textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Near record or record cold temperatures forecast for Saturday and possibly Sunday.

- Snow showers will persist along the Divide and points west through the weekend. Roadways and mountain passes could become snow-covered at times, especially overnight and in the early morning.

- Gradual warm-up with afternoon showers is expected Tuesday and Wednesday. Frosty mornings are possible.

- High temperatures reaching the 70s by Friday across north- central Idaho and western Montana lower valleys.

An unseasonably cold airmass will remain entrenched across the Northern Rockies through early next week. Cold temperatures aloft will maintain steep lapse rates and daily instability showers. Additionally, higher surface pressure east of the Divide coupled with lower pressure to our south and west will drive nocturnal easterly winds. This easterly flow and associated orographic lift will support persistent snow showers along and west of the Divide, particularly across the Glacier Region.

Freeze Warnings and Frost Advisories remain in effect for many lower valleys. While patchy cloud cover and channeled easterly winds may limit cooling in some localized areas, optimal radiational cooling elsewhere will allow temperatures to plummet into the teens and low 20s. This poses a continuing threat to unprotected outdoor plumbing and irrigation systems.

Model guidance highlights a secondary upper-level trough dropping into Idaho Sunday, focusing precipitation across southern Idaho and Lemhi counties. This feature brings the potential for winter driving conditions and snow-covered roads at Lost Trail Pass and other Southwest Montana passes by Monday morning.

Broad cyclonic flow persists through Wednesday as the primary upper low shifts toward the Great Lakes. This pattern will maintain below-average temperatures, frosty mornings, and daily afternoon instability showers.

A significant pattern shift arrives late in the week as a highly amplified upper ridge builds into the region. Ensemble probabilities strongly highlight a return to the 70s for western Montana valleys, with temperatures approaching 80 degrees in Hells Canyon.

AVIATION

Patchy fog will be possible this morning for valley locations that see clearing towards sunrise, mostly notably for KMSO and KGPI. Any fog that develops will be short lived. An unseasonably cold airmass will maintain steep lapse rates across the Northern Rockies, promoting scattered diurnal rain and snow showers that will yield localized MVFR to IFR conditions and widespread mountain obscurations. These orographic snow showers will be most persistent along and west of the Continental Divide, particularly impacting the Glacier Region airspace. Overnight, a tightening pressure gradient will drive enhanced easterly winds through terrain-channeled valleys, introducing the potential for low-level mechanical turbulence and localized crosswinds at impacted terminals. Otherwise, predominantly VFR conditions are expected outside of shower activity and patchy morning cloud cover.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Freeze Warning until 8 AM MDT Sunday for Bitterroot/Sapphire Mountains...Flathead/Mission Valleys...Kootenai/Cabinet Region...Lower Clark Fork Region...Missoula/Bitterroot Valleys.

ID...Freeze Warning until 8 AM MDT /7 AM PDT/ Sunday for Eastern Lemhi County...Northern Clearwater Mountains...Southern Clearwater Mountains.

Frost Advisory until 8 AM MDT /7 AM PDT/ Sunday for Orofino/Grangeville Region...Western Lemhi County.