Backcountry travelers may trigger avalanches on wind loaded slopes near and above treeline. Wind slab avalanches may step down into persistent weak layers, causing larger more destructive avalanches. A large natural avalanche occurred on the NE face of Fremont Peak, perhaps in the last day or two. It failed on a layer of facets which developed near the ground. Wind loading in combination with a persistent weak layer was likely the cause of the avalanche on Fremont.
Winds of 15-25 mph are forecast for Thursday 1/5 and gusts may blow as hard as 45 mph. With new snow available for transport wind slab remains the number one problem. Watch for wind loaded areas and avoid starting zones with large wind deposits.
Two persistent weak layers currently hide within the snowpack.
Basal facets are hidden at the bottom of the snowpack everywhere that early season snow remained prior to the storm cycles starting Dec. 28th. In some locations this has developed into full blown depth hoar. Although present in isolated areas, these facets have the continuity to produce avalanches, such as the recent slide on Fremont Peak, which ran to ground. Without digging a snow pit, it is difficult to figure out where this potentially dangerous weak layer exists. Shaded and wind protected gullies originating near and above treeline are the most suspect.
When thick wind slab sits on top of depth hoar, it can potentially bridge the weak layer, however, a skier/rider's weight can cause this layer to fail and the resulting avalanche can be catastrophic. This creates a low likelihood, high consequence scenario. The only sure way to stay safe is avoiding suspect terrain or evaluate carefully by digging pits, performing stability tests and treating this condition with great respect and prudence.
The second layer of concern is at a rain crust formed on Sunday night. This crust may have extended to 11,500 ft. or higher. Like the basal facets, this rain crust layer will be further stressed by wind transported snow and may trigger with the additional weight of a skier/rider.
Storm cycles between Dec 28th and January 3rd delivered approximately 49" at 10,800' and 29" at 9,700'. The most recent storm came in warm and turned cold. Rain Sunday evening, at the head of the storm, created a rain crust and a new potential persistent weak layer at elevations up to and beyond 11,500'. Much of the above treeline and near treeline terrain is covered in rime, as this melts additional natural avalanches may occur.
Like the recent natural avalanche on Fremeont, other wind loaded slopes may release on this weak layer .The combination of windloading on top of weak layers is a deadly recipe. If traveling in the backcountry use caution and avoid wind loaded pockets and larger wind loaded areas. Remember to observe safe travel practices, such as skiing one at a time and avoid staging in run out zones. As the sun emerges look for additional natural avalanches, an obvious sign of instability.
