textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Valley inversions through Thursday this week, with near normal temperatures. Areas of degrading valley air quality.

- Increasing chances for areas of valley fog/stratus by Tuesday/Wednesday.

- Ridge breakdown by Friday, with risk for valley freezing rain.

Valley inversions are the main highlight this week as a ridge of high pressure builds over the West Coast. Satellite imagery currently reveals remnant subtropical moisture rotating around this ridge, bringing high-level clouds. While these clouds could prevent temperatures from reaching their potential minimums, most locations will remain colder than most of December thus far tonight. Waves of mid-high level cloud cover complicates the temperature forecast; the primary challenge lies in whether cloud cover will be more prevalent at night, keeping lows slightly warmer, or during the day, which would suppress afternoon highs.

Numerical models remain in strong agreement that the ridge axis will shift directly over the Northern Rockies by Wednesday. As the atmosphere stabilizes, an expansion of valley fog and/or low stratus is probable. These stagnant air masses will lead to areas of degrading air quality through Thursday.

Looking towards Friday, the ridge is predicted to break down. Approximately 75% of ensemble members favor a Friday evening arrival for the incoming system, while the remaining 25% suggest a slower progression into Friday night. A significant concern during this transition will be snow or freezing rain in the valleys, depending on the strength and persistence of the established cold pools. Snow levels will initially be near or above valley floors, before rising above 4,500 feet. Further details on precipitation types will be provided in the coming days.

AVIATION

As the ridge axis shifts toward the Northern Rockies, atmospheric stability will increase through the next few days. High clouds may delay the onset of widespread fog tonight, with localized areas of fog along valley river systems. Expect an increasing probability of valley fog and/or stratus by Tuesday into Wednesday as valley inversions strengthen.

Winds/Turbulence: Winds will remain light and variable at the surface. Light to moderate northwesterly flow aloft will persist as the region sits on the front side of the ridge. No significant turbulence or LLWS is anticipated within the first 24 hours.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None.

ID...Air Stagnation Advisory until 12 PM PST Thursday for Lower Hells Canyon/Salmon River Region...Northern Clearwater Mountains...Orofino/Grangeville Region...Southern Clearwater Mountains.