textproduct: Missoula

DISCUSSION

KEY MESSAGES:

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

- Mid-Week Valley Fog Potential: High pressure building on Wednesday and Thursday increase the likelihood of valley inversions.

- Wetter and Mild Weekend: A Pacific system will tap into atmospheric river moisture beginning late Friday.

Through Tuesday Night: A moist, zonal-to-southwesterly flow remains established across the Northern Rockies today. An initial, weak shortwave trough will cross northwest Montana this morning, generating scattered light showers. Snow levels remain around 5,000 feet and above. While surface winds will remain relatively light for most of the area, a tighter pressure gradient will support localized gusts of 15 to 20 mph from Lemhi County into southwest Montana.

A more robust Pacific disturbance will traverse the region from this evening through late Tuesday, expanding precipitation coverage. Snow levels will be near 6,500 feet by Tuesday morning, ensuring that valley precipitation will fall as rain. Westerly winds will also increase on Tuesday, with valley locations seeing afternoon gusts of 15 to 25 mph. Given the high snow levels and light QPF, overall sensible weather impacts through Tuesday night will remain minimal.

Wednesday through Sunday: Transient upper-level ridging will build across the Intermountain West from Wednesday through Thursday. This stabilizing air mass will foster the development of valley inversions, leading to areas of low stratus and patchy fog. High temperatures during this timeframe will be heavily dependent on the persistence of this stratus deck; valley locations that remain trapped under the inversion will struggle to warm, while higher elevations enjoy clear skies and milder conditions.

Ensemble guidance is in excellent agreement that the ridge axis will migrate eastward by Friday. This shifts the Northern Rockies into a southwesterly flow regime, which will effectively mix out any lingering valley inversions. Simultaneously, this southwest flow will tap into an Atmospheric River impacting the West Coast, advecting a plume of anomalous moisture into the Inland Northwest. Snow levels will remain notably high through the weekend, generally holding between 6,000 and 7,000 feet. Overall precipitation amounts appear light to moderate in the mountains and very light in the valleys as much of the precipitation is intercepted by the Cascades.

AVIATION

A moist Pacific flow will persist across the Northern Rockies through Tuesday, resulting in periods of light precipitation and periodic reductions in flight categories. An initial, weak shortwave disturbance will track across northwest Montana this morning, bringing occasional light showers and widespread mountain obscuration. Elsewhere, localized valley fog will create patchy IFR/LIFR conditions early this morning, giving way to prevailing VFR conditions by late morning. Surface winds will remain relatively light for most terminals, though localized gusts of 15 to 20 kts are expected from Lemhi County into southwest Montana.

A more potent disturbance will sweep across the region this evening through late Tuesday, spreading widespread light precipitation across the airspace and reintroducing extensive mountain obscuration. Snow levels will be near 6,500 feet MSL by Tuesday morning, ensuring valley precipitation falls entirely as rain. Westerly surface winds will also strengthen on Tuesday, with afternoon gusts of 15 to 25 kts expected at most valley terminals.

MSO WATCHES/WARNINGS/ADVISORIES

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