textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Areas of frost on Tuesday morning.

- Drying and warming conditions by mid-week.

- Trending wetter late Thursday into Friday.

An upper-level low continues to track east toward the Northern Plains, leaving lingering mountain terrain showers along the Continental Divide. Scattered snow showers will continue for Logan Pass in Glacier NP into this evening leading to slushy conditions at times. Westerly winds gusting to 20-25 mph will diminish near sunset, yielding a cool, autumnal afternoon.

Clouds will persist across northwest Montana tonight, but clearing skies south of Interstate 90 will create optimal conditions for radiational cooling by early Tuesday. Areas of frost are expected in the higher valleys along the Divide and north- central Idaho, as well as cold-prone spots within the Missoula, Bitterroot, and Clearwater valleys. Protect sensitive vegetation.

Scattered showers remain confined near the Canadian border on Tuesday with periodic mid- to high-level cloudiness elsewhere. Daytime temperatures will gradually warm back to seasonal averages through Wednesday.

A wetter pattern returns Thursday into Friday as another upper- level trough slowly traverses the Northern Rockies. Deep southwesterly flow will advect monsoonal moisture into the region, elevating precipitable water (PWAT) values to 150-200% of normal. Weak bulk shear will limit organized severe weather, but diurnal heating will generate modest instability for scattered thunderstorms. Weak steering currents and high PWATs make slow- moving thunderstorms containing locally heavy rainfall and localized runoff the primary hazards.

Ensemble guidance agrees well (90%) on a warming and drying trend from Saturday through next Monday.

AVIATION

Persistent showers are expected across western Montana through the early evening hours. Cloud cover will gradually diminish tonight for most areas except northwest Montana. An increasing pressure gradient will promote breezy westerly surface winds across western Montana this afternoon, producing gusts of 15 to 25 kts, except closer to 30 kts over the Divide. For Tuesday, there could be patches of valley fog over north-central Idaho and west-central/southwestern Montana in the morning giving way to partly cloudy skies and VFR conditions. A northerly wind gradient may help to enhance the early evening north winds at KMSO, and KGPI may experience more variable wind rather than the typical southerly lake breeze.

MSO WATCHES/WARNINGS/ADVISORIES

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