FOUS11 KWBC 230706
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 AM EDT Thu Apr 23 2026

Valid 12Z Thu Apr 23 2026 - 12Z Sun Apr 26 2026

...Northern Rockies to the High Plains... Days 1-2...

A 500mb vorticity maximum diving south from Alberta will interact
with a negatively-tilted disturbance over eastern MT to produce a
large and strengthening closed-low over south-central Canada
Thursday night. The 500mb pattern over North America by Friday
features a +PNA and -NAO regime that effectively weakens the 500mb
zonal flow over the continent and causes this closed low to remain
quasi-stationary into the upcoming weekend. A steady stream of
700-300mb moisture on the western flank of the strengthening upper
low will be placed over the Northern Rockies while, at the same
time, surface high pressure over western Canada builds in. This
combination of easterly upslope flow via strengthening high
pressure to the north, along with a more than sufficient 850-700mb
CAA aloft will support periods of moderate-to-heavy snow over much
of the Northern Rockies, including ranges as far south as the
Tetons, Wind River, and Big Horns. The heaviest snow will reside
over the Bitterroots, Lewis Range, and Absaroka where upslope flow
and the core of the coldest temperatures aloft will reside the
longest with snow lasting through the day on Friday. Snow levels
will crash as low as 2,000ft in western MT and northern ID.

Perhaps the trickiest aspect of this forecast is the deformation
zone banding on the western and southwest flank of the develop
upper low this morning. Around 12Z, latest 00Z CAMs show the
atmospheric column cooling enough to support moderate-to-heavy snow
in northeast MT. After subsiding Thursday afternoon, 500mb
vorticity maxima revolving around the western flank of the closed
low looks to reinvigorate snow showers Thursday night and into
Friday morning. The NAM CIPS snow squall parameter shows a staunch
signal for snow squalls early Friday morning that could still
persist through the day given the unusually cold temperatures aloft
when combined with strong surface based heating during the day.
Snowfall totals are likely only to be around a coating to 2"in
these areas, but given north of the MT border, a narrow axis of >6"
snowfall totals is depicted in southern Saskatchewan. Any 50-100
mile shift south in the TROWAL axis over southern Canada could lead
to locally heavier amounts over northern MT that surpass 6".

WPC probabilities for this event show moderate-to-high chances
(50-80%) for snowfall totals >12" in the peaks of the Northern
Rockies above 6,000ft. Some hazardous travel impacts at pass level
are likely, although with NOHRSC showing many of these mountain
ranges with below normal snow depth for late April per NOHRSC, most
of the expected snowfall will be welcome. WPC probabilities also
snow a small portion of northeast MT with moderate-to-high chances
(50-80%) where snowfall totals >4" are depicted, suggesting some
members of the WPC super ensemble do in fact show the heavy snow
banding potentially protruding into northeast MT. The WSSI
generally shows Minor to locally Moderate Impacts in all of these
aforementioned locations with Snow Amounts being the primary driver
in impact over the Northern Rockies. The WSSI suggest Blowing Snow
is the bigger potential impact from the Little Belt and Big Snowy
Mountains on east into the High Plains of northern MT.

...Sierra Nevada & Central Rockies... Day 3...

A 500mb shortwave trough rotating around the southern flank of a
500mb low off the CA coast will track into the Southwest U.S. on
Saturday, bringing with it a slug of 700-300mb moisture.
Temperatures aloft are not overly cold, but healthy 500mb
vorticity advection coupled with the diffluent left-exit region of
a 250mb jet streak aloft supports vertical ascent. Plus, a 250-500
kg/m/s IVT over the Southwest supplies the Pacific moisture needed
to foster mountain snow. The lack of a very cold air-mass and the
progressive nature of this shortwave trough will keep snowfall
amounts more beneficial than harmful at a time where these mountain
ranges could use more snowpack. WPC probabilities show moderate-
to-high chances (50-70%) for snowfall totals >4" in southern
Sierra Nevada above 8,000ft and the more remote reaches of the
Wasatch, San Juans, Uinta, and central CO Rockies.

The probability for significant icing over one-tenth of an inch is
less than 10 percent.

Mullinax

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