textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Prolonged atmospheric river event from Wednesday night through Saturday.

-Strong and potentially damaging winds late Wednesday night through Thursday, with the strongest gusts expected across northwest Montana and along the Continental Divide.

-Very difficult backcountry conditions due to strong winds and heavy snow, leading to localized blizzard conditions.

The Northern Rockies will remain under a westerly (zonal) flow for the next 24 to 36 hours, bringing breezy westerly winds and intermittent light snow showers. The exception will be around the Glacier National Park area, where relatively light but persistent snow is expected to bring an additional 1 to 7 inches of accumulation.

An atmospheric river will start to push into the region on Wednesday, increasing the coverage and intensity of precipitation. Snow levels will steadily rise during this period, reaching around 4,000 feet in northwest Montana and 6,000 feet in Lemhi County by Thursday morning. Late Wednesday night into Thursday, a strong low-pressure system will slide along the Canadian border, pushing a cold front south across the region. This cold front will bring extremely strong westerly winds, especially across northwest Montana and along the Continental Divide.

The latest high-resolution ensemble guidance shows a 40% to 100% probability of wind gusts exceeding 50 mph across western Montana and Lemhi County on Thursday. The higher terrain around Glacier National Park has a 30% to 80% chance of experiencing wind gusts in excess of 70 mph by Thursday morning. The combination of strong winds and heavy snow in the higher terrain has led to the issuance of Winter Storm Warnings for elevated areas, though a few of these locations are being monitored for potential upgrades to Blizzard Warnings.

Areas expected to experience strong winds with minimal snow impacts are currently under a High Wind Warning. However, a few localized areas within these High Wind Warning zones such as Condon Summit on Highway 83, the Georgetown Lake region, and the Happys Inn to Marion corridor on Highway 2 could still experience winter storm conditions on Thursday. As the cold front pushes the moisture feed southward on Thursday, Lookout and Marias Passes will experience a decrease in snowfall intensity.

The moisture plume will continue to stream into the Northern Rockies from Thursday night through Saturday, with the moisture feed slowly making its way northward again. The Clearwater, Bitterroot, and Sapphire Mountains appear to be under the heaviest portion of this moisture feed during this prolonged event. Snow levels will continue to be the most challenging aspect of the forecast; models show levels hovering near 2,000 feet in northwest Montana and around 6,000 feet over Lemhi County after the cold front passes.

Confidence is high that the higher terrain and mountain passes will receive 2 to 5 feet of new snow by Saturday morning. However, valley impacts, especially in Idaho and Clearwater counties, remain uncertain. If snow levels remain high, flooding of small streams and creeks will be a primary concern. Conversely, if snow levels fall, the flood risk diminishes, but snow accumulations on roadways will become a significant travel issue.

The moisture plume appears to exit the region by late Saturday, but the Northern Rockies will remain in a breezy and active weather pattern through the remainder of the weekend and into next week.

AVIATION

The Northern Rockies will remain under westerly flow for the next 24 to 36 hours. Breezy westerly winds will continue at regional terminals during the afternoon and early evening hours today. Additionally, numerous snow showers will impact the region this afternoon, potentially causing periods of IFR conditions at affected terminals. Both wind and shower activity are expected to diminish after sunset this evening.

An atmospheric river is expected to push into the region on Wednesday, bringing increased precipitation coverage. Precipitation is expected to start as snow before transitioning to rain as snow levels rise. Gusty westerly winds are expected to develop once again during the afternoon hours.

Late Wednesday through Saturday, the Northern Rockies airspace will be impacted by a high intensity Atmospheric River system.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Warning from 6 PM Wednesday to noon MDT Saturday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region...West Glacier Region.

High Wind Warning from midnight Wednesday night to midnight MDT Thursday night for Butte/Blackfoot Region.

High Wind Warning from midnight Wednesday night to 6 PM MDT Thursday for Flathead/Mission Valleys...Kootenai/Cabinet Region...Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

Winter Weather Advisory until 6 PM MDT Wednesday for West Glacier Region.

ID...Winter Storm Warning from 5 PM Wednesday to 11 AM PDT Saturday for Northern Clearwater Mountains...Southern Clearwater Mountains.

High Wind Warning from midnight Wednesday night to midnight MDT Thursday night for Eastern Lemhi County...Western Lemhi County.