Persistent weak layers are present in the snowpack. The storm that occurred two days ago deposited dense wind driven snow onto a layer of well-developed surface hoar. That new snow and strong winds also increased and redistributed the load on persistent weak layers of faceted snow deeper in the snowpack that formed much earlier in the season. Wednesday’s storm triggered the natural release of some large slab avalanches. Notable events occurred on Mt. Hunt, Fossil Mountain, Housetop Mountain, in Granite Canyon and in a drainage on the south end of Saddle Butte. Similar events could be triggered by humans today. Snowpack stability assessments may be inconclusive in these conditions and the consequences of your terrain choices could be fatal. Whumphing and the collapse of the snowpack are strong indications of instability.
Persistent slab avalanches could be triggered by the first person on a slope or on a slope with multiple tracks. These dangerous slabs could be triggered on a variety of aspects. They could be 2 to 5 feet deep at the mid and upper elevations and up to 2 feet deep at the lower elevations.
A weak pacific trough and cold front will pass overnight producing light snowfall and gusty westerly winds. Snowfall is expected to end by the morning with an accumulation of one to three inches in the mountains. Tomorrow skies will be partly cloudy and clearing throughout the day. After the cold front passes, winds will veer to the northwest and are expected to be 15 to 30 miles per hour. Mountain temperatures will start out in the low teens and rise into the upper teens and twenties. Persistent weak layers are tricky to predict and take more time than one might think to become unreactive to triggers such as skiers and riders. In the three forecast areas, the likelihood of triggering slab avalanches varies, but the poor structure of weak faceted snow exists in all of them. Where the snowpack is deeper, like at the upper elevations, it will take a larger trigger such as a snow machine or crossing a thin spot in the slab to trigger these deeper slab avalanches. In the Grey’s River/Southwest Trails forecast area, the avalanche hazard is expected to be Considerable above 7,500 feet, where dangerous avalanches exist and backcountry travelers could trigger slabs 2 to 4 feet in depth. In the Tetons and Togwotee Pass forecast areas, the avalanche hazard is expected to be Moderate, where it is possible for skiers and riders to trigger persistent slab avalanches. Slab depths could range from 4 to 6 feet in areas where the snowpack is deep, like the upper elevations of the Tetons. At the mid and lower elevations in the Tetons and on Togwotee Pass, avalanches will be easier to trigger but slab depths will be smaller like 1 to 3 feet. Careful snowpack and terrain evaluation are essential to safely travel through avalanche terrain.
