textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- High Pressure to bring prolonged dry pattern through early next week (January 20).

- Areas of valley fog and stratus through Friday.

- Trend toward a major pattern shift to colder and wetter starting Jan 21-22. A blocking ridge of high pressure will remain the dominant weather feature across the Western US and Northern Rockies through early next week. GOES satellite imagery early this morning shows widespread valley fog/stratus across southern British Columbia. Recent images highlight fog development expanding southward into northwest Montana as high cloud cover has began to exit.

Model guidance is in strong agreement for the ridge axis to shift inland toward the ID/MT border this afternoon. As temperatures warm aloft and clear skies enhance radiational cooling, valley inversions will significantly strengthen tonight into Thursday. This will lead to expanding fog and stratus, particularly near river systems and large bodies of water such as Flathead Lake. Widespread stratus is likely across the Flathead and Mission valleys, a solution strongly supported by current guidance. These stagnant conditions will result in degrading air quality with initial concern for lower elevations of north-central Idaho thus Air Stagnation Advisories remain in effect. Additional air stagnation advisories may be required in the coming days across western Montana but the pattern is less favorable at least initially for immediate concerns.

Global ensembles indicate this pattern will persist through early next week. However, latest long-range guidance now suggests high likelihood, near 70% chance of the ridge breaking down by mid-next week (January 21-22). Details still vary on how far west colder air will reach and the amount of moisture available for precipitation production. Overall, this pattern is favorable for colder air spilling over the Divide with both mountain and valley snow across Northwest Montana. Impacts to areas further west and south are less certain given the variability mentioned earlier. Stay tuned as this would be a major shift from the mild and dry weather through early next week.

AVIATION

GOES satellite imagery this morning shows high-level clouds streaming overhead in northerly flow, with recent clearing skies across northwest Montana. Valley fog and stratus has been quick to develop across Northwest Montana as clearing occurred, with IFR/LIFR impacts at KGPI by 14/0800Z. The ridge axis will shift overhead tonight, causing valley inversion to strengthen. This will result in more expansive and persistent fog and stratus development on Thursday morning, especially along and north of the I-90 corridor.

MSO WATCHES/WARNINGS/ADVISORIES

MT...ID...Air Stagnation Advisory until 10 AM PST Friday for Northern Clearwater Mountains...Orofino/Grangeville Region... Southern Clearwater Mountains.