textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Stratus cloud cover and isolated fog will persist through today in north- central Idaho and northwest Montana.

- Colder Air Arrives Beginning this Evening: A modified Arctic front will slide into western Montana tonight and Friday, bringing gusty easterly winds and a drop in temperatures for the weekend.

The main story for today remains the persistent cloud cover in north-central Idaho and northwest Montana. High pressure aloft and light winds at the surface are maintaining a status quo forecast. While a weak shortwave passing overhead may provide just enough lift to raise the stratus deck by afternoon, significant clearing is unlikely for the lower valleys.

By Friday, the synoptic pattern begins to shift. A potent Arctic high pressure system sliding down the lee of the Rockies will push a cold front westward across the Continental Divide. As this colder, dense air spills over the Divide, expect gusty easterly winds to develop, particularly in favorable gap areas like Hellgate Canyon (impacting Missoula) and Bad Rock Canyon (impacting Kalispell) beginning this evening. This mechanical mixing should finally scour out the stagnant valley air.

Moisture is limited with this system, confining the best chances for light snow to the Glacier Region and the immediate Continental Divide. Valleys with lingering cloud cover may have flurries or a light dusting.

Highs on Friday will struggle to reach freezing in north-central Idaho and low 20s in western Montana, as the colder airmass settles in.

Saturday through Sunday: The region will remain under the influence of the cooler airmass. While not a brutal Arctic outbreak (the core of the coldest air remains well to our east over the Dakotas), temperatures will run 5 to 10 degrees below normal. Highs will generally be in the 20s with lows in the single digits to teens. The flow aloft turns northerly/northwesterly, keeping chances for orographic light snow showers in the forecast for the mountains, but widespread valley precipitation is not expected.

Monday through Wednesday: Forecast confidence decreases significantly heading into next week. Models are struggling to resolve the interaction between a potential Pacific trough moving inland and the lingering cold air over the continent.

Scenario A (Wetter): Some ensemble members suggest the Pacific system could override the cold air, leading to an overrunning precipitation event (snow) Tuesday into Wednesday.

Scenario B (Drier): Recent deterministic runs and a cluster of ensembles keep the high pressure ridge stronger, keeping the Northern Rockies dry and cool.

Current NBM (National Blend of Models) guidance leans toward the drier solution, keeping PoPs (Probability of Precipitation) generally low (20-30%) for the start of the week.

AVIATION

Primary aviation concern remains persistent MVFR to localized IFR stratus across north-central Idaho and northwest Montana through the period. High pressure and inversions are trapping low- level moisture, keeping mountain obscurations prevalent across the Cabinet, Mission, and Glacier ranges. In contrast, KMSO and southern terminals (KBTM/KHRF/KSMN) are currently VFR, though a window of stagnant, hazier air may allow for MVFR ceilings to develop at KMSO between 15Z and 19Z. The pattern shifts abruptly this evening (02Z-06Z) as a modified Arctic front crosses the Divide. This boundary will introduce moderate Low- Level Turbulence and gusty easterly winds, particularly through Hellgate Canyon (KMSO) and Bad Rock Canyon (KGPI). While these winds will effectively scour out valley inversions/stratus, they will be accompanied by lower ceilings and potential light snow or flurries near the Divide and Glacier region.

MSO WATCHES/WARNINGS/ADVISORIES

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