textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Snow Levels Dropping: Rain and mixed precipitation will transition to all snow for valley floors starting Monday as a series of cold fronts push through the region.

- Heavy Mountain Snow: Significant snow accumulations are expected Tuesday afternoon through Wednesday morning, especially at Lookout Pass.

- Thursday Snow Potential: Confidence is increasing for a secondary trough on Thursday, which could bring the best chance for widespread valley snow accumulations this week.

A very active and moist Pacific flow remains over the Northern Rockies this afternoon. We are currently in a transition phase as warmer southwest air clashes with lingering cold air trapped in some of the valleys, but a shift toward a colder, more wintry regime is on the horizon.

The primary concern for the rest of today remains the persistent cold pools in the valleys of northwest Montana. While most areas have warmed into the mid-30s or low 40s, northwest Montana, specifically near Kalispell, Libby, and Troy, continues to see cooler air stubbornly trapped at the surface. This creates a lingering threat of patchy freezing rain and fog through tonight as waves of moisture move overhead.

By Monday, a cold front will begin to push south, lowering snow levels to the valley floors across western Montana and north- central Idaho. While Monday's snow will largely be nuisance accumulation for most valleys (an inch or less), it marks the end of the rain/snow mix in most places and the start of a more consistent winter travel period.

The most impactful part of the week arrives Tuesday afternoon and lasts through Wednesday morning. A potent surge of moisture will hit the region, and with snow levels already low, we expect significant mountain travel impacts.

Mountain Passes: Lookout Pass is currently the bullseye, with an 80% chance of seeing over 8 inches of snow in 24 hours. Marias and Lolo Passes are forecast to receive roughly 6 inches.

Valleys: Northwest Montana (Yaak, Troy, and Heron) could see substantial valley snow, with 4 to 8 inches possible. Most other valleys in western Montana and central Idaho will see 1 to 3 inches.

Wind: Gusty winds will accompany the front on Wednesday, potentially creating areas of blowing snow on mountain passes and reducing visibility.

Guidance is coming into much better agreement regarding a secondary trough on Thursday. Latest model clusters, including AI- enhanced guidance, now favor a deeper and more organized system dropping into the region. This evolution would provide the necessary lift and cold air reinforcement to produce more meaningful valley snow than the previous flatter model solutions suggested. If this trend holds, Thursday could offer the best chance for widespread, accumulating snow across valley locations.

The pattern begins to calm down by Friday, and by next weekend, models are in high agreement that a ridge of high pressure will build over the region. This will end the active snow cycle but will likely lead to the return of valley inversions, morning fog, and stagnant air conditions.

AVIATION

An increasing pressure gradient and southerly winds are helping to erode valley cold pools, though KGPI remains a notable exception with temperatures remaining in the 30s. As winds increase this afternoon, rain shadow effects may limit valley precipitation. Snow levels will begin to drop toward valley floors late tonight into Monday morning, but pockets of freezing rain will remain possible. High dewpoints and evening showers also point to increased potential for valley fog to return overnight, potentially impacting visibility through Monday morning.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Watch from Tuesday afternoon through Wednesday morning for Kootenai/Cabinet Region...Lower Clark Fork Region.

ID...Winter Storm Watch from Tuesday morning through Wednesday morning for Northern Clearwater Mountains.