Area Forecast Discussion

DISCUSSION

KEY MESSAGES:

- Scattered showers, isolated thunderstorms, and cooler temperatures this weekend.

- Frost potential in northwest Montana Sunday morning, becoming more widespread on Monday morning.

- Cool and wet conditions Tuesday through Thursday

A cold front moving through the Northern Rockies today will maintain mostly dry and breezy conditions, though isolated thunderstorms are possible in southwest Montana near Butte. Due to dry air near the surface, these storms may produce strong wind gusts up to 50 mph. Tonight, lingering cloud cover will persist for most locations, but clearing skies in far northwest Montana will allow temperatures to drop into the low to mid 30s. This has prompted a Frost Advisory for Lincoln County, where valley frost is expected.

On Sunday, a low-pressure system moving directly over western Montana and north-central Idaho will generate widespread showers, primarily south of Interstate 90 from late tonight through Sunday morning. Snow levels will drop to around 6,000 feet, resulting in 1 to 2 inches of light accumulation in the backcountry. By Sunday afternoon, scattered rain showers will develop across the rest of the region. As skies clear and drier air settles in Sunday night into Monday morning, widespread frost will threaten sensitive vegetation in the valleys of western Montana and the high valleys of north-central Idaho, which may require additional frost advisories.

Monday will provide a brief period of calm weather before another system approaches on Tuesday, increasing thunderstorm activity across all of western Montana and north-central Idaho. By Wednesday and Thursday, weather models suggest this system will move progressively through the region, unlike previous model runs which showed a slower moving system. There remains a high probability (70 to 90 percent) of at least half an inch of rain across portions of north-central Idaho and northwest Montana along the Continental Divide, with lighter precipitation expected in the broader valleys of western Montana. There remains a lower 10 to 20 percent chance of heavier rainfall totaling 1 to 2 inches or more along the Divide. If this wetter scenario develops, localized flooding could become a concern for recently saturated areas, including the Mission Mountains and surrounding valleys.

AVIATION

An upper-level trough will settle over the Pacific Northwest today, gradually pushing a cold front through the Northern Rockies. While mid-level moisture streams overhead, limited dynamic forcing and weak instability will keep the majority of regional precipitation restricted to scattered light showers. VFR conditions will generally prevail at terminals, though occasional mountain obscurations can be expected due to lowering cloud bases, particularly across northwest Montana and along the Continental Divide. High- resolution guidance supports a 20% to 30% chance of thunderstorm development in southwest Montana this afternoon, primarily between 20Z and 02Z, which could briefly impact KBTM.

A tightening westerly gradient will support sustained surface winds of 10 to 15 knots with afternoon gusts reaching 15 to 25 knots. Localized, erratic gusts up to 35 knots will be possible in the vicinity of any afternoon thunderstorms in southwest Montana. Surface winds are forecast to diminish and decouple within the valleys by late evening, but they will remain elevated along regional ridgetops overnight. This will maintain a risk for localized mechanical turbulence over high terrain through Sunday morning. Meanwhile, ongoing light shower activity will transition to areas primarily along and south of the I-90 corridor overnight into Sunday morning.

MSO WATCHES / WARNINGS / ADVISORIES

MT...Frost Advisory from midnight tonight to 11 AM MDT Sunday for Kootenai/Cabinet Region.

ID...None.