textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Patchy dense fog this morning in Northwest Montana, improving by late morning.

- Seasonably mild and showery conditions continue through Monday with generally light precipitation and snow levels above valley floors.

- A colder system Tuesday through Thursday brings plowable snow to mountains and potentially in valleys east of US-93 with a rain/snow mix to all other valley locations.

Weakening thermal inversions remain established across the region this morning with cooling aloft over the past 24 hours. Mid and high- level cloud cover streaming in from the southwest continues to inhibit efficient radiational cooling in valley bottoms south of the I-90 corridor, limiting widespread dense fog formation in locations like Missoula and Hamilton. However, dense fog remains entrenched in the Flathead and lower Clark Fork valleys. Additionally, localized clearing will continue to allow for temperature and dewpoint depressions to remain close, resulting in periodic development of patchy mist and low stratus through mid- morning.

A progression of weak troughs embedded in southwesterly flow will bring widely scattered light showers to the region today through Monday. Precipitation amounts will remain light, generally ranging from a few hundredths up to 0.20 inches in the valleys. Snow levels will remain elevated between 5,000 to 6,000 feet through the weekend. Light southwesterly winds will continue, with the strongest gusts focused along mountain passes, in higher terrain, and north-south oriented valleys and gaps.

A stronger thermal gradient and transient surface low will track through the souther half of the region Tuesday into Thursday, introducing a colder airmass. Snow levels will become highly diurnal, dropping to near 3,000 feet or lower during overnight periods. Precipitation will generally fall as predominantly rain or a rain/snow mix in the valleys along and west of I-90 during the daytime periods, possibly transitioning to light snow showers overnight.

Storm total precipitation amounts (liquid and melted snow) through Thursday could reach as high as 1 to 1.5 inches in valley locations favoring the southern track of the surface low south of Flathead Lake and along/east of US-93. For the higher terrain, including the highest peaks of the Bitterroots, southern Idaho County mountains, Lemhi County mountains, and mountains east of US-93, high- end snowfall totals of 10 to 12 inches are possible through Thursday evening. Valley locations east of US-93 could see a couple inches of wet, dense snow and periods of impacts to road travel. While valley floor snow accumulations will be minimal west of US-93, any overnight snowfall will likely melt during the day with the potential to re-freeze into patchy ice on elevated roadways during subsequent overnight periods.

AVIATION

Mid and high-level cloud cover continues to prevent optimal radiational cooling and full saturation south of I-90, maintaining predominantly VFR conditions at most regional terminals, though a brief window of patchy fog or lower stratus remains possible around sunrise. Localized dense fog and low stratus will persist at KGPI and surrounding Northwest Montana valleys through at least mid- morning, continuing IFR/LIFR visibility and ceiling reductions. A series of weak troughs embedded in southwesterly flow will continue to trigger scattered light rain showers across the region today, occasionally causing brief MVFR visibility reductions. Surface winds will remain relatively light and terrain-driven today before a slight increase in south and southwesterly winds develops across ridgetops and in north-south oriented valleys and gaps this afternoon.

MSO WATCHES/WARNINGS/ADVISORIES

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