textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Valley rain and mountain snow Sunday into Monday morning, focused along and south of I-90.

- High pressure bringing stable conditions and valley fog next week.

GOES Satellite imagery this afternoon highlights a closed low spinning off the Oregon and Washington coastline, with southerly flow aloft sending mid-high level clouds over the Northern Rockies. Regional radar imagery shows shower activity across south-central Idaho, with high-resolution guidance suggesting this activity will shift northward through Lemhi County and west- central Montana into this evening. Overall, this shower activity will bring hit or miss rain amounts of a trace to up to 0.10 inches.

On Saturday, the aforementioned closed low begins to move southeastward along the California coast. Shower activity will shift south of the area, with the majority of locations seeing mostly cloudy skies and dry conditions for Halloween activities. The exception will be across southern portions of Idaho and Lemhi Counties and along the divide in southwest Montana, where scattered showers will bring light precipitation amounts.

The closed low will move across the Great Basin on Sunday, with moisture and lift focusing precipitation along and south of the I-90 corridor. The majority of precipitation will fall with snow levels ranging from 6,000-7,000 feet during the day on Sunday. As the low begins to pivot eastward into the central Rockies Sunday night, cold air advection aloft under northwesterly flow will bring lower snow levels, with snow falling down near 5,000 feet as precipitation rates begin to decrease through Monday morning. Overall, this doesn't appear to be a significant snow event, with minor snow amounts (trace to locally up to 4 inches) above 6,000 feet across southwest Montana and Lemhi County. This includes the US-93 and I-90 corridor over Lost Trail and Homestake Passes.

Ensemble model guidance is in strong agreement for high pressure to build across the western US Monday night into midweek. This stable pattern will lead to morning fog and stratus across many valley areas. The weather pattern through much of the week will be characterized by a split flow regime, with weaker systems attempting to break through the ridge of high pressure by Thursday into Friday, perhaps bringing periodic showers to the area. No significant and/or strong systems are currently on the horizon through next weekend.

AVIATION

A disturbance is on track to pass through western Montana and north-central Idaho this afternoon and evening. This will bring scattered showers, lowering ceilings, and terrain obscuration across the region. As the disturbance exits tonight decreasing mid-level clouds will combine with a stable, saturated boundary layer. This environment points to a return of a mix of fog and low stratus developing overnight impacting sites like KGPI, KMSO, and KHRF.

MSO WATCHES/WARNINGS/ADVISORIES

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