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FXUS65 KMSO 022049 AFDMSO

Area Forecast Discussion National Weather Service Missoula MT 149 PM MST Sat Dec 2 2023

DISCUSSION
A multiple day winter weather event is continuing today. Snow has been falling across the Northern Rockies through the day, especially for mountains and mountain passes like Lookout, Lolo, and Lost Trail Pass. Snow will continue to push west to east across the region into this afternoon. Minor accumulations and snow covered roads are possible in valleys during this time. By the middle part of the afternoon a cold front will be pushing across north-central Idaho and western Montana, which will provide support for additional snow development. Snow development will also be supported by the Northern Rockies sitting under the left exit region of a 140 knot upper-level jet streak. This atmospheric set-up is favorable for the development of snow bands and snow squalls this afternoon and evening. Brief bursts of heavy snowfall with rates up to and even exceeding 1 inch per hour are likely, with wind gusts of 30-40 mph working to greatly reduce visibility.
Roads will quickly become snow covered and slick underneath the heavier bands. The drop in temperature expected behind the cold front, although not drastic, will also help create icy spots on area roadways. Use caution if traveling across west-central and southwest Montana through this evening.

Another round of precipitation is expected Sunday afternoon. High resolution models depict another period of moderate to heavy snow for mountain passes like Lolo, Lookout, and Lost Trail. With increased confidence in this scenario, another round of winter weather advisories have been issued for those aforementioned locations. The forecast for valleys on Sunday is much more nuanced. There will be milder air pushing in from the south, leading to an increase in surface air temperatures. With precipitation likely changing to rain or drizzle for area valleys Sunday evening, ground and road temperatures will be critical in determining the possibility of icy road conditions. Currently it appears that northwest Montana valleys have the best chance of seeing impacts from slick roads Sunday evening, as the surface temperatures take longer to warm up. Even if rain or drizzle does not freeze on contact Sunday evening, slick spots will be a possibility overnight and into Monday morning when road temperatures drop once again.

Much milder conditions will be in place Monday through Wednesday of next week. A ridge of high pressure will be setting up over western Montana and north-central Idaho. Model guidance suggests temperatures will be near record warm during this period. This includes daytime highs as well as record warm overnight lows.
Although a ridge will be in place, this will not be a dry period.
This is because an atmospheric river will be pushing a large plume of sub-tropical moisture into the Northern Rockies. Precipitation will be mainly rain, with snow levels between 7000 and 8000 feet through Wednesday. Snow levels will begin to come back down late Wednesday into Thursday, as a cooler weather system approaches from the west. /KS

AVIATION
Widespread light to moderate snow is occurring across much of western Montana and north central Idaho this afternoon.
Most all valleys, other than the lowest valleys of north central Idaho, will be experiencing snow. Low ceilings and visibility due to snow will be common with the higher terrain being obscured.
Gusty southeast to south winds will become more westerly to northwesterly this afternoon, with gusts up to 35kts possible.
Behind the front, precipitation will become more showery and banded in nature. Intense snow squalls are possible late this afternoon through the evening hours (02/23z through 03/03z), especially from the northern Clearwater Mountains and points east into west-central Montana including KMSO and KBTM. Low level wind shear is also a concern this evening and tonight, particularly in the vicinity of KGPI and KMSO.



MSO WATCHES/WARNINGS/ADVISORIES
MT...Winter Storm Warning until 5 PM MST this afternoon for Lower Clark Fork Region.

Winter Weather Advisory from 5 PM this afternoon to 5 AM MST Sunday for Lower Clark Fork Region.

Winter Weather Advisory from 11 AM Sunday to 5 AM MST Monday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

Winter Weather Advisory until 5 PM MST this afternoon for Bitterroot/Sapphire Mountains
Kootenai/Cabinet Region
West Glacier Region.

Winter Weather Advisory until 11 PM MST this evening for Flathead/Mission Valleys
Missoula/Bitterroot Valleys
Potomac/Seeley Lake Region.

Winter Weather Advisory until midnight MST tonight for Butte/Blackfoot Region.

ID...Winter Storm Warning until 4 PM PST this afternoon for Northern Clearwater Mountains...Southern Clearwater Mountains.

Winter Weather Advisory from 4 PM this afternoon to 4 AM PST Sunday for Northern Clearwater Mountains...Southern Clearwater Mountains.

Winter Weather Advisory from 10 AM Sunday to 4 AM PST Monday for Northern Clearwater Mountains...Southern Clearwater Mountains.

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