Mega storms are over (for now). Storm totals at 45" for 12/12 to 12/16 and 60" for 12/21 to 12/26. Snow surface still mostly soft bumps off the groomers. Still getting plenty of core shots even though the mountain is reporting a 60" base.
BC conditions starting to stabilize out, but I'd still stay away from Northerly aspects for a while. Lots of stuff out near the PC ridgeline ripping to the ground. Some zones that people consider a bit safer are ripping out big, too.
This slide was remotely triggered just outside of the Alta ski resort boundary, a very popular resort-accessed backcountry spot that you very rarely see proper backcountry protocol. This is a perfect example of why people dislike the word "sidecountry". People ride the ski lifts to access a slope that is uncontrolled by any of the ski areas. They feel like they're in a safe area because they rode the lift to get there, but it is still 100% uncontrolled backcountry. Rocky Point, the location of this obs, can be skied off of the Supreme chair and takes generally less than 10 mins to hike out of.
Our biggest instability of concern is a persistent weak layer; an instability that lingers anywhere from weeks up to the entire winter months. In this case, we're dealing with basal facets or depth hoar (basal meaning at the base of the snowpack, depth hoar meaning the facets are now buried underneath a slab of new snow). Facets are totally uncohesive snow created by a large temperature gradient in the snowpack. They're created because the earth's surface is generally warm, while the air temp on clear nights can drop very low. With a thin snowpack, this temperature gradient of, say near freezing at the ground level and 5°F at the snow surface pushes water vapor towards the surface of the snow. This vapor pressure turns our stronger rounded snow into weaker facets. This explains the famous "thin snow is weak snow" saying that you'll hear in any sort of avalanche education (that, and the bridging capabilities are less). Another spot you may see faceting is around a crust layer (melt-freeze or rain crust), because a crust tends to isolate temperatures and create a steep temperature gradient. Science-y stuff - Just as snow can turn from rounded precipitation particles to facets, it can turn from facets back into strong rounds. The critical threshold is 1°C / 10 cm of snow. If the temperature gradient is less than that, say 0.6°C per 10 cm, the snowpack is gaining strength as the facets are turning into rounds. If the gradient is more, say 2°C per 10 cm of snow, the snowpack is getting weaker as the precip particles are turning into facets.
Facets can actually make for a great skiing surface that can be found in sheltered, north-facing slopes well into a high pressure period. It's when the new snow falls on top of the facets (now depth hoar) and there is a slab of relatively stronger, more cohesive snow on top of the weaker, facet layer underneath when things start to get dangerous. Imagine a bunch of champagne flutes with a layer of plywood on top of them. Now load that layer of plywood with bricks. Once the stress of the added load of bricks is more than the strength that the flutes can handle, the stems of the glasses will break and the load of bricks collapses. Put all of this on an incline, and the bricks go flying down the slope. Also to be noted is the rate that the stress (bricks) is added. If you stack them one at a time, it will be able to handle a greater load than if you were to drop them all on at once. A few inches of snow a day is great as the snowpack has time to adjust to the load, but when you have a situation when 100" of snow is added, with wind loading, all over the course of ten days, things get especially sketchy. This is the exact setup you have when a big storm loads new snow on top of a weak layer, or winds start cranking and blow a layer of wind slab over the facets. Furthermore, put this snowpack structure on some undulating mountains. You can be on a ridgeline or a basin connected to a steep slope above or below you, collapse the snow while you're not on a slope steep enough to slide but the fracture propagates across the weak layer and the steep slope adjacent to you goes crashing down. In some cases, the slab is strong enough that the snow pulling down on the steep slope will break the slab all the way back in the flats, which can take you for an unexpected ride when you think you're in a safe zone. The snow collapsing underneath the weight of a skier is known as "whoomphing" is a red flag that you shouldn't be anywhere near avalanche terrain.
The Rocky Point slide posted above was triggered from 300 feet away, was 5 feet deep, and terminated in a stand of trees. Much like the Uintas video from above, no airbag pack or Avalung will help you survive this slide.
Full obs here.
Video Obs.Full disclaimer: I've never taken a formal avalanche class. I started with reading
Snow Sense, then
Staying Alive in Avalanche Terrain, check the UAC report daily, live up in the mountains so I generally get to see every change in the weather. Also, I have spent a lot of time stomping and digging around in the backcountry around here. I am, by no means, an expert, but I have probably spent more time in the backcountry in the past three years than most people will have in ten.
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