textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Significant closed low pressure system moving through the region Wednesday night through Friday bringing widespread snow and cooler temperatures.

- High pressure returns this weekend, bringing warming and drying.

The Northern Rockies will remain under a moist west-southwest flow through Wednesday ahead of an approaching Pacific trough. High mountain passes south of I-90, including Lolo, Lost Trail, and Homestake, may see nuisance snow accumulations through tonight, though impacts will remain localized. While widespread showers develop Wednesday, rising snow levels (5000 to 6000 feet) will limit accumulations to the highest terrain.

The primary forecast challenge arrives late Wednesday night through Friday as the trough evolves into a closed low. While a cold front is expected to drop snow levels to 3000 feet by Thursday morning, uncertainty remains regarding the exact track and timing of the closed low. Small latitudinal shifts in the center of this feature will dictate where the heaviest precipitation falls. Current guidance favors a heavy, wet snow event for the Seeley/Swan and Butte/Blackfoot regions Thursday into Friday morning, where upslope flow and wrap-around moisture are most likely to overlap. Given the high-moisture content of snowfall, tree damage and power outages are a concern. However, because the exact position of these heavy precipitation bands often remains volatile until 24 hours before onset, residents and travelers should anticipate adjustments to the current Winter Storm Watches.

On Friday, the system will exit the region, bringing continued light snowfall under northwest flow aloft and cooler temperatures. High confidence ( >80%) exists for a transition to a warmer, drier regime this weekend as a high-pressure ridge builds over the region.

AVIATION

The Northern Rockies will experience periods of mountain obscurations and localized valley MVFR conditions through Wednesday as a moist west-southwest flow prevails ahead of an approaching Pacific trough. Primarily south of the I-90 corridor, including terminals near KMSO and KBTM, intermittent light snow will impact mountain passes through tonight, though valley floors should remain VFR. By Wednesday, a more widespread increase in shower activity is expected; however, rising snow levels to between 5000 and 6000 feet will transition most terminal impacts to rain, with snow-related IFR/LIFR risks and icing becoming increasingly confined to the higher terrain and mountain crests.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Watch from Thursday morning through late Thursday night for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...Potomac/Seeley Lake Region.

ID...Winter Storm Watch from Wednesday evening through late Thursday night for Southern Clearwater Mountains.