textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Additional Valley Snow Tonight: Steady snow is expected through the Flathead and Mission Valleys tonight. Snow is again possible in the Missoula and northern Bitterroot Valleys tonight but less certain with slightly warmer temperatures.

- Intense Saturday Snow Squalls: High-impact snow squalls and a flash-freeze possible across northwest and west-central Montana Saturday afternoon and evening.

The primary forecast challenge tonight remains the precise lowering of snow levels as heavy precipitation rates continue to drive top-down cooling across the region. Consistent snow is expected to develop and persist through the night for the Flathead and Mission Valleys. Residents should expect several inches of accumulation by daybreak Saturday. As the night progresses, the steady nature of the snow will begin to transition into more intense, cellular showers as the atmosphere destabilizes ahead of the arriving shortwave.

The overall airmass is slightly warmer this afternoon, meaning that while rain is expected to change back to snow in the Missoula and northern Bitterroot Valleys tonight, accumulating snow on paved surfaces will be more difficult to achieve. Slushy conditions are the most likely impact on roads. However, any higher intensity bursts could still lead to quick, temporary accumulations before the more significant front arrives Saturday afternoon.

Confidence is high that a frontal boundary will plunge south out of Alberta Saturday morning, reaching northwest Montana by midday and surging through west-central Montana during the afternoon. This setup is highly favorable for the development of frontal snow squalls. There is a 80 percent chance that intense bands of snow will impact the corridor between Kalispell and Missoula and along Highway 200. The timing is particularly concerning for I-90 from Lookout Pass to Homestake Pass; a period of heavy snow, gusty winds up to 45 mph, and rapidly dropping temperatures is likely. Any roads wet from earlier daytime melting will be prone to a "flash freeze," creating treacherous and unpredictable travel conditions through Saturday night.

Conditions will briefly stabilize on Sunday as a Arctic air mass settles in the region. By Sunday night, overrunning moisture from a new Pacific system will reintroduce wintry travel concerns, particularly for the mountains and northern valleys.

Looking further ahead, ensemble guidance strongly favors a shift toward a high-amplitude ridge starting Tuesday. While some "dirty" ridge cloud cover remains possible, a significant warming trend will take hold mid-week, with valley high temperatures potentially rebounding into the 50s and 60s by next Friday.

AVIATION

Temperatures are slowly moderating in the valleys this afternoon with places like KMSO switching to a rain or rain/snow mix. There are indications that snow levels will lower overnight to near most valley floors in western Montana. Snow is likely to continue in northwest Montana with long-duration IFR conditions, including at KGPI. Widespread MVFR/IFR conditions will prevail across the region as consistent precipitation reduces visibility and ceilings. Terrain will remain obscured across all north- central Idaho and western Montana ranges. A trough passage on Saturday will transition precipitation to intense snow showers during Saturday morning, followed by a line of intense snow squalls (LIFR likely) and erratic winds along the Highway 200 and I-90 corridor from Saturday afternoon through Saturday night. These squalls are likely to impact KMSO and KBTM, causing sudden whiteout conditions and potential for rapid surface icing.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Warning until noon MDT Saturday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region... Potomac/Seeley Lake Region...West Glacier Region.

Winter Weather Advisory from 9 PM this evening to 6 PM MDT Saturday for Flathead/Mission Valleys.

Winter Weather Advisory until midnight MDT Saturday night for Butte/Blackfoot Region.

ID...Winter Storm Warning until 11 AM PDT Saturday for Northern Clearwater Mountains...Southern Clearwater Mountains.

Flood Watch through Saturday afternoon for Lower Hells Canyon/Salmon River Region...Northern Clearwater Mountains...Orofino/Grangeville Region...Southern Clearwater Mountains.

Wind Advisory from noon to 9 PM MDT Saturday for Eastern Lemhi County...Western Lemhi County.