Area Forecast Discussion
Issued by NWS Missoula, MT
Issued by NWS Missoula, MT
673 FXUS65 KMSO 091905 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 1205 PM MST Sat Nov 9 2024 .DISCUSSION... KEY MESSAGES: - Pacific moisture arrives tonight, bringing wet snow above 4,500-5,000 feet along the ID/MT border and northwest MT - Active Pacific storm track beginning Monday night, bringing snow to mountain passes by Tuesday morning. The lower valleys in north-central Idaho and western Montana are currently trapped under inversions, with fog and low clouds dominating the weather. Model soundings indicate that these inversions will persist, though some afternoon improvement in fog is expected before it redevelops in the evening. Above the inversion layer, there`s a mix of sun and clouds along with mild temperatures. Looking to the west, satellite imagery shows increasing cloud cover ahead of an approaching shortwave trough in the southwest flow aloft. Snow levels are projected to remain steady around 4,500-5,000 feet, which should limit travel impacts on mountain passes to minimal, with only up to an inch of slushy, wet snow accumulation. The next Pacific system is expected to move onshore by Monday, spreading precipitation along the Idaho-Montana border by mid- afternoon. Snow levels will rise to 5,000-6,000 feet with southwesterly flow ahead of this system, dropping to around 4,000 feet overnight into Tuesday morning as a cold front moves through. Winter driving conditions are likely in mountain passes by Tuesday morning, with Lookout and Lolo Passes seeing a 70% chance of 4 inches or more of snow, and Lost Trail and Marias Passes a 40% chance. A broad trough will remain over the western U.S. from Wednesday through Friday, bringing intermittent mountain snow with snow levels fluctuating between 4,000 and 5,000 feet. None of these days are expected to bring particularly high travel impacts to mountain passes and higher elevations. However, ensemble models suggest a trend toward a split-flow pattern during this period, with more concentrated precipitation moving south into California and the Great Basin. This pattern warrants close monitoring, especially for those planning backcountry activities or travel through the passes. && .AVIATION...Stable conditions under high pressure has caused valley fog to remain persistent across the Northern Rockies, with the lowest visibility and ceiling occurring at KMSO and KGPI. Some locations have broken out of the fog, such as KHRF, but with conditions expected to remain stagnant, redevelopment of fog is expected this evening and overnight. This afternoon, minimal improvements to visibility and ceiling are possible between 09/2200Z and 10/0100Z for KGPI and KMSO, before lowering thereafter. With incoming cloud cover and moisture from the Pacific Coast, improvements will be more likely later in the morning Sunday or early afternoon Sunday. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. && $$