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. This week large human-triggered avalanches, capable of killing or severely injuring a person, occurred on Gunsight Pass and northwest of Blind Bull Summit. 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.
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 foot in depth and limited to isolated terrain features.
( Discussion is generated from the previous day's afternoon forecast. )The high-pressure ridge will move eastward ahead of a cold front that is expected to impact the forecast area by Saturday morning. Snow should develop first over the Tetons and Togwotee Pass and then quickly spread south to the Southwest Trails. Accumulation is expected to be 4 to 10 inches across the region by Sunday morning. Temperatures will start in the teens with a slight valley inversion and then warm into the twenties. Ridgetop winds are expected to be southwesterly at 10 to 20 miles per hour with stronger gusts. On steep slopes, dangerous avalanche conditions remain a concern, where the weight of a single person could trigger a one to three foot deep avalanche. Above 7,500 feet, the general avalanche hazard is expected to be Considerable in the Southwest Trails and Teton areas and Moderate on Togwotee Pass. Below 7,500 feet, where avalanches are unlikely, the general avalanche hazard is Low. As tomorrow’s new snow loads slopes, the increased weight could add to the potential of triggering persistent slabs. These killer slabs could be triggered by the first person or after a slope has been crossed many times. Careful assessment of terrain is important for safe travel in the mountains.
