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FXUS65 KMSO 091048 AFDMSO

Area Forecast Discussion National Weather Service Missoula MT 348 AM MST Sat Dec 9 2023

DISCUSSION
The next surge of abnormally high atmospheric moisture (150-200 percent of normal) is expected to move into north central Idaho and western Montana early this evening into Sunday. Most elevations will start out cold enough to see precipitation fall as snow, before transitioning to rain or a wintry mix by mid day Sunday. Significant snowfall is expected in the mountainous terrain, especially through north central Idaho and along the Idaho/western Montana border, where a foot of snow or more is expected through Sunday afternoon for elevations above roughly 5000 feet.

The forecast for snow amounts in the valleys is more difficult as final amounts will depend heavily on how quickly snow levels rise. In a warm front scenario such as this, forecast models tend to increase snow levels a bit too quickly. We have trended the forecast towards a slower progression of the warm air, which is allowing for several inches of snow accumulation tonight into Sunday morning, especially in northwest Montana where cold air will be last to erode. Numerous winter weather advisories are in place to account for widespread snow accumulations and slick travel into Sunday afternoon.

Northwest flow aloft will maintain showers across western Montana and northcentral Idaho Sunday night into Monday. Snow levels are expected to range between 3000 to 4000 feet, keeping most valley locations as rain or a rain snow mix. However, cold ground temperatures could allow for some localized ice formation Sunday night into Monday morning.

During the day Monday, forecast models show a weak Canadian cold front crossing the Continental Divide into northwest Montana. The main result will be some enhanced snowfall potential along the Continental Divide.

Forecast models and their ensembles continue to support a ridge of high pressure building over the region by Wednesday, lasting through at least the end of the week. This scenario would support valley inversions and the potential for fog/low stratus to develop.

AVIATION
Transient high pressure is building over the Northern Rockies this morning, resulting in patchy fog and low stratus.
Another system will begin to affect the region this evening into Sunday as anomalous moisture move in from the west. Widespread lowered ceiling and visibility is expected as snow develops across northcentral Idaho and western Montana. Snow levels will be low enough to allow snow into the valleys initially, before increasing to around 4000 feet Sunday. KGPI and KMSO in particular will likely see some snow develop, with low ceilings and visibility between 10/0300z and 10/1500z. A southerly gradient will develop this afternoon, bringing gusty winds to the Camas Prairie in Idaho and the southern Bitterroot valley, up to 25kts. Winds will also be increasing at ridge top level this afternoon and evening from the west up to 30 kts.

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

Winter Weather Advisory from 5 PM this afternoon to 11 AM MST Sunday for Flathead/Mission Valleys...Kootenai/Cabinet Region
Potomac/Seeley Lake Region
West Glacier Region.

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

Winter Weather Advisory from 4 PM this afternoon to 10 AM PST Sunday for Lower Hells Canyon/Salmon River Region...
Orofino/Grangeville Region.

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