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 areas of blizzard conditions.

Changes Over Last 24-Hours:

* Upgraded Glacier Park region to Blizzard Warning late tonight into Thursday morning; significant impacts from whiteout conditions from heavy snow and strong winds.

* Replaced High Wind Warning in Butte/Blackfoot region with Winter Storm Warning; where significant heavy snow, strong winds, and periods of blizzard conditions are expected across pass areas and the Montana Highway 1 corridor near Georgetown Lake. IMPORTANT: A significant wind event remains on track across all elevations.

* Issued Winter Storm Warning for Seeley Lake/Potomac region Thursday night into Saturday morning, where snow amounts have notably trended higher.

Discussion:

A significant and prolonged atmospheric river event remains on track to impact the Northern Rockies later today through Saturday. Satellite and regional radar imagery this morning show the atmospheric river making landfall across the Pacific Northwest, with precipitation rates beginning to increase along the Idaho and Montana border. The primary impact window begins tonight into Thursday morning, as a 990-995 mb surface low tracks along the USA-Canadian border west-to-east into southern Alberta by Thursday morning. Model guidance has been very consistent with a significant threat for strong and damaging winds as a near-record mid-level jet stream moves across the region after midnight tonight. Models suggest 700-mb winds (10,000 ft MSL winds) will reach near 90-100kt (~100-110 mph), which is near the all-time record climatology within historical data. These strong winds aloft, coupled with a surface cold front tracking through northwest Montana early Thursday morning maximize the threat for strong and widespread damaging winds, particularly north of the I-90 corridor, with the greatest impacts will focus across Lincoln, Flathead Counties, and the highest terrain. Further south, strong and damaging winds will be more localized, driven by strong winds aloft being mixed down to the surface through convective gusts and mountain wave activity. This includes areas along and south of I-90. Historically, the Bitterroot Valley and the I-90 corridor from Garrison to Anaconda are the most susceptible to the strongest winds, with gusts up to 70 mph a concern during this event.

A long duration winter storm will bring major to extreme impacts across the Glacier Park region, the Bitterroot, Clearwater, and Sapphire Mountains, and Seeley Lake, Potomac, Butte/Blackfoot regions tonight into Saturday morning. Storm impacts will peak after midnight into early Thursday afternoon across mountain passes and backcountry areas, with periods of blizzard conditions driven by heavy snow rates and strong winds, with gusts of 50 to 80 mph. The worst conditions and impacts will be felt across the Glacier Park region, specifically on US Highway 2 east of West Glacier to Marias Pass and in the higher terrain, where several hours of whiteout conditions are expected. Model guidance suggests over a 80-90% probability for snow rates of near 1 inch per hour and strong winds in excess of 50 mph as the cold front passes. Lookout, Lost Trail, MacDonald Passes, and the Georgetown Lake areas will also see blizzard-like conditions late tonight into Thursday morning. Those planning to travel across these areas should consider delaying or canceling plans as these conditions will be life threatening.

The atmospheric river will continue to bring widespread heavy mountain snow Thursday afternoon through Saturday. Ensemble guidance has been consistent with a 24-48 hour period of Integrated Vapor Transport Values of 200-400 kg/ms reaching the divide. This provides a high confidence scenario for feet of snow above 4,000-5,000 feet across mountain areas, with the highest accumulations of 2-5 feet across the Bitterroot and Clearwater Mountains. The combination of heavy snow, strong winds, and significant precipitation totals will cause dramatic changes to the snowpack and those heading into the backcountry and in the mountains should closely monitor the latest avalanche conditions.

Rising temperatures and snow levels during the atmospheric river event in Clearwater and Idaho Counties of Idaho will bring the risk for sharp rises of small streams and creeks as 1 to 5 inches of precipitation falls below the snow line. Roadways will also be at risk for rock and mudslides. Please see our hydrologic outlook for more information.

This storm system will begin to exit the region on Saturday, with the passage of a Pacific cold front and upper level trough from the northwest. Snow levels will quickly drop late Friday night into Saturday morning, with snow showers bringing minor-moderate winter weather impacts. Valley snow amounts remain variable during this period; please monitor the forecast closely in the coming days. /AL

AVIATION

The atmospheric river will continue to move into the Northern Rockies tonight and last through Saturday. This moisture will cause cloud cover to steadily increase and lower over the next 12 hours at all terminals. Snow levels will be climbing during the evening hours so any precipitation at the terminals are expected to be rain. Then tonight a cold front will begin to push through the region from the north to south causing strong to damaging winds, all terminals are expected to have gusts in excess of 35kts on Thursday. Low level wind shear will also be a concern late tonight through Thursday. The cold front will also cause snow levels to drop to valley floor, especially across northwest Montana. Any snow that impacts KGPI, anticipate at least IFR conditions along with blowing snow.

MSO WATCHES/WARNINGS/ADVISORIES

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

High Wind Warning from midnight tonight 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 this evening for West Glacier Region.

Winter Storm Warning from 6 PM this evening to midnight MDT tonight for West Glacier Region.

Winter Storm Warning from 9 AM Thursday to noon MDT Saturday for West Glacier Region.

Blizzard Warning from midnight tonight to 9 AM MDT Thursday for West Glacier Region.

Winter Storm Warning from midnight tonight to noon MDT Saturday for Butte/Blackfoot Region.

Winter Storm Warning from 6 PM Thursday to noon MDT Saturday for Potomac/Seeley Lake Region.

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

Wind Advisory from 8 PM this evening to 11 AM PDT Thursday for Lower Hells Canyon/Salmon River Region... Orofino/Grangeville Region.

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