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FXUS65 KMSO 040923 AFDMSO

Area Forecast Discussion National Weather Service Missoula MT 223 AM MST Mon Dec 4 2023

DISCUSSION
Winter weather advisories remain in effect until 900 AM MST across the Bitterroot/Sapphire Mountains and the Lower Clark Fork, Potomac/Seeley Lake and West Glacier Regions and the Clearwater Mountains, where freezing rain is a concern given cold sub-surface temperatures causing untreated roadways to freeze.

A deep trough (501-dam) of low pressure will move into the Gulf of Alaska today, causing downstream amplification of a high pressure ridge across the Pacific Northwest and Northern Rockies.
An atmospheric river will set up between the interface of the deep low and downstream ridge with moisture focusing across northwest OR, northeast WA, and northwest Montana through Wednesday. The influx of subtropical moisture and warm air from the southwest will cause temperatures to approach near record values (40s and 50s). This is supported by near record 850mb temperatures. The 00z run of the ECMWF has +9C over Spokane, WA on Tuesday. The all- time highest December 850mb temperature recorded by upper air balloon soundings (records to 1948) released by NWS Spokane in December is +13.7C and +6.2C on December 6th.
As you might expect, this air mass will send snow levels climbing, reaching 8000-9000 feet. Snow levels will eventually lower closer to 6,000 feet Wednesday afternoon as the Pacific trough begins to approach eastern WA.

Precipitation associated with the atmospheric river will focus primarily across the northern Clearwater Mountains and western portions of Lincoln, Sanders, and Mineral Counties along the ID/MT border. 50th percentile (mostly likely amounts) precipitation amounts from the NBM4.2 project 1.5-2.0” of rain for Heron, Troy, and Bulk Lake areas, with 3-4” in the Cabinet Mountains. Valley areas further to the east, such as the Flathead and Mission, would see 0.25-0.50” under this scenario. Streams and rivers will see rises, but are projected to remain below action stage.
Localized flooding is possible in poor drainage areas. It’s worth mentioning that considerable uncertainty remains within this precipitation forecast given the narrow, but intense nature of the atmospheric river. Current model guidance puts the aforementioned areas on the eastern edge of the atmospheric river. If the models where to trend further east with the moisture, then precipitation amounts would increase, possibly towards the 90th percentile NBM, which has 4-5”+ of rain in the terrain and 2-4” of in the valleys right along the ID/MT border.

A broad Pacific trough remains on track to move across the Northern Rockies late Wednesday into Friday. Snow levels will drop down to 3,500 feet by Thursday morning, with moderate snow impacting travel across mountain passes. A secondary shot of colder air arrives Thursday night into Friday morning, with snow levels reaching 2,000 feet. This period will need to be monitored for valley snow and impacts, given unstable northwesterly flow conducive to convective snow showers. By Friday PM, 48-hour snow totals from the NBM4.2 50th percentile reach 1-2 feet above 5,000 feet across the northern Clearwaters, Cabinet, Mission, and Swan Mountains, with 6-12” for the Bitterroot Mountains and Whitefish Range. /Lukinbeal

AVIATION
Shower activity will decrease between 04/1200-1600Z across north- central ID and southwest MT as a shortwave trough departs the region to the east. A southerly pressure gradient will remain in place today, with 15-25kt winds developing along the divide and over the higher terrain. Areas of mountain wave turbulence will continue given 700mb winds on the order of 40-55kts. Rain showers return across Idaho and Clearwater Counties and in northwest Montana after 05/0000Z as pacific moisture returns from the southwest. /Lukinbeal



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

ID...Winter Weather Advisory until 8 AM PST this morning for Northern Clearwater Mountains...Southern Clearwater Mountains.

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