textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Minimal Impacts Today: Periodic light precipitation and breezy westerly winds.

- Wednesday and Thursday Fog/Stratus Potential: Strong high pressure increasing likelihood of valley inversions.

- Pattern Shift: A Pacific system will tap into atmospheric river moisture beginning Saturday, snow levels stay relatively high.

Today and Tonight: A Pacific system is bringing a wave of light precipitation to the Northern Rockies early this morning, producing up to a tenth of an inch per hour in the favored mountains. With snow levels remaining above 6,000 feet, winter impacts will be minimal. Precipitation will transition to scattered showers after sunrise. Meanwhile, westerly valley winds will increase late this morning into the afternoon, gusting 15 to 25 mph before diminishing this evening. Tonight, a glancing blow from an atmospheric river will push another round of light precipitation into northwest Montana. Wednesday through Sunday: Strong upper-level ridging builds across the Intermountain West Wednesday into Thursday. Mid-level warm air advection and subsidence will yield highly stable conditions, setting the stage for valley inversions. The primary forecast challenge is the strength of these inversions and their subsequent impact on valley high temperatures, which hinges on the extent of overnight fog and stratus Tuesday night into Wednesday morning and again Wednesday night.

Abundant mid- to high-level cloud cover may limit radiational cooling, potentially restricting widespread fog development. However, areas north of I-90, particularly near Flathead Lake, where warm surface water will enhance boundary layer moisture remain the most likely target for fog and stratus. If persistent stratus materializes, high temperatures in these localized areas will trend several degrees cooler than currently forecast. Conversely, higher elevations will break out of the inversions and experience anomalously warm temperatures for this time of year.

By Friday and into the weekend, ensembles are in excellent agreement that the ridge axis will migrate eastward. This transition places the Northern Rockies under a southwesterly flow regime, introducing cold air aloft that will help mix out any lingering valley inversions on Saturday. Under a broader split- flow pattern, an atmospheric river impacting the West Coast will advect a plume of moisture into the Inland Northwest on Saturday. Snow levels will remain notably high through the weekend, generally hovering between 5,500 and 6,500 feet. Precipitation amounts will be light in the mountains and very light in the valleys, as the Cascades intercept much of the moisture and the primary trough energy digs well south of our region.

Next Week: Ensemble guidance favors a continuation of this split- flow pattern through much of next week. Successive upper-level troughs and atmospheric rivers will crash into the West Coast, but the bulk of the dynamic energy will dive south into California and the Desert Southwest. Consequently, the Northern Rockies can expect generally seasonable temperatures and relatively light precipitation.

AVIATION

A weak Pacific system is pushing a wave of light precipitation across the Northern Rockies early this morning, producing widespread terrain obscurations and degraded ceilings across north-central Idaho and northwest Montana. With snow levels remaining above 6,000 feet MSL, precipitation will fall strictly as rain at all valley terminals.

Steady precipitation will transition to scattered showers after sunrise. Meanwhile, westerly surface winds will steadily increase by late morning. Expect gusts of 15-25 knots at most valley terminals through the afternoon before the boundary layer decouples and winds diminish this evening.

Tonight, a moisture plume associated with a West Coast atmospheric river will deliver a glancing blow to the region, bringing a secondary round of light precipitation and renewed mountain obscurations, primarily across northwest Montana. As this system exits, gradual clearing overnight into Wednesday morning will promote radiational cooling. This will lead to a moderate probability of valley fog and low stratus, but there is still a potential for lingering high/mid level clouds limiting fog/stratus development.

MSO WATCHES/WARNINGS/ADVISORIES

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