Yesterday a skier triggered a slab avalanche in a steep couloir that drops into Granite Canyon in Grand Teton National Park. Skiers and riders who venture into steep wind loaded avalanche starting zones could trigger dense slab avalanches today. Larger, persistent slabs could also be triggered if a particularly weak spot in the snowpack is impacted or if larger loads such as groups of people or snow machines are present. Carefully evaluate terrain and the snowpack. At the lower elevations small pockets of wind slab lie upon a base of weak faceted snow and could be triggered by humans in very steep, wind loaded avalanche starting zones.
The weight of a single person could trigger a dense slab avalanches up to two feet in depth on a weak layer of buried surface hoar. The sensitivity of these slabs to human triggers may increase as the day warms. Some potential may remain for skiers and riders to trigger deeper persistent slab avalanches with depths to three feet on a layer of faceted snow that is lower in the snowpack.
A fast moving Pacific trough will bring snow to the region. Snow will develop late tonight. Snowfall rates of an inch an hour are possible during the morning hours. The snowfall will taper off to light snow showers in the afternoon. Accumulations of 4 to 8 inches are possible in localized areas. Most areas should get 2 to 4 inches. Strong southwest winds with gusts to 40 miles per hour will accompany the heavy snowfall and will create areas of blowing snow. Temperatures will in the teens and 20s. Tomorrow’s storm will begin to form pockets of new wind slab on leeward aspects. New snow instabilities are likely on steep slopes that experience rapid loading for a sustained period. Skiers and riders who venture into these areas could trigger pockets of new wind slab up to a foot in depth.The new snow will also be increasing the load on existing buried weak layers. Skiers and riders could trigger dense wind slabs up to two feet in depth on a persistent weak layer that formed earlier in December and larger slab avalanches to three feet in depth on a persistent weak layer that formed in November. Monitor conditions and carefully evaluate terrain and snow conditions. The general avalanche hazard is expected to moderate and to be on the increase.
