Winter Storm Warning 14, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
344 AM MST Tue Dec 17 2024

.DISCUSSION...

KEY MESSAGES:

- Mixed precipitation, including freezing rain possible Tuesday
  night into Wednesday morning

- Strong gusty winds Wednesday afternoon, especially in the higher
  terrain and in southwest Montana

- High pressure late in the week introduces fog and low stratus

Light showers persist this morning across north-central Idaho and areas
south of the Interstate 90 corridor in Montana. Winter Weather Advisories
remain in effect through this morning due to slick conditions caused by
light snow. Snow showers may continue into the afternoon; however, overall
impacts should be minimal as temperatures are expected to rise above
freezing in many valley locations later today.

Snow levels will rise Tuesday night into Wednesday, reaching between 5,000
and 6,000 feet by Wednesday afternoon. Simultaneously, an atmospheric river
will funnel significant moisture into the region. In the valleys,
precipitation will transition from snow to rain during this time, with a
10-15% chance of freezing rain in areas where cold air remains trapped near
the surface, primarily late tonight into Wednesday morning. The greatest
risk for freezing rain is in northwest Montana, where ground temperatures
are coldest and moisture is most concentrated.  Winter weather highlights
will likely be needed to account for slick conditions due to snow and ice
for the Wednesday morning commute.

In higher elevations, heavy, wet snow is expected. Forecast models show a
70-80% chance of 6 inches or more of snow accumulating above 5,000 feet in
northwest Montana during the 24 hours ending at 5 PM Wednesday.

Additionally, a strong jet stream on Wednesday will bring an increased risk
of gusty westerly winds across western Montana. Valleys have a 50-70%
chance of seeing widespread gusts exceeding 30 mph in the afternoon and
evening, with a 20-30% chance of gusts surpassing 40 mph in the Bitterroot
Valley and Butte/Anaconda regions.  The strongest winds may be realized in
the Butte/Anaconda area where gusts over 50 mph are not out of the
question.  A high wind watch has been added.  The strong winds will likely
subdue precipitation amounts for locations south of Interstate 90 due to
downsloping effects.

Looking ahead, a ridge of high pressure is expected to build over the
Northern Rockies Thursday and Friday, increasing the potential for fog and
low stratus clouds each day. About 65% of forecast models suggest the ridge
will persist into Christmas Eve, which could disrupt holiday travel.
However, some models indicate a trough of low pressure could move in,
bringing mountain snow and a rain/snow mix to the valleys.

&&

.AVIATION...Snow showers persist this morning across north central
Idaho into southwest Montana. Brief lowering of the visibility and
ceiling is possible, including for KHRF, KBTM and KSMN through
17/1500z. Widespread obscurations of the higher terrain are
expected across all of the Northern Rockies region due to the
development of low to mid level stratus. Some isolated fog is also
possible this morning, mainly north of the Interstate 90 corridor.
The next weather system quickly approached by this afternoon,
spreading snow and low elevation rain into the region. Snow levels
will begin to increase overnight tonight into Wednesday, allowing
for a transition from snow to rain. A period of freezing rain is
likely to occur as this transition takes place tonight into
Wednesday morning, mainly impacting KGPI, but also possibly KMSO.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 9 AM MST this morning for
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...
     Potomac/Seeley Lake Region...West Glacier Region.

     High Wind Watch from Wednesday morning through Wednesday evening
     for Butte/Blackfoot Region.

ID...Winter Weather Advisory until 8 AM PST this morning for Northern
     Clearwater Mountains...Southern Clearwater Mountains.

&&