textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Travel Impacts Through Friday: Expect periodic snow-covered mountain passes (Lookout, Lolo, Marias) and brief periods of low visibility and a quick 1-2 inches of snow in some valleys through tonight.

- Ridge Building This Weekend: Drier weather and warming temperatures aloft will move in Saturday. This pattern typically traps cold air in the valleys, leading to persistent fog and potential air stagnation by early next week.

- Complex System Monday-Tuesday: A surge of elevated moisture will move into the region from the northwest. While this will bring rain and snow, the combination of warm air aloft and trapped cold air in the valleys may cause mixed precipitation, including freezing rain.

A weather disturbance is moving through the region today, keeping snow showers across western Montana and north-central Idaho. The atmosphere is becoming more unstable this afternoon, which means snow showers are more convective and aligning into snow bands. These bands can be hard to track but may drop a quick inch of snow in a short amount of time, briefly lowering visibility and covering roads even in the lower valleys.

Total accumulations through Friday will generally remain light (trace-2 inches for most valleys), but mountain passes could see higher totals of 4-6 inches. Temperatures will remain seasonably cool, keeping snow levels at the valley floors.

The overall weather pattern shifts significantly this weekend. A strong ridge of high pressure will move over the Northern Rockies on Saturday and likely persist through the middle of next week.

The most challenging part of the forecast arrives Monday into Tuesday. While high pressure usually brings dry weather, forecast models suggest a "dirty ridge" pattern, where clouds and moisture move through an area of high pressure. A stream of well-above normal moisture (200-250% of normal) is expected to move over the ridge and dive into western Montana and north-central Idaho from the northwest.

As temperatures a few thousand feet above the ground warm significantly, snow levels are expected to rise to 5,000-6,000 feet. While this would normally mean rain for the valleys, there is a catch: if the layers of cold air trapped in the valleys from the weekend (inversions) remain in place, the cold air at the surface may not be pushed out easily. This creates a messy wintry mix in the valleys before eventually changing to rain.

Current forecasts show light amounts. However, these "overrunning" systems, where warm, moist air flows over a layer of cold air, are notorious for producing more moisture than models initially predict. We are watching for the potential for higher precipitation totals than currently advertised.

AVIATION

Scattered snow showers and bands will continue to impact the region through Friday morning bringing brief, periodic reductions in ceilings and visibilities. Winds will become increasingly northwesterly heading into Friday (ridgetops 30-40 knots) with the potential for any heavier showers or bands to mix these winds down to the surface as gusts.

MSO WATCHES/WARNINGS/ADVISORIES

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