textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Through Sunday: Intermittent rain and isolated thunderstorms; wind gusts to 35 mph with storms. Snow levels remain high (>6,000 ft).

- Monday Morning: 55% chance of >1" slushy snow on mountain passes above 5,500 ft (e.g., Lost Trail, Gilmore, Georgetown Lake).

- Wednesday/Thursday: Strong cold front brings sharp cooling; snow levels plummeting to 1,500-2,000 ft. Potential for accumulating valley snow; impact depends on moisture/cold air overlap.

Through Sunday:

Moist southwesterly flow will transport a series of weak disturbances across the region through Sunday. Combined with afternoon instability, this pattern will trigger rounds of rain showers and occasional thunderstorms, particularly during the peak heating hours. Thunderstorms and organized shower lines may produce gusty winds up to 35 mph.

While snow levels will gradually trend downward through Sunday, they are expected to remain above 6,000 feet. Consequently, impacts will be limited to the higher peaks with no significant travel concerns anticipated through the weekend.

Monday through Tuesday:

Widespread precipitation will continue as the upper-level low currently off the West Coast moves inland. As the system de- amplifies and crosses the Northern Rockies in several waves, snow levels will drop to approximately 5,500 feet. This creates a 55% probability of over 1 inch of slushy snow on Monday morning, specifically impacting high-elevation routes such as Lost Trail Pass, Gilmore Summit, and the Georgetown Lake area.

Wednesday through Thursday:

The primary forecast concern remains a potent cold front slated for mid-week. High confidence exists regarding a significant temperature drop; current guidance shows snow levels plummeting to 1,500-2,000 feet by Thursday morning. This setup introduces the first legitimate potential for accumulating snow on valley floors this month. Pass level roads stand a decent chance (>70%) of impacts in the form of winter driving conditions.

Uncertainty & Impacts: The main forecasting challenge is the moisture-cold air overlap.

1. Scenario A: If the front moves through quickly, the coldest air may arrive after the deepest moisture has exited, resulting in minimal valley accumulation.

2. Scenario B: If the moisture lingers as the cold air settles in, more widespread travel impacts and accumulating snow on valley floors become likely.

We will continue to monitor model trends to refine the timing of this transition.

AVIATION

Southwest flow will continue to transport weak disturbances and moisture across the Northern Rockies through Sunday. Diurnal instability will support rounds of showers and occasional thunderstorms (primarily south of I-90 (30% probability)).

Main Hazards:

- Wind: Erratic gusts to 30 kt possible near thunderstorms and organized shower lines. - Visibility/Ceiling: Prevailing VFR, though temporary MVFR conditions at terminal sites and frequent mountain obscurations are expected during precipitation episodes.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.