Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Mon Oct 23 2023

Valid 12Z Mon Oct 23 2023 - 12Z Thu Oct 26 2023

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

An early season significant winter storm continues to appear more
likely beginning tonight with multiple rounds of moderate to heavy
snow possible.

The guidance has gotten into at least somewhat better agreement
tonight which is producing increased confidence that a significant
winter storm is going to impact portions of the Pacific Northwest
through the Northern High Plains beginning tonight. The GFS/GEFS
cluster, which was in its own camp last night, is now supported by
many of the CMCE members with a deeper trough by D3-4 across the
Inter-Mountain West according to the DESI site, with the EC/ECE
members driving a majority of the slower cluster members. The
downstream ridge over the eastern 1/3 of the CONUS is strong (by
12Z Thursday more than 40% of the NAEFS members suggest extreme
heights over the Carolinas/Mid-Atlantic) which could result in a
slower progression to the east. This suggests that the ECE cluster
cannot be ruled out, but is used with a small percentage in the
thermal blend for this update.

The primary driver of this event will be a potent closed low that
will dig into the Pacific Northwest late Tuesday, shed a vorticity
lobe eastward into the High Plains late Wednesday, and then
re-strengthen and evolve a neutral-to-negative tilt near the
northern Great Basin by the end of the forecast period. A complex
jet structure will accompany this evolution, with weakly coupled
jet streaks eventually merging into a large polar jet streak
placing impressive, but transient, upper diffluence over the
region. At the same time, a surface cold front will drop out of
Canada Tuesday morning and then continue to sink southward towards
the Central Rockies and Northern Great Basin by Thursday. The
influence of this front will be to drive increasing post-frontal
upslope flow into the terrain, while also providing intensifying
850-600mb fgen to drive more robust lift. The result of this
overlap of ascent will likely be two waves of precipitation, one
moving eastward from Tuesday night through Wednesday, with a
second, potentially more intense area developing east of the
northern Great Basin D3.

This should produce multiple waves of snowfall through the
forecast period. The first will be associated with the shedding
vort and lead but weak surface wave. This should bring heavy snow
to the Cascades and into the Northern Rockies where WPC
probabilities for more than 6 inches peak at 60-80%, highest in
the Cascades and Olympics, with snow levels gradually falling to
just 1500-2500 ft. This could result in accumulating snow at pass
level, including Snoqualmie and Stevens Passes. Additionally, as
the wave moves eastward late Tuesday night into Wednesday, the
synoptic setup seems to support a narrow band of heavy snow moving
northeast through much of MT and into western ND. There is low
confidence in this evolution, but with a cooling airmass and
deepening DGZ, high SLRs within this band could produce a stripe
of heavy snow well into the Plains. Although confidence in
placement and SLRs is lower than normal, current WPC probabilities
across this area reach as high as 30-60% for 6+ inches of snow,
with locally higher amounts possible should this band translate
efficiently.

Finally, during D3 the secondary low begins to organize with more
impressive moisture advection surging northeastward from the
Pacific noted by impressive isentropic ascent to fuel additional
heavy snowfall within the strong dynamics due to fgen/upslope
flow, especially in terrain around NW WY. Confidence in placement
and amounts is even lower this far out in time, but confidence is
high in heavy snow at least above 6000 ft near Yellowstone N.P.
WPC probabilities for more than 6 inches of snow are above 80% in
the Tetons, Absarokas, and other surrounding ranges on D3, with
locally 1.5 feet possible in the highest terrain.

This event will likely continue into D4 and spread additional
heavy snow farther east into the High Plains.

Weiss