textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Slippery Travel: Occasional freezing rain and snow will result in slick travel conditions during the overnight and early morning for valleys and mountain passes through Saturday morning.

- Lemhi County River Ice: The Lemhi River near Lemhi continues to slowly rise due to ice build up. Minor flooding will continue to be possible.

- Weekend Warmth: Temperatures will trend upward through the weekend and peak on Sunday, with widespread highs in the 40s and 50s likely.

- Monday System: A weak disturbance returns Monday bringing valley rain and mountain snow, followed by warming temperature trends for the remainder of the week.

Satellite imagery this afternoon shows a stalled low pressure system parked over the Gulf of Alaska. A weak ridge centered on the Pacific Northwest coastline is helping push some moisture inland. The Northern Rockies are situated downstream of this ridge, placing the region under light west-northwesterly flow aloft. A couple of pushes of very light moisture will continue to move into the region through Saturday morning.

Snow levels are relatively high for this time of year, rising to 4,000-5,000 feet during this weak precipitation event. With stable valley cold pools and cloud cover in place, expect temperature trends to persist, hovering right around freezing for most valley locations through the morning with warming into the upper 30s near 40 by the afternoon. This will lead to a mix of precipitation types, including snow, rain, and freezing rain. Consequently, isolated to widely scattered slick travel conditions will be possible, especially overnight into Friday morning and again late Friday into early Saturday morning.

The ridge along the coast will continue to progress eastward and strengthen over the Northern Rockies through the weekend. Daytime high temperatures will begin to creep well into the 40s and 50s by Saturday and Sunday for many valley locations, approaching daytime record highs. The peak of this short-term warming trend looks to be on Sunday, where confidence is highest. During this timeframe, the typically warmer valleys (lower Salmon River near Riggins, Bitterroot Valley) will have the highest probability of nearing the 60-degree mark.

A brief breakdown of the ridge arrives on Monday as a shortwave trough moves through the Northern Rockies. Snow levels will remain high, generally above 4,000 feet, so valley precipitation will likely fall as rain. Be aware of possible slick spots overnight or early morning, while mountains and passes see minor snow accumulations.

While earlier forecasts spoke of a strong ridge rebuilding for the middle to end of next week, confidence in the hottest temperature scenarios has decreased slightly. Though warmth and even record warmth is still anticipated, much of the forecast area is expected to remain under west-northwesterly flow aloft, with North Central Idaho seeing the best chances for a southwesterly wind gradient, better mixing, and the warmest temperatures throughout the region.

AVIATION

Light precipitation will continue to bring brief reductions in visibility (down to 2-3SM) with a rain/snow mix and the potential for freezing rain (10-20% chance) overnight into Friday morning. A mid-level cloud deck around 8,000ft MSL will remain in place across the region through the day, lowering to approximately 6,000ft MSL overnight. Anticipate terrain obscurations and gusty west winds over mountain crests, especially near the Continental Divide where gusts may reach 30-40 knots.

HYDROLOGY

Due to persistent cold temperatures, area rivers and streams are beginning to see significant ice build up in Lemhi County, Idaho. Some properties are experiencing minor flooding downstream of the Lemhi River gauge, near Lemhi. Watch for increased river levels as we approach the expected warm temperatures this weekend and early next week.

MSO WATCHES/WARNINGS/ADVISORIES

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