The National Weather Service has issued a winter storm warning through Monday morning. Temperatures will warm throughout the storm creating an upside down snowpack. During the day, light density snow and strong southwesterly to westerly ridgetop winds will begin to form new soft slabs in leeward starting zones at the upper elevations. Backcountry travelers could trigger these shallow slabs, along with loose snow sluffs, later in the day. Wind loading will also add weight to persistent slabs, which will become more susceptible to failure as the day progresses. Monitor conditions closely, and be willing to alter your plans if circumstances warrant.
Persistent slabs lie upon sun crusts, wind pack, near surface facets and surface hoar at varying depths throughout the snowpack. These potential sliding surfaces formed during previous dry spells, creating a snowpack with poor structure. The likelihood of triggering these slabs to depths of one to three feet will increase during the day with the additional load of the new snow.
The second wave of this storm is expected to arrive during the early morning hours and peak during the day. This pulse will be accompanied by very strong west winds at speeds of 25 to 40 with gusts over 60 miles per hour at the pass and summit levels that will create areas of blowing and drifting snow. New snow accumulations of 4 to 8 inches are expected in the mountains overnight and will continue to favor the Salt River Range, the Snake River Range and the southeast highlands of Eastern Idaho with the lesser amounts likely on Togwotee Pass. Another 6 to 8 inches of accumulation are likely during the day before a possible decrease in these snowfall rates on Saturday night. Temperatures will rise into the 20s in the mountains during the day and into the 30s in the lowest elevations where a mix of snow and rain is possible in the afternoon.The general avalanche hazard is expected to increase to HIGH in the Southwest Trails/Greys River and at the upper elevations of the Tetons, areas which are expected to continue to receive the most snowfall. Travel in avalanche terrain is not recommended in those areas. The general avalanche hazard in the Togwotee Pass/Continental Divide area is expected to increase but remain CONSIDERABLE. Expect knowledge and conservative terrain choices will be necessary for safe travel in avalanche terrain where the hazard is considerable. In all areas the hazard will be increasing. The new snow, warming temperatures and strong winds will create dangerous avalanche conditions. Natural avalanche activity is likely or possible and large human triggered avalanches are likely. Poor snowpack structure presents the likelihood for surface slab avalanches to step down to deeper weak layers with total depths of three to five feet.
