textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Atmospheric River tonight into Friday, bringing wind impacts and heavy mountain snow.

- Lingering light showers late Friday into Sunday morning.

- Another disturbance late Sunday into Monday.

Another pulse of the atmospheric river is beginning to push into the Northern Rockies this afternoon. This moisture will bring widespread precipitation to the region. Snow levels are expected to start off around 2000-3000 ft MSL and rise to around 6000 ft MSL by midnight tonight, meaning most locations will start with snow then the valleys transition to rain with mountain snow. The heaviest precipitation associated with this round of moisture appears to be across north central Idaho into west central Montana. The Clearwater, Bitterroot and Sapphire mountain ranges are anticipated to receive over a foot of new snow by Friday afternoon. While higher elevation valleys, such as Elk City, Essex and Georgetown Lake should anticipate 2 to 6 inches of snow accumulation.

A cold front is anticipated to push across the Northern Rockies between midnight tonight to mid Friday morning. This front will cause gusty winds ahead and along the front. The winds will not be as strong as Wednesday's wind event, but widespread wind gusts of 30 to 50 mph should be anticipated in the valleys with localized areas potentially getting up to 60 mph. The higher terrain, especially along the Continental Divide will be experiencing wind gust potential up to 70 mph. Several wind advisories have been issued for the region and a winter storm warning has been issued for the Bitterroot and Sapphire mountains for the combination of snow and wind.

A low amplitude ridge of high pressure builds over the region late Friday into Saturday. This ridge will cause a reduction in the precipitation, limiting the light snow showers to mainly northwest Montana.

The next weather disturbance appears to move into the Northern Rockies by late Sunday into Monday time frame. This active weather pattern is anticipated to last through much of next week.

AVIATION

Another atmospheric river (AR) will impact the Northern Rockies over the next 24 hours, characterized by significant moisture flux, strengthening low to mid-level jets, and a passing cold front late tonight into Friday morning.

*Wind/Turbulence: A tightening pressure gradient by afternoon will result in widespread surface wind concerns by this evening into Friday. South-southwest winds will gust 15-25 kts across most valley terminals (KMSO, KGPI, KBTM) starting after 18/2100Z. Higher concern exists for Camas Prairie (near Grangeville), where gusts may exceed 450-50 kts. Expect Mechanical Turbulence and Low-Level Wind Shear (LLWS) as a 50+ kt 700mb jet moves overhead tonight. The risk for peak gusts >40 kts increases Friday morning for the I-90/US-93 corridors in SW Montana (KHRF, KBTM, KSMN) ahead of and along the frontal passage.

*Ceilings/Visibility: VFR conditions this afternoon will deteriorate to MVFR this evening and overnight as the AR plume arrives. Primary impacts will be obscured terrain and heavy precipitation over the mountains.

*Icing: icing is a concern within the AR plume tonight, especially between 5,000 ft and 12,000 ft MSL.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory until 11 AM MST Saturday for Lower Clark Fork Region...West Glacier Region.

Wind Advisory from 11 PM this evening to 8 AM MST Friday for Kootenai/Cabinet Region.

Wind Advisory from 2 AM to 11 AM MST Friday for Butte/Blackfoot Region...Flathead/Mission Valleys...Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

Winter Storm Warning from 5 PM this afternoon to 5 PM MST Friday for Bitterroot/Sapphire Mountains.

ID...Winter Weather Advisory until 10 AM PST Saturday for Southern Clearwater Mountains.

Winter Weather Advisory until 5 PM MST Friday for Eastern Lemhi County...Western Lemhi County.