textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Steady precipitation today south of about I-90.

- Warming and drying trend Sunday into the first part of next week.

Satellite imagery early this morning indicates a cutoff upper level low pressure circulation moving across southern Idaho. A deformation zone north and east of the low pressure center is bringing rain to north-central Idaho, along with west-central and southwest Montana. Rainfall is heavy at times, with rates of up to about 0.30 inches per hour in the Clearwater and Bitterroot ranges.

The upper low will slide across southern Idaho throughout the day today, spreading a band of wrap-around stratiform rain with embedded convection from I-90 south into Lemhi County. Precipitation will taper off late this afternoon and evening. Storm total rainfall will range from 0.50-1.50 inches from northern Lemhi County east to Butte, with isolated totals exceeding 2.0 inches.

Clearing skies tonight will favor valley fog and low stratus, particularly in areas that received substantial rainfall. Upper- level ridging builds across the region Sunday and Monday, establishing a warming and drying trend.

While deterministic models diverge on the details mid to late next week, ensemble clusters favor southwesterly flow, mild temperatures, increased afternoon breezes, and a gradual return of isolated afternoon thunderstorm chances starting as early as Tuesday. By Sunday (Sep 27), ensemble guidance (60-70% confidence) points toward a deeper Pacific Northwest trough pushing inland, bringing cooler temperatures and higher shower chances.

AVIATION

Mostly stratiform rain will continue this morning for terminals along and south of about Interstate 90. Expect decreases in visibility and ceilings down to IFR, especially during times of heavier rainfall. Winds at KMSO will become easterly to southeasterly with the wraparound nature of the precipitation band. Rain and low stratus will obscure terrain across the Clearwater, Bitterroot, and mountain ranges of southwest Montana. Low-level moisture will be conducive to fog formation and low- level stratus this morning and again on Sunday morning, especially in valleys that receive ample rainfall early this morning and today.

MSO WATCHES/WARNINGS/ADVISORIES

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