textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- High Pressure to bring prolonged dry pattern next 7-8 days.

- Valley stratus and fog development begins Wednesday into Thursday.

- 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 for the next 7-8 days. Satellite imagery shows the ridge centered over Northern California with the axis along the Pacific Coast. This setup maintains north-northwest flow aloft over the Northern Rockies, sending waves of mid-to-upper level clouds across the region through Wednesday morning. Despite the cloud cover, temperatures will remain well above normal to near or eclipsing record values, with afternoon valley highs reaching the 40s and 50s! As of 2 pm MST, Missoula was already 53 degrees which has broken the previous record of 50 degrees set back in 1983. Butte is on its way of reaching the previous record of 52 set back in 1996. Kalispell hasn't been able to get any wind to mix out the temperature inversion there, and they are about 5 degrees cooler than they were yesterday at this time. Hamilton is 59 at the airport which is crazy for this time of year! We saw robins here at the airport this afternoon, which is unheard of. The Stevensville Airport is sitting at 64 degrees! Over in north- central Idaho, temperatures are approaching 50 degrees. The only place with strong temperature inversions is the Salmon Valley where highs may only touch the freezing mark. Spring in the Northern Rockies...oh wait, it's early January!

Model guidance is in strong agreement that the ridge axis will shift inland toward the ID/MT border by Wednesday afternoon. As temperatures warm aloft and clear skies enhance radiational cooling, valley inversions will significantly strengthen Wednesday night into Thursday. This will lead to expanding fog and stratus, particularly near river systems and large bodies of water like 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 were issued. 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

VFR conditions will prevail into tonight as high pressure remains in control. Mid and high-level clouds will continue to stream overhead in northerly flow, though clearing skies by early Wednesday morning will support the development of valley fog and stratus. While some improvement is expected Wednesday afternoon, the ridge axis shifting overhead will strengthen valley inversions. This will likely result in more expansive and persistent fog and stratus development on Thursday morning.

MSO WATCHES/WARNINGS/ADVISORIES

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