textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Patchy dense freezing fog this morning with reduced visibility and slick areas of roadway in Northwest Montana.

- Mild and showery conditions with overnight fog and low stratus continue through Monday with generally light precipitation amounts.

- Colder air arrives Tuesday and Wednesday, bringing mountain snow and a mix of rain and snow to valleys.

Strong thermal inversions are established across western Montana this morning, with mid-slope terrain temperatures running up to 15 degrees warmer than the valley floors. However, mid and high- level cloud cover streaming in from the southwest has inhibited the efficient radiational cooling and saturation necessary for prolonged and widespread dense fog formation south of the I-90 corridor. Areas of dense fog and freezing fog remain present in the Mission, Flathead, and lower Clark Fork valleys. Road surfaces, especially bridges and overpasses, may become slick in these areas. The likelihood of prolonged periods of widespread overnight and morning fog will continue to linger, but eventually decrease, heading through the weekend.

A series of weak troughs embedded in southwesterly flow will bring widely scattered light showers to the region today through Monday. Precipitation amounts will remain light. The 25th to 75th percentile spread for precipitation totals through Monday evening generally ranges from a few hundredths up to 0.20 inches in the valleys, with up to 0.50 inches possible in the central Idaho mountains. Snow levels will remain above 5,000 to 6,000 feet through the weekend. Light southwesterly winds will develop Sunday and Monday, with the strongest winds and gusts focused along mountain passes and higher terrain.

A stronger thermal gradient and transient surface low will track through the region Tuesday into Wednesday, introducing a colder airmass. Snow levels will become highly diurnal, dropping to near 3,000 feet, or lower, during overnight periods. Minor snow accumulations (up to a couple inches) are expected for the higher elevations of Central Idaho, Lemhi County, and Southwest Montana, including locations such as Lost Trail, Georgetown Lake, and Homestake Pass. Valley precipitation will generally fall as predominantly rain or a rain/snow mix during the day, possibly transitioning to light snow overnight near and above that 3,000 foot mark.

For this Tuesday through Thursday system, the 25th to 75th percentile spread for precipitation totals ranges from 0.10 to 0.50 inches in western Montana valleys, with heavier amounts across central Idaho and southwest Montana. While valley floor snow accumulations will be minimal, any overnight snowfall will melt during the day with the potential to re-freeze into patchy ice particularly on elevated roadways, bridges, and overpasses during subsequent overnight periods. Another low pressure system dropping south into California by Thursday will maintain southwesterly flow and periodic light showers across the Northern Rockies.

AVIATION

Despite strong surface thermal inversions across the Northern Rockies airspace this morning, mid and high-level cloud cover has prevented optimal radiational cooling and full saturation south of I-90. Marginal temperature and dewpoint spreads will maintain VFR and MVFR conditions at KMSO, KBTM, KHRF, and KSMN with remaining low potential for patchy fog or lower stratus especially around sunrise. Localized freezing fog and low stratus will remain a threat for KGPI and surrounding Northwest Montana valleys through at least mid-morning, bringing 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 through 14/2100Z. Surface winds will remain relatively light and terrain-driven today with the potential for more overnight fog and lower ceilings tonight before a slight increase in southwesterly winds develop across ridgetops higher terrain through the day on Sunday.

MSO WATCHES/WARNINGS/ADVISORIES

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