textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Today: Patchy morning fog gives way to afternoon showers and isolated thunderstorms. Main threats: Gusty winds to 40 mph and small hail.

- Tonight-Monday: Transition to steady valley rain, slushy snow for passes above 5000 feet.

- Wednesday-Thursday: Significant cold front. Snow levels dropping to 1,500-2,000 ft by Thursday morning. Light valley snow accumulations and moderate travel impacts on mountain passes.

Today through Tuesday:

Early morning showers have largely tapered off, watch for patchy dense valley fog through the mid-morning hours where clearing occurs. The focus now shifts to a closed upper-level low currently moving onto the Northern California coast. As this system moves inland, it is expected to weaken and fragment, sending a series of disturbances and a plume of Pacific moisture across the Northern Rockies.

This will result in widespread showers and isolated thunderstorms developing late this morning and spreading south to north through the evening. Primary threats with any thunderstorms include:

- Gusty outflow winds up to 40 mph.

- Brief heavy rainfall and small hail.

Precipitation will transition to a stratiform regime tonight into Monday morning as a weak surface low develops over southern Idaho and tracks into southwest Montana. A focused band of precipitation is expected to set up from north-central Idaho into west-central Montana.

- QPF: Total moisture through Monday morning will range from 0.50" to 1.50" for Idaho and Clearwater counties, with 0.25" to 1.00" across west-central Montana.

- Snow Levels: Levels will hover between 5,000 and 6000 feet by early Monday. High-elevation passes, including Lolo, Lost Trail, Homestake, along with Georgetown Lake may see 1-2 inches of slushy snow accumulation.

By Monday afternoon, the region transitions to a westerly (zonal) flow that will persist through Tuesday. This pattern will keep light, terrain-driven showers in the forecast, particularly across northwest Montana, while lower elevations remain mostly dry.

Wednesday through Friday:

Confidence remains high that a potent cold front and associated low pressure system will impact the Northern Rockies beginning Wednesday. This system will usher in an unseasonably cold air mass, causing temperatures to drop significantly below seasonal averages. Snow levels are projected to plummet to 1,500-2,000 feet by Thursday morning. Latest guidance indicates a high probability for light snow accumulations at all elevations, including valley floors. Mountain passes and higher-elevation valleys especially in southwest Montana are the most likely to experience moderate travel impacts from accumulating snow during this period.

AVIATION

A closed low moving onto the Northern California coast will break apart sending several disturbances and Pacific moisture across the Northern Rockies into Monday. A few showers continue early this morning but overall a diminishing trend. Patchy dense fog may impact terminal sites where areas of clearing occur early this morning.

Showers become widespread from south to north late this morning through this evening. There is a potential for isolated thunderstorms mainly south of I-90. Watch for gusty winds to 35 kts, brief heavy rain, and small hail with any thunderstorms.

Precipitation will become stratiform tonight into Monday morning along a band expected to stretch from west-central Montana into north-central Idaho as a weak surface low develops over southern Idaho and moves into southwest Montana. MVFR and occasional IFR conditions are likely to impact terminal sites tonight into Monday morning.

MSO WATCHES/WARNINGS/ADVISORIES

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