Winter Storm and Heavy Snow in Idaho, February 2 to 5, 2025

Area Forecast Discussion
National Weather Service Missoula MT
242 PM MST Sun Feb 2 2025

.DISCUSSION...

KEY MESSAGES:

- Sunday-Monday: Arctic airmass will move west of the Continental
  Divide, bringing colder temperatures and snow

- Mid day Monday through Wednesday: Arctic overrunning event
  producing widespread moderate to heavy snow and wind, especially
  along and south of Interstate 90

Little has changed regarding the forecast going into Tuesday. An
arctic airmass is currently banked up against the Continental
Divide this afternoon, and is expected to filter into northwest
Montana this evening and through the rest of western Montana
Monday morning.

At the same time, satellite imagery depicts a broad plume of
subtropical moisture that will be moving into the region from the
southwest. With the arctic boundary remaining stalled across
western Montana for much of the upcoming week, the incoming
moisture will ride up and over the boundary producing periods of
moderate to heavy snow. There are two main periods of concern we
are monitoring.

The first comes Monday afternoon into Tuesday morning. Overall, we
have higher confidence in a high impact event occurring during
this period, generally along and south of the Interstate 90
corridor from roughly Missoula to Butte. Moderate to heavy snow
will be accompanied by gusty northeast to east winds, as high as
35 to 40 mph through aligned terrain features. This will translate
to blowing and drifting snow and low visibility. Through north
central Idaho, winds are not anticipated to be as strong. However,
heavy snow could make travel difficult, especially over mountain
passes. Further south, in Lemhi county, warmer air in place may
cause precipitation type to switch over to rain for locations such
as Salmon on Tuesday. With very cold ground temperatures in
place, it is possible that icy conditions will develop. Finally,
the Camas Prairie in Idaho will likely see a prolonged period of
moderate snowfall. Overall, numerous winter weather products are
in place to cover challenging winter conditions.

The next period of concern comes Tuesday night into Wednesday
morning. Confidence is still somewhat lower with this push in
terms of how far north and west it will traverse into the region.
This is especially true across northwest Montana, where 50
percent of models show little-to-no snow while the other 50
percent show a moderate to potentially heavy snowfall. Because of
these remaining questions, we have elected to keep the winter
storm watch in place until details become more clear. Along and
south of the Interstate 90 corridor, there is a 60 percent chance
that another round of heavy snow develop. Some forecast models are
showing significant amounts of moisture, which could bring even
more impressive snowfall amounts that the first push.

Bottom line: travel will be difficult and changeable this week. If
you have travel plans this week, we recommend keeping an eye on
the forecast as further adjustments/changes are likely.

Beyond Wednesday: temperatures will likely moderate slightly.
However, more surges of moisture from the southwest look likely,
thus snow will likely not be coming to an end anytime soon. Stay
tuned!

&&

.AVIATION...The approaching Arctic front is currently banked up
against the Continental Divide in northwest Montana and will
progress south and west into northwest Montana through the evening.
Northeast winds will increase after 03/0300z gusting to 35kts, at
KGPI. Through 03/1200z, there is less confidence that Arctic
winds will begin to affect other terminals which we produce TAFs
for though most locations will see a change to generally more of
a northeast to east direction. For locations along and south of
Interstate 90, including KMSO,KBTM, KHRF and KSMN, light-to-moderate
snow will continue this afternoon, but gradually decrease through
the evening. For locations across northwest Montana, including
KGPI, light snow will likely develop early this evening into the
overnight hours as moisture overrides the approaching arctic
boundary. Obscurations of the terrain will be widespread across
the Northern Rockies into Monday due to lowered ceilings and snow.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 11 AM Monday to 11 AM MST Wednesday
     for Bitterroot/Sapphire Mountains...Butte/Blackfoot
     Region...Potomac/Seeley Lake Region.

     Winter Weather Advisory until 11 AM MST Monday for
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...
     West Glacier Region.

     Winter Weather Advisory until 11 AM MST Monday for
     Flathead/Mission Valleys...Kootenai/Cabinet Region.

     Winter Storm Watch from late Monday night through Wednesday
     morning for Flathead/Mission Valleys...Kootenai/Cabinet
     Region...West Glacier Region.

     Winter Weather Advisory from 11 AM Monday to 11 AM MST Wednesday
     for Lower Clark Fork Region.

     Winter Storm Warning from 5 PM Monday to 11 AM MST Tuesday for
     Missoula/Bitterroot Valleys.

ID...Winter Storm Warning from 10 AM Monday to 10 AM PST Wednesday
     for Northern Clearwater Mountains...Southern Clearwater
     Mountains.

     Winter Weather Advisory until 10 AM PST Monday for Southern
     Clearwater Mountains.

     Winter Weather Advisory from 4 PM Monday to 10 AM PST Wednesday
     for Orofino/Grangeville Region.

     Winter Weather Advisory until 11 AM MST Wednesday for Eastern
     Lemhi County...Western Lemhi County.

&&