869
FOUS11 KWBC 290658
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Mon Apr 29 2024

Valid 12Z Mon Apr 29 2024 - 12Z Thu May 02 2024

...Pacific Northwest and Northern Rockies...
Days 1-3...

Active winter weather will continue across a lot of the western
CONUS through mid-week as dual shortwaves evolve into a larger
scale trough emerging across the region. The lead shortwave will
race east, moving onshore the OR coast this morning and then
pivoting into eastern MT while maintaining amplitude through
Tuesday morning. Immediately in the wake of this shortwave, a
second impulse will race into OR Tuesday morning, and then the
interaction of these features will force a closed mid-level low to
broaden over the northern High Plains by Wednesday. This trailing
shortwave, despite being absorbed into the larger gyre aloft, will
then spin east into the Central Rockies, continuing large scale
ascent through D3.

While the primary forcing will be PVA/height falls associated with
this mid-level evolution, there will be additional contributions to
lift through upper diffluence as a pair of jet streaks downstream
of each shortwave also pivots to the northeast providing diffluence
aloft. The mid-level flow will generally be zonal until the trough
closes off D2-D3, which will additional enhance lift via upslope,
so many of the mountain ranges will see enhanced precipitation this
period, with shadows likely downstream. This precipitation will
fall as moderate to at times heavy snow above generally 3000-4000
ft, but will lower to below 2000 ft in the Cascades at times,
resulting in impactful snow at area passes.

While most of the snowfall appears disorganized except in
persistent upslope regions, there continues to be increasing
confidence in an axis of heavy snow pivoting south into the
Northern Rockies near Glacier NP on Wednesday. This enhancement
will be due to an inverted trough extending from an occluded and
retrograding surface low, which will drop into the area from
Canada. This trough will be accompanied by enhanced moisture as a
modest TROWAL pivots above it, with ascent also intensifying
through E/NE upslope flow and an axis of fgen. A significant late-
season snow event is becoming more likely, and this is reflected by
WPC probabilities for more than 6 inches reaching 60-80% across the
Lewis Range and Little Belt Mountains, with locally more than 12
inches possible, especially above 4000 ft.

Elsewhere, WPC probabilities D1 and D2 are moderate to high
(50-80%) for more than 6 inches across the Cascades and Olympics,
although D1 will likely feature more considerable impacts. During
this time, snow accumulations below pass level will likely result
in hazardous travel at many passes including Snoqualmie, Stevens,
and Santiam. Heavy snow exceeding 6 inches is also likely into
parts of ID around the Bitterroot and Salmon River ranges on D1.


...Minnesota...
Day 1...

A band of precipitation associated with WAA along an elevated front
will be lifting across northern MN to start the period, although
the heaviest precipitation should wane quickly Monday morning.
However, enough isentropic ascent between 290K-300K through the
afternoon will still result in showery precipitation through the
day. Although this precip should generally be light, it could still
result in some light icing as the DGZ dries out leaving periods of
freezing drizzle, especially in the higher terrain of the Iron
Ranges where additional upslope flow could enhance lift. Additional
icing accretion should be light, only a few hundredths of an inch
as shown by WPC probabilities for 0.01" of just 10-30%, but storm-
total icing could exceed 0.1" in a few isolated locations.

Weiss


$$