495
FXUS65 KMSO 170942
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
342 AM MDT Sat May 17 2025

.DISCUSSION...

KEY MESSAGES:

- Wet system to bring moderate precipitation rates Saturday night
  into Sunday

- Moderate to high confidence in accumulating snow for the higher
  terrain down to 5000 feet, including mountains passes

- 20 to 30 percent chance for accumulating snow down to 3000 feet
  for Missoula valley, Mission valley and Seeley/Swan valleys
  Sunday morning

Little has changed over the last 12 hours in terms of the upper
level set up for an incoming low pressure system later tonight
into Sunday. Forecast models continue to show support for a
moderate to heavy precipitation event as the upper level low cuts
off over north central Idaho and taps into a surge of gulf
moisture. At the same time, a mid level circulation spins up in
southwest Montana, with the majority of global forecast models
developing a deformation band through portions of Idaho county and
eastwards into west central Montana. As high resolution models
have recently begun to extend out into this timeframe, they are
largely in agreement with the global forecast models. Middle of
the road precipitation amounts show precipitation amounts ranging
from 0.75 inches to 1.25 inches by Sunday evening, mainly focused
along and north of Highway 12.

Confidence is high in moderate to heavy snowfall amounts
occurring in the higher terrain down to roughly 5000 feet. Slushy
back country roads are anticipated and thus numerous winter storm
watches are in effect. Even high mountain passes such as Lolo and
Lost Trail will likely see a period of winter like travel by
Sunday morning.

The more difficult call is how low will snow fall. Even the
coldest forecast models only show snow levels dropping to roughly
4500 feet. However, Spring storm systems such as this one have a
history of lowering snow levels as much as 1000 to 2000 feet lower
than forecast models suggest under the heavier deformation band
areas. This means snow is still not out of the question Sunday
morning down as low as 3000 feet, especially for the Missoula and
northern Bitterroot valleys, Mission valley, and Seeley/Swan
valley. Current guidance shows a roughly 20 to 30 percent chance
of these locations receiving snowfall. Though confidence remains
low, it should be noted that heavy wet snow on foliage could cause
tree damage and possibly bring areas of power outages if heavies
valley snow does develop.

With abundant moisture, patchy fog is possible Monday and Tuesday
mornings. A cool, showery pattern continues into Wednesday,
followed by a warming trend later next week.


&&

.AVIATION...With yesterday`s precipitation in the Flathead, KGPI
may again have fog in the vicinity causing reductions in
visibility and/or ceilings through roughly 17/1500z.

A wet system is forecast to affect the Northern Rockies airspace
beginning around 18/0600z. All terminals are expected to
experience reduced ceilings and visibilities, with KMSO, KHRF,
KGPI, and KBTM potentially experiencing moderate to heavy
precipitation. Snowfall could occur down as low as 3000 feet by
Sunday morning in a narrow band across west-central Montana.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from late tonight through Sunday afternoon
     for Bitterroot/Sapphire Mountains...Potomac/Seeley Lake
     Region.

ID...Winter Storm Watch from late tonight through Sunday afternoon
     for Eastern Lemhi County...Southern Clearwater Mountains...
     Western Lemhi County.

&&

$$