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

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

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

In wake of a cold frontal passage this afternoon, a series of
upper level disturbances rotating beneath the base of an upper
level low along the Montana/Canada border will bring about periods
of mountain snow through the first half of the work-week. At 250mb,
the mountain ranges of the Pacific Northwest and Northern Rockies
will become located beneath the divergent left-exit region of a jet
streak to help maximize vertical ascent within the column through
Tuesday afternoon. At the same time, a steady stream of 850-700mb
moisture will be readily available from the Washington Peninsula
to the Northern Rockies through Tuesday. Add in mean layer winds
within the 850-300mb layer are primarily out of the west and this
should support some upslope enhancement (especially in the Olympics
and Cascades.

By Wednesday morning, a positively tilted vorticity max moving
through the Northern Rockies will spur additional high elevation
snowfall in mountain ranges that include the Bitterroots, Lewis
Range, Tetons Absaroka, and as far east as the Little Belt and Big
Snowy Mountains of central Montana. Cyclonic flow on the back side
of the upper low will continue to support periods of mountain snow
in these regions through Wednesday night. Some mountain snow may
linger into Thursday, but with the air-mass in place gradually
modifying, accumulations will be tougher to come by on Thursday.
Meanwhile, the next upper level disturbance in the northeast
Pacific approaches the Pacific Northwest coast early Thursday
morning. Expect some additional snowfall in the highest peaks of
the Cascade Range, but little in the way of accumulations <5,000ft.

For the duration of the short range (through 00Z Friday), the Lewis
Range of northwest Montana (including Glacier NPS) sports
moderate-to-high chances (50-70%) for snowfall totals >18". The
WSSI does indicate the potential for Moderate Impacts in and around
Glacier NPS. Similar probabilities are present in the tallest
peaks of the Oregon Cascades for snowfall totals >12", as well as
the
Big Snowy, Little Belt, and Absaroka >7,000ft. Lastly, moderate
chances (40-60%) for snowfall totals >8" are present in parts of
the Big Horns, Wind River Range, and northern Colorado Rockies.
Given the time of year, these totals will vary significantly by
elevation with the WSSI showing some Minor to highly localized
Moderate Impacts in portions of these mountain ranges through
Thursday evening.

The probability of significant icing is less than 10 percent.

Mullinax