textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Ridge Building This Weekend: Drier weather and warming temperatures aloft will move in Saturday. This pattern typically traps cold air in the valleys, leading to persistent fog and potential air stagnation by early next week.

- Complex System Monday-Tuesday: Moisture moving into stable air/valley inversions will bring a wintry mix in precipitation, especially Monday morning.

A few mountains are experiencing showers under northwest flow, this activity may continue through late afternoon. Northwest flow will weaken by Saturday as high pressure propagates eastward. Atmospheric expansion under the ridge of high pressure will allow for warming temperatures in the mid- slopes. Models are predicting periods of clouds rotating through the ridge, timing of these clouds will be crucial in how strong valley inversions develop. There is some confidence that well developed cold pools will be possible north of I-90 due to snow cover, while south of I-90 remains some questions.

Strength of cold pools is important because a weak atmospheric river is expected to impact the Northern Rockies Monday into Tuesday. This AR will be capable of producing wind gusts overnight Monday of 35 to 45 mph in the mountains. Additionally, there is a spread in precipitation amounts of 0 to 0.50 inch north of I-90 in the mountains and 0.00 to 0.15 of an inch in the valleys. If valley cold pools are as strong as expected, a period of mixed precipitation (freezing rain) will be possible Monday morning and less likely but again Tuesday morning. Impacts in valleys, especially north of I-90, are scattered areas of icy roadways. Watch future forecast updates for the potential issuance of winter weather products. While a brief period of snow is expected at pass level, the transition from snow to rain may also include a period of freezing rain.

The ridge rebuilds Tuesday into the middle of the week. In locations where cold pools are weakened or scoured out, high temperatures by mid-week could be mid-50s in MT and 60s in north- central ID.

AVIATION

Ongoing snow showers will cause obscured terrain and intermittent lowered visibility through around 10/02z. As high pressure builds tonight, the northwesterly flow will likely maintain a persistent stratus deck overnight, which is typical with an exiting weather system. While continued cloud cover is most likely, fog development cannot be ruled out by Saturday morning especially in valley locations with fresh snow.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.