Mark Staples
2015-2024 - Director, Forest Service Utah Avalanche Center
Brief history:
Snow fell in October and became faceted. In early to mid-November, warm weather formed a crust on top of this layer which remained on north and east-facing slopes. Warm weather melted snow off of south and west-facing aspects. A little more snow fell on top of the crust around Nov 20th, and this snow faceted. All these facets formed the persistent weak layer that is now at the bottom of the snowpack.
When heavy snowfall arrived during the week of Thanksgiving, many avalanches fractured on this persistent weak layer. Some avalanche failed on facets above the crust. Some failed on facets under the crust. Some slopes avalanched, reloaded with more snow and avalanched again. Following the Thanksgiving snowfall, two smaller storms occurred around Dec 5th and Dec 8th. The most recent storm occurred Dec 12-14th.
A more detailed review of these storms and the avalanche activity is recorded in our "Week in Review" found HERE.
More info on how this layer formed and where it exists HERE.
Current situation as of December 19th:
This persistent weak layer is generally buried 2-4 feet deep. It is buried less deep on slopes that previously avalanched.
Avalanche activity on this persistent weak layer has tapered off. Obvious signs of instability like collapsing and cracking have stopped. Stability test results have been improving. These are all good signs, and the likelihood of triggering an avalanche on this persistent weak layer is decreasing. The odds of triggering an avalanche on this layer don't go away; instead, they go down a little bit each day. Unfortunately, the consequences of being caught in an avalanche on this layer don't change. They remain severe, and even get worse as the slab of snow on top of this layer gets thicker and harder.
What will it take for this layer to go away or heal?
The short answer is that it doesn't go away for a very long time. Three years ago, we were in a similar situation. On Dec 19, 2016 following a large storm with strong winds, a massive avalanche in the Birthday Chutes in Little Cottonwood Canyon caught a snowboarder who miraculously walked away unharmed. The Salt Lake Tribune wrote a story about this avalanche. See a video HERE. Photo of the Birthday Chutes avalanche with a crown that ranged from 2 ft to 10 ft:
The persistent weak layer near the ground this winter can easily come back to life and produce another widespread avalanche cycle if we get another major winter storm. For now, it is insulated by a thick layer of snow. This thick layer of snow is good and bad. It's good because it is insulating the persistent weak layer and allowing it to slowly gain strength. It's bad because this hard slab can make a large and deadly avalanche.
A little patience is the best strategy. According to Trent-
What to do now?
"I get it - the snow looks amazing on steep northerly facing aspects, the desire to ride these lines is pulling me in. However, I am just fine looking at it and saying "soon enough." The risk does not equal the reward in my book."
"What will it take for me to trust this weak layer? More time. No more avalanches occurring on this layer. Steadily improving stability test scores. Another large loading event (aka big storm) without avalanches failing on this layer."
What to do now?
The snowpack is phenomenal for December with great coverage and great riding conditions. The challenge is that riding conditions on south-facing slopes that don't have this persistent weak layer have deteriorated. These conditions will push us onto northerly facing slopes that have good powder, but these slopes also have this weak layer lurking near the ground. If you want to ride on northerly facing slopes:
(1) Start with slopes less than 30 degrees in steepness where avalanches rarely occur.
(2) If you choose to ride steeper slopes, stack the odds in your favor. Every day that you wait, the odds of triggering a slide go down more. Also, choose slopes with a clean runout zone so that if you do trigger an avalanche it won't smash you into trees or rocks and cause deadly trauma. Make sure you only expose one person at a time. Make sure you and your partners watch each other from safe locations.
(3) Read the avalanche forecast every day. A lot may change in coming days and weeks.
The current persistent weak layer will heal and will go away (mostly), we just need to give it time and respect.
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