textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Dry weather will continue through early next week (January 20). Expect persistent valley fog, freezing fog, and low clouds, which may cause icy spots on roads and low visibility.

- A 60 percent chance of a shift to colder and wetter conditions is expected mid-to-late next week (January 2123).

The upper level pattern will remains stagnant for the most part through early next week as high pressure remains in place along the west coast and and a cold trough of low pressure digs into the Northern Plains. This will place western Montana and north central Idaho under stable north to northwesterly flow.

The main forecast challenge under this pattern will be valley temperatures and the formation of fog and low clouds. Current satellite imagery reveals widespread fog and low stratus already in place through northwest Montana, the Camas Prairie in Idaho, the southern Mission valley and in the vicinity of Salmon. As temperatures warm aloft (700 mb) tonight into Saturday morning, fog and stratus is anticipated to expand, likely filling into the Flathead valley, Missoula valley and portions of the Bitterroot valley. Once fog develops tonight, it will likely linger through much of the day on Saturday. Due to cold temperatures in place, some ice development will be possible for areas that see persistent fog. Locations out of the fog will see generally sunny skies and relatively mild temperatures.

On Sunday, though the upper level pattern remains similar, another push of cooler air out of Canada into central Montana may provide enough lift to improve fog conditions, similar to what occurred today. This will especially be true for valley locations along Highway 93 and east, including Kalispell and Missoula.

The forecast becomes more challenging mid to late next week, as forecast models continue to show the potential for an arctic cold front to bank up against the Continental Divide, spilling into northwest Montana and possibly further west. There is a 50 percent chance that this begins across northwest Montana as early as Wednesday, January 21st. Higher probabilities (50 to 80 percent) exist for another, possibly colder surge across all of western Montana by Friday, January 23rd with the highest chances across northwest Montana. There are also indications of increased precipitation chances, especially towards Thursday and Friday. This could translate to mountain snow and mixed precipitation in the valleys. Stay tuned as this period is still highly volatile, especially in terms of timing and westward progression of the colder air.

AVIATION

Valley fog and low stratus will once again be the main impact for area aviation terminals into Saturday morning. Persistent fog and low stratus remains in place across far northwest Montana (KS59, K88M), the Camas Prairie in Idaho (KGIC), the southern Mission valley (K52S), and in the vicinity of KSMN. This fog and stratus is expected to expand this evening into the overnight hours as warming occurs aloft, affecting KMSO, KGPI and KHRF. This looks more likely to occur around/after 17/1000z. Fog and low stratus will likely linger through much of Saturday for areas that see it develop overnight tonight. Obscured terrain due to low ceilings and low visibility will be the main aviation hazards. Above the fog and stratus, generally clear skies are anticipated.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None.

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