The National Weather Service has issued a winter weather advisory today. New snow will load slopes and increase the potential for backcountry travelers to trigger large, dangerous, persistent slab avalanches. These dangerous slabs are failing on persistent weak layers (PWL) and could be triggered by the first person or after the slope has been crossed multiple times. Although the persistent slab avalanche remains the main concern, strong winds and heavy snowfall rates could develop new surface slabs. Backcountry travelers should monitor the storm and choose safe terrain, especially as snowfall rates increase. Below 7,500 feet, avalanches are expected to be isolated to steep terrain in areas with a deeper snowpack. If a significant amount of snow falls at the lower elevations, a layer of buried surface hoar could become reactive. Careful terrain assessment and conservative decisions will be essential to travel through the mountains today.
At the mid and upper elevations on a variety of aspects, large slab avalanches up to three feet in depth can be triggered, on buried persistent weak layers (PWL), by the weight of a single person. At the lower elevations, slabs are expected to be less than a foot in depth.
Periods of light to moderate snowfall will continue overnight before a drying trend begins Sunday morning. Overnight lows in the mountains will be in the low teens before rising into the upper teens during the day. Westerly ridgetop winds at 20 to 30 miles per hour will veer to the northwest and decrease to 5 to 15 by morning.Dangerous avalanche conditions exist in the Teton and Southwest Trails forecast zones, and the general avalanche hazard is expected to remain Considerable on Sunday. While skiers and riders could trigger small pockets of newly develop wind slab near upper elevation ridgelines, the primary concern will be persistent slab avalanches failing on crusts and faceted snow from the December 11th interface. In steep, avalanche prone terrain above 7,500 feet, skiers and riders could trigger these large to very large slabs on a variety of aspects. The underlying persistent weak layers and sliding surfaces vary significantly across the landscape; some slopes are stable, others are not, and it is difficult to distinguish between the two. Conservative terrain choices and route finding will be essential for safe travel in avalanche terrain on Sunday.On Togwotee Pass, where a fraction of the snowfall has accumulated since the 11th, the general avalanche hazard is expected to remain Moderate above 7,500 feet. There, backcountry travelers have the potential of triggering small to large persistent slabs on steep, avalanche prone slopes.
