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FXUS65 KMSO 100930 AFDMSO

Area Forecast Discussion National Weather Service Missoula MT 230 AM MST Sun Dec 10 2023

DISCUSSION
A plume of abnormally high atmospheric moisture is moving through western Montana and north central Idaho this morning. Widespread snow is occurring with this feature, as low as roughly 1500 feet. As expected, snow levels have been slower to rise than forecast model guidance depicted. This is resulting in more snow accumulation than originally forecasted for areas such as the Missoula/Bitterroot valleys and the I90/Highway 12 corridors east of Missoula. Numerous winter weather advisories remain in place to account for accumulating snow and slick travel this morning.

Snow levels are expected to slowly rise from south to north through the day today, gradually changing precipitation over to rain or a wintry mix for most valley locations. At the same time, precipitation will become more showery in nature and focused over the terrain. The mountains, particularly through north central Idaho and along the Idaho/western Montana border, will continue to see snow through the day and into tonight. Therefore, while conditions will be improving in the valleys, mountain passes will likely continue to see slick/snowcovered roads into Monday morning. One thing to note for valleys: cold subsurface temperatures could allow for the light valley rain to freeze in shaded areas or even after sunset on this afternoon.

A relatively weak Canadian cold front is expected to cross the Continental Divide into northwest Montana Monday morning. The front will then stretch into west-central Montana by mid day. A short burst of snow will likely accompany the front, with snow levels dropping back to the valley floors. Overall accumulations should be relatively light with this feature, but another round of slick conditions is possible. This is especially true across northwest Montana where the cold front passage will time better with the early morning hours, making surfaces more susceptible to accumulations.

A ridge of high pressure is still anticipated to build back over the Northern Rockies Tuesday into Wednesday. Morning fog and low stratus will be possible under this pattern. Forecast models continue to show a weak disturbance moving through the ridge on Thursday, which will result in some snow showers across mainly northwest Montana and the Idaho panhandle.



AVIATION
A plume of atmospheric moisture continues to produce widespread snow across western Montana and north central Idaho this morning. Moderate to heavy snow is occurring for the areas along the western Montana/Idaho border, obscuring mountains and passes through today. Snow levels will be rising throughout the period which will cause the precipitation type to transition over to rain for most valley locations by this afternoon. KGPI is expected to see an end to steady snow by 10/12z and KMSO by 10/15z with more showery conditions after. KSMN, KBTM and KHRF will experience more downslope effects during this event so the impacts from the snow will be more limited.

MSO WATCHES/WARNINGS/ADVISORIES
MT...Winter Weather Advisory until 5 AM MST Monday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

Winter Weather Advisory until 11 AM MST this morning for Flathead/Mission Valleys
Kootenai/Cabinet Region
Missoula/Bitterroot Valleys
Potomac/Seeley Lake Region
West Glacier Region.

Winter Weather Advisory until 5 PM MST this afternoon for Butte/Blackfoot Region.

ID...Winter Weather Advisory until 4 AM PST Monday for Northern Clearwater Mountains...Southern Clearwater Mountains.

Winter Weather Advisory until 10 AM PST this morning for Lower Hells Canyon/Salmon River Region...Orofino/Grangeville Region.

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