textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Tonight/Wednesday: Pass level snow, potential minor travel impacts.

- A potent cold front drops snow levels to the valley floors Wednesday night and Thursday, bringing the potential for sudden snow squalls in southwest Montana and travel impacts over mountain passes.

- A brief warmup with drier weather is expected for the upcoming weekend before active weather returns.

Expect breezy west winds this afternoon, gusting around 25 mph. Tonight, a stronger low-pressure system and a plume of Pacific moisture will begin moving into the Northern Rockies. The heaviest precipitation will focus across north-central Idaho and northwest Montana. Snow levels will hover around 4,500 feet, so impacts will mostly be confined to higher terrain. Lookout, Marias, and Lolo passes have a high chance (60 to 80 percent) of seeing a few inches of new snow, though roads just below the passes should remain mostly wet.

A strong cold front will sweep across the region Wednesday, continuing pass level snow and valley showers. By Wednesday afternoon and evening this front will cause a burst of heavier precipitation and rapidly falling snow levels, dropping all the way to the valley floors. Be on alert for sudden snow squalls as the front moves through southwest Montana on Wednesday evening, which can cause rapidly dropping visibility and slick roads. This will create moderate travel impacts on mountain passes Wednesday night and Thursday morning.

There is still some uncertainty regarding how much snow will ultimately accumulate in the valleys. Current models show the bulk of the moisture arriving directly along the cold front, which typically limits valley accumulations, but heavy bursts are still possible. Furthermore, there is a 25 percent chance that a weak, closed low-pressure system could develop over southwest Montana. If this happens, precipitation amounts in that area would be enhanced, leading to higher snow totals than currently forecast. Finally, we will closely monitor the potential for snow squalls Wednesday evening, as their exact timing and intensity are highly localized and complex to pin down in advance.

Showery conditions will remain through Thursday with the trough over the region, and lingering mountain snow showers will taper off through Friday. A ridge of high pressure will build over the region this weekend, bringing a brief but welcome return to warmer and drier weather. However, this break will be short-lived, as another weather disturbance is currently expected to arrive by early next week.

AVIATION

VFR conditions will generally prevail today, though some mountain obscuration will continue, especially across northwest Montana. Southwest winds will increase to 10 to 15 knots with gusts of 20 to 25 knots at terminal sites this afternoon. Winds will relax in most valleys south of the I-90 corridor by sunset but will remain elevated in northwest Montana. Precipitation (rain and high-elevation snow) will quickly increase along the Idaho/Montana border by early evening and spread eastward overnight. Downsloping winds will limit precipitation in many western Montana valleys. Snow levels will lower to 4,000 to 5,000 feet by Wednesday morning.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 6 PM Wednesday to 9 AM MDT Thursday for Butte/Blackfoot Region...Potomac/Seeley Lake Region.

Winter Weather Advisory from midnight tonight to 9 AM MDT Thursday for Lower Clark Fork Region...West Glacier Region.

Winter Weather Advisory from 3 PM Wednesday to 9 AM MDT Thursday for Bitterroot/Sapphire Mountains.

ID...Winter Weather Advisory from 6 PM MDT /5 PM PDT/ Wednesday to 9 AM MDT /8 AM PDT/ Thursday for Eastern Lemhi County... Northern Clearwater Mountains...Southern Clearwater Mountains...Western Lemhi County.