textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Areas of valley fog/stratus into early next week.

- Scattered showers and gusty winds for Sunday.

- High pressure the middle of next week bringing more valley inversions, fog/stratus potential.

Today through Sunday Night:

A weak disturbance traversing the Northern Rockies this morning will trigger some light showers across northwest Montana. Thus far, considerable mid- to high-level cloud cover has largely suppressed fog and low stratus formation. However, localized patches of fog and stratus will likely redevelop in areas that experience partial clearing.

By this afternoon, the development of southerly winds will promote mixing in several valleys, leading to significant warming, particularly across portions of north-central Idaho, Lemhi County, southwest Montana, and the Bitterroot and Mission valleys. Conversely, the Missoula Valley may struggle to mix out. If the inversion holds, temperatures will remain notably cooler, though maximum heating will heavily depend on the extent of any lingering fog or stratus.

On Sunday, a cold front will sweep across the region, generating scattered showers primarily over northwest Montana and north-central Idaho. The most significant impact from this frontal passage will be the complete scouring of persistent valley inversions. This front will also usher in gusty winds, with widespread surface gusts of 20 to 30 mph expected by Sunday afternoon. Higher terrain along the Continental Divide, particularly near Glacier National Park, can expect gusts exceeding 40 mph.

Monday through Friday:

A secondary, subtle disturbance is progged to track through the Northern Rockies late Monday into Tuesday. This system will maintain enough atmospheric mixing to inhibit strong valley inversions while reintroducing a chance for light showers, again favoring northwest Montana.

By Wednesday, long-range guidance strongly favors the building of a high-amplitude upper-level ridge over the region, dominating the weather pattern through the remainder of the workweek. This quiescent pattern carries a high probability for the redevelopment of strong, persistent valley inversions. Consequently, areas of valley fog and low stratus are anticipated, which will moderate diurnal heating and keep valley temperatures below their full mixed potential.

AVIATION

A weak shortwave traversing northwest Montana this morning will produce scattered light showers and considerable mid-level cloudiness. Elsewhere across the Northern Rockies, expect a mix of mid- to high-level clouds. Weak boundary layer inversions persist across several western Montana valleys today. If transient clearing occurs, there is a high probability for localized fog and low stratus development this morning, bringing the potential for IFR/LIFR conditions, particularly at KMSO and KHRF.

Any morning fog or stratus is expected to mix out by early afternoon, yielding widespread VFR conditions with high clouds and relatively light surface winds for the remainder of the day. Looking ahead to Sunday, a cold frontal passage will introduce gusty westerly surface winds, additional shower activity, and potential mountain obscurations across the region.

MSO WATCHES/WARNINGS/ADVISORIES

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