Area Forecast Discussion

DISCUSSION

KEY MESSAGES:

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

- Frost potential Sunday morning, more widespread on Monday morning.

- Increasing potential for widespread precipitation by Tuesday and Wednesday next week with a closed low pressure system.

An upper-level trough will settle over the Pacific Northwest today, gradually pushing a cold front through the Northern Rockies. Daytime temperatures will be roughly 10 degrees cooler than yesterday. While mid-level moisture will stream overhead, overall dynamic forcing and instability remain limited.

Impacts (Today through Sunday): * Thunderstorms: High- resolution model guidance supports a 20-30% chance of thunderstorms across southwest Montana this afternoon. * Winds: A westerly gradient will generate afternoon gusts of 15-25 mph, with localized gusts up to 40 mph possible near any thunderstorm activity in southwest Montana. Watch for localized elevated fire weather concerns this afternoon in southwest Montana. * Precipitation & Snow: Showers will focus in northwest Montana and along the Divide today, then south of the I-90 corridor tonight through Sunday morning. As the upper trough passes overhead on Sunday, additional scattered instability showers will develop. Total precipitation looks light, though minor snow accumulations of 1-2 inches are possible above 5,000 feet. * Temperatures: Highs on Sunday will drop 10-15 degrees below normal. Combined with afternoon wind gusts of 15-25 mph, conditions will feel unseasonably chilly.

Frost Potential (Sunday and Monday Mornings): Drier air and clearing skies will introduce frost concerns for sensitive vegetation. * Sunday Morning: Patchy frost is possible within the valleys of northwest Montana. * Monday Morning: A more widespread frost event is anticipated for the lower valleys of western Montana and the higher valleys of north-central Idaho.

Mid-to-Late Next Week Outlook: Ensemble guidance is in excellent agreement regarding a large, potent weather system arriving by mid-week, bringing the potential for significant, widespread precipitation across western Montana and north-central Idaho.

Ahead of this system, thunderstorm activity is expected to become more numerous across the region, particularly by Tuesday. Most locations have a decent probability (50-70% chance) of receiving over half an inch of rain between Tuesday and Thursday.

While less likely, there is a 10-20% chance of this system tapping into deeper, subtropical moisture. This scenario could deliver 1-2+ inches of rain along the Continental Divide. If these heavier amounts materialize, localized flooding will become a concern, especially in recently saturated basins such as 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.

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...None. ID...None.