture
into the region, and this will encounter continued lowering of
snow levels. This will have a two pronged effect of spreading
heavy snow back across areas that received snow from the first
wave, but this time with more efficient accumulation due to higher
SLRs thanks to the colder column and a DGZ depth for which the
SREF suggests has a >50% probability for exceeding 50mb in many
areas. The low developing near the Rockies will more effectively
transport moisture northward within a modest theta-e ridge
building downstream, which will be wrung out through strong fgen
and upslope flow overlapping near the NW WY ranges and into
southern MT. This secondary wave appears to remain progressive as
well as it gets embedded into stronger westerlies shearing out the
primary trough, but there could again be a band of heavy snow that
develops on the NW side of this wave shifting eastward again into
ND by the end of the forecast period. The highest WPC
probabilities for more than 6 inches of snow D2-D3 are across the
NW WY ranges and into the Absarokas where they exceed 90%, and
some of the higher terrain above 6000 ft could receive up to 2
feet of snow. Lesser additional accumulations are likely in the
Cascades and Sawtooth/Salmon River ranges as moisture begins to
erode from west to east, especially by D3. As the surface low
tries to consolidate across the Rockies and move eastward, the
overlap of better ascent and some increasing moisture could result
in a second stripe of heavy snow from eastern MT into ND where WPC
probabilities for 6+ inches peak above 70%. Locations that receive
both of these waves could see 12-18 inches of snow.


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

-- A long duration winter storm will begin today across the
Pacific Northwest and Northern Rockies, and continue through the
end of the week through 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 Rockies as far south as Wyoming. This
will cause hazardous travel at many of the mountain passes.

-- Multiple waves of heavy snow will stretch across Montana and
into North Dakota Tuesday night 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.

-- These bands will likely result in difficult travel due to snow
covered or icy roads, and poor visibility.

Weiss