Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sun Oct 22 2023

Valid 12Z Sun Oct 22 2023 - 12Z Wed Oct 25 2023

...Pacific Northwest through the Northern High Plains...
Days 2-3...

Widespread moderate to heavy snowfall is becoming more likely from
portions of the Cascades eastward through the Northern Rockies and
potentially into the Northern High Plains by the end of the
forecast period. A complex and multi-faceted evolution is
producing lower than typical confidence, however.

The primary driver of this event will be a deepening shortwave
which will dig into a closed low near across the Pacific Northwest
Tuesday night. This will result in downstream divergence for
ascent, overlapping with at least a weakly coupled jet structure
as one jet streak arcs into Manitoba while a more impressive jet
max approaches from the Pacific. There is quite a bit of spread in
the amplitudes of these features, and the DESI clusters suggest
two primary camps. The GFS/GEFS makes up one camp which is faster
and more amplified than the CMC/ECMWF and their ensembles. The NAM
is also in this slower, less amplified, camp. Synoptically it
appears less realistic that the deeper trough would move
progressively into the downstream ridge over the eastern CONUS,
and the trends in the GEFS from DESI have been slower. This
suggests a solution more towards the ECMWF/CMC is preferred, but
with some influence for the more robust GFS included. With a
surface front dropping southward out of Canada D2-D3, there will
likely be robust ascent as enhanced 850-600mb fgen occurs beneath
the strong synoptic lift, which is additionally aided by
post-frontal upslope flow. This is likely regardless of the model
choice, so periods of heavy snow appear likely, with the challenge
focused around snow levels and how quickly the cold air can sink
southward, in addition to the placement of the most intense ascent.

Due to the significant spread, the NBM was used heavily for the
forecast, both for snow levels and SLR. The snow levels should
fall from around 5000 ft to start D2 to 2000-3000 ft in the
Cascades/below 1000 ft in the Northern High Plains by the end of
D3. SLRs should climb to near Baxter climo values as the column
cools due to the lowering of the fully saturated DGZ with SREF
indicated DGZ depths featuring a greater than 30% chance for
exceeding 50mb. This will support rounds of heavy snow, first near
Glacier N.P. on D2, where WPC probabilities for more than 4 inches
reach 50%, before expanding significantly on D3 to include the
Cascades, more of the Northern Rockies, and into the Northern High
Plains. WPC probabilities for more than 4 inches on D3 reach
70-80% across the WA Cascades, with a secondary max also in the
Big Snowy, Little Belt, and Crazy Mountains. Although confidence
is moderate at this time, local maxima above 12 inches are likely
in some of the higher mountain peaks.

Due to the uncertainty and spread, WSSI-P for this event is
currently quite modest for moderate impacts, but it is becoming
more likely that impactful snow will affect the Cascade passes of
Snoqualmie and Stevens, as well as the areas around Great Falls,
MT.


Weiss