174 

FXUS65 KMSO 300757

AFDMSO



Area Forecast Discussion

National Weather Service Missoula MT

1257 AM MST Tue Dec 30 2025



.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.



Observations around the region are beginning to define the valley

inversions. For instance, Point Six above Missoula at 8000 feet

should be 15 to 18 degrees cooler than Missoula, its only 3 

degrees cooler. Finleys Meadow at 4400 should be 5 degrees cooler 

than Missoula, it is 5 degrees warmer. While this is a single 

locations it is indicative of the entire forecast area. Decent 

valley inversions are already in place and will strengthen as 

Wednesday approaches, as midslopes warm slightly under high 

pressure.



What does this mean for the forecast? Expect valley fog and

stratus to develop and increase in both coverage and longevity as

the week continues. The stagnant air masses will allow for a slow

degradation of air quality. Air stagnation advisories have been 

issued for north-central ID, while western MT will be monitored. 



Ridge breakdown is still on track for Friday with 80% of the

models in agreement. The remaining 20% would suggest a slightly 

later breakdown, overnight Friday into Saturday. The key concern 

would be established cold pools allowing for a period of freezing 

rain to occur in valleys, prior to changing over to rain. Snow 

levels are forecast to be just above valleys at onset of 

precipitation are then expected to quickly rise above 5000 feet. 

More details to come on the evolution of the ridge breakdown.







&&



.AVIATION...As the ridge axis shifts toward the Northern Rockies,

atmospheric stability will increase through the next few days. 

High clouds are delaying the onset of widespread fog. However, 

there may be localized areas of fog along valley river systems 

after sunrise today. No major impacts to area terminals are 

expected today, as reflected in the TAFs. Expect an increasing 

probability of valley fog and/or stratus overnight into Wednesday 

as valley inversions strengthen.





&&



.MSO WATCHES/WARNINGS/ADVISORIES...

MT...ID...Air Stagnation Advisory until noon PST Thursday for Lower Hells 

     Canyon/Salmon River Region...Northern Clearwater 

     Mountains...Orofino/Grangeville Region...Southern 

     Clearwater Mountains.



&&



$$