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FXUS65 KMSO 080950 AFDMSO

Area Forecast Discussion National Weather Service Missoula MT 250 AM MST Fri Dec 8 2023

DISCUSSION
Considerable instability under an upper trough will bring areas of snow showers throughout today. A weak disturbance will provide a focus for showers through mid morning across west- central Montana into Lemhi County, while a secondary wave brings showers to northwest Montana. Road conditions will be slick on most passes and changeable in the valleys, allow a little extra travel time for the morning commute. Showers diminish later this afternoon and evening for much of the region but continue on the terrain into early Saturday.

Brief ridging on Saturday will increase the potential for morning fog in the valleys.

Remnants of an atmospheric river will begin to impact the Northern Rockies Saturday evening continuing into Sunday. Snow levels start out at valley floors then gradually rise on Sunday.
Probabilities support a moderate snow events for the passes along the Idaho/Montana border with a 70-90% chance of at least 4 inches of snow at Lookout, Lolo, and Lost Trail passes.

It's not uncommon with these overrunning scenarios for guidance to raise snow levels too quickly, especially with inversions slow to break. Wintry impacts for the valleys of north- central Idaho will be very snow level dependent with the potential for several inches of snow if snow levels stay lower than currently forecast.
Downsloping will limit snow amounts in some western Montana valleys with most areas receiving a trace to 2 inches, but MT-200 and US-2 west of the Flathead Valley look to receive 2 to 4 inches with locally higher amounts in excess of 4 inches.

Areas of snow continue on the terrain Monday as a weak disturbance in northwest flow passes through. Valleys of northwest Montana may also see another round of light snow. Overall for Saturday night through Monday, watch for periods of snow packed roads on the passes, especially along the Idaho/Montana border.
Valleys may see slick roads as well, particularly on Sunday morning.

Ensemble clusters continue in very good agreement for a ridge of high pressure to develop the middle of next week but disagree on how quickly it will break down. There is a potential of valley inversions with fog and stratus under the ridge.



AVIATION
An upper level trough will remain overhead today with areas of snow showers causing brief lowered ceilings and reduced visibility. Showers will gradually diminish tonight and become confined to the terrain. Westerly winds will gust 15-20 mph at times today as well. Brief high pressure takes over on Saturday with an increased potential for valley fog in the morning.

MSO WATCHES/WARNINGS/ADVISORIES
MT...Winter Weather Advisory until 9 AM MST this morning for Bitterroot/Sapphire Mountains
Flathead/Mission Valleys
Lower Clark Fork Region
Potomac/Seeley Lake Region
West Glacier Region.

ID...Winter Weather Advisory until 8 AM PST this morning for Lower Hells Canyon/Salmon River Region...Southern Clearwater Mountains.

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