textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- High pressure will bring a long period of dry weather through early next week (January 20). Valley inversions and areas of fog/low clouds are anticipated.

- Sixty percent chance of a shift towards colder and wetter conditions starting mid next week (Jan 21-23).

High pressure remains in place today. Satellite imagery shows a mix of sunny skies and valley fog/low clouds, mainly positioned across northwest Montana and through north central Idaho. Locations not in the fog and low clouds are seeing temperatures warm well into the 40s and even low 50s, while locations in the fog are only warming into the low 30s. Valley fog and temperatures will be the main forecast challenge going into next week as high pressure remains in place with valley inversions persisting.

There is some chance for improvement in foggy conditions later tonight into Friday morning across northwest Montana as a push of colder Canadian air bumping up along the Continental Divide will create a light northeasterly gradient. This may be enough to at least subtlety improve the low visibility and ceilings in place, especially through the Flathead valley. However, as the gradient relaxes tomorrow morning, there is a good chance fog and low clouds will redevelop.

Air stagnation advisories have been extended through 4pm PST on Tuesday through north central Idaho. Stable conditions with valley inversions will continue and may cause degraded air quality with time.

There is still around 60 percent confidence in a pattern change of sorts next week, though the timing of when this will occur is still questionable. The majority of models are indicating January 22-23rd as the period of change as an arctic airmass may cross the Continental Divide into northwest Montana. However, some models bring this in as early as the 21st and others are slower. There is also significant differences in how far south and west this colder air will push. Bottom line: the details are still fuzzy, but a change towards colder and potentially snowy conditions is possible. Stay tuned!

AVIATION

Valley fog and low clouds forecast challenge for aviation terminals this today through the next few days as high pressure remains over the West Coast. KGPI and other locations across northwest Montana continue to see fog and low stratus lowering ceiling and visibility. However, there is some sign of the stratus eroding over Flathead Lake and the Swan valley. This may allow from some improvement as we approach 16/0000z for KGPI. In addition, a colder airmass sinking southwards out of Canada into central Montana will allow for a light northeasterly wind gradient to develop this evening and overnight, which may allow for further improvements to the fog. However, there are indications that the clearing may only be temporary, potentially redeveloping towards 16/1200z. KMSO has cleared out nicely and will likely remain that way through 16/0600z. Thereafter, redevelopment of patchy fog/low clouds is possible, though overall confidence isn't high. As the colder air out of Canada collides with the Continental Divide, some lowering ceilings will be possible, including at KBTM. Otherwise, generally light winds will prevail across the region.

MSO WATCHES/WARNINGS/ADVISORIES

MT...ID...Air Stagnation Advisory until 4 PM PST Tuesday for Northern Clearwater Mountains...Orofino/Grangeville Region... Southern Clearwater Mountains.