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FXUS65 KMSO 030939 AFDMSO

Area Forecast Discussion National Weather Service Missoula MT 239 AM MST Sun Dec 3 2023

DISCUSSION
A series of pacific storm systems will impact the Northern Rockies through the upcoming week. The next Pacific trough, as seen in GOES-18 Water Vapor imagery, is beginning to move onshore in the Pacific Northwest and British Columbia early this morning.
A warm front, associated with this trough, will spread precipitation across Idaho and Clearwater Counties by the late morning hours. Precipitation will spread along the ID/MT border during the early afternoon, before tracking across the rest of western Montana by the mid-afternoon. Winter weather advisories remain in effect for Clearwater Mountains, the Bitterroot and Sapphire Mountains, and the Lower Clark Fork Region, where higher confidence exists for moderate snow and travel impacts, primarily for mountain pass areas. Snow levels will start this morning at 2,000 feet, but will quickly climb above 4,000 feet in Idaho by late this morning. Snow levels in Montana will climb above 4,000 feet by the early to mid-afternoon as a southerly pressure gradient brings mild air northward.

Black ice is a concern Sunday night into Monday morning. As precipitation changes over rain, cold ground temperatures may lead to icy conditions for untreated surfaces Sunday night.
Northwest Montana valleys and areas along Montana Highway 83 and 200 will be the most susceptible for black ice formation.

A transitory ridge of high pressure will amplify across the Northern Rockies late Monday into Tuesday. Southwesterly flow will bring a mild Pacific air mass across the region, with temperatures approaching record values (upper-40s to 50s) and snow levels climbing above 8,000 feet! Model guidance continues to show an atmospheric river along the western periphery of the ridge. This plume of moisture will extend across northwest OR into northeast WA and across north-central ID and northwest MT. Precipitation through Tuesday night will primarily focus across the Cabinet and Purcell Mountains.

A broad Pacific trough will move across the Northern Rockies late Wednesday into Friday. Snow levels will fall down into the 2000-3000 feet range by Thursday night as a cold front tracks across the region. The trajectory of this system will favor mountain snow along the ID/MT border and in northwest Montana.
Snow showers will continue into Friday under northwesterly flow.
/Lukinbeal



AVIATION
The next pacific storm system arrives late this morning into the afternoon across the Northern Rockies airspace.
Precipitation, associated with a warm front, will track west to east across Idaho and Clearwater Counties between 03/1600-1800Z, reaching western Montana after 03/1900Z. Snow will initially fall down to 2,000 feet, before snow levels quickly rise above 4,000 feet south of I-90 by 03/2100Z. Snow levels across northwest Montana will rise above 4,000 feet after 04/0000Z as the warm front continues northward. Southwest-west winds will increase after 04/0000Z along the Continental Divide and over the higher terrain. /Lukinbeal

MSO WATCHES/WARNINGS/ADVISORIES
MT...Winter Weather Advisory from 11 AM this morning to 5 AM MST Monday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

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

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