Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
132 PM EST Mon Nov 13 2023

Valid 00Z Tue Nov 14 2023 - 00Z Fri Nov 17 2023

...Northern Rockies...
Days 2-3...

The guidance has become a bit more aggressive with snow from the
Northern Rockies into parts of the Northern High Plains beginning
late Wednesday and persisting into early Thursday.

A confluence of moisture streams will merge over the Northern
Rockies Wednesday in response to a deepening trough west of CA and
a secondary shortwave digging out of British Columbia. This second
shortwave is progged to deepen as it moves into MT Wednesday
night, combining with increasingly coupled jet streaks to drive
impressive synoptic ascent over the area. Although PWs are
forecast to be near normal according to NAEFS ensemble tables by
Thursday, any available moisture should be wrung out efficiently
by this ascent. At the same time, a warm front will lift northward
during Wednesday, with the associated WAA providing additional
ascent through modest 295K isentropic ascent and enhanced moisture
advection. The overlap of the robust synoptic lift and this front
could result in surface low pressure development as well, which
will move eastward from ID to WY by D3.

In general, these features are expected to be transient which will
limit total snowfall. However, some upslope flow into the terrain
of the Northern Rockies, especially as the front sags back to the
south as a cold front resulting in post-frontal westerly upslope
snow, could accumulate significantly, especially above 3000 ft
which could impact some of the local passes. WPC probabilities D2
and D3 are around 30-40% for 6+ inches, peaking above 80% in the
24-hr period ending D2.5 /12Z Thursday/. However, event total
snow, especially in the higher terrain around Glacier N.P. could
exceed 12 inches.

Additionally, the synoptic evolution is somewhat representative by
D3 of supporting a translating SW to NE snowband across the High
Plains of MT and ND. An axis of strong 850-600mb fgen north of the
warm front may overlap with some mid-level deformation, which will
enhance the ascent already provided via the synoptic setup. Cross
sections through this region suggest at least a threat of CSI as
theta-e lapse rates exceed the geostrophic momentum, with the best
ascent aligning directly into the saturated DGZ. The depth of the
DGZ is modest as reflected by SREF DGZ depth probabilities for
50mb, and there is potential for some rapid column drying to limit
precipitation duration, but some enhanced snowfall rates are
possible Wednesday night as this band pivots northeastward. WPC
probabilities for more than 1 inch are modest across
central/eastern MT outside of terrain features, but should be
monitored with future model cycles for a possible increase.

Weiss