"I can’t remember a time that I’ve seen more red flags in one ride. We observed widespread cracking and collapsing. Avalanches were easily triggered on any test slope steeper than 30 degrees."
Snowpro, avy educator extraordinaire,Wasatch SAR member, pro sledder, and all around great guy Tyler St. Jeor states it best in his comment from Saturday.
Here's the deal-
Remember the big dryspell during January/February? (You're thinking to yourself... hmmm, "I forget what time I drop the kids off for soccer practice today.") Well, that midwinter drought helped create a weak layer of faceted, sugary snow which is now buried under several layers worth of storm snow from last week. And while we might forget everyday occurrences, the snowpack has an amazing memory. In fact, now that this layer is buried, it's our new problem child, or persistent weak layer (PWL).
Chads viddy above illustrates the problem. We've got an unusual, late season, set up... strong snow on top of weak snow. And the weak layer is easily failing under our additional weight and will produce deep, dangerous avalanches today. Like a layer of dominoes with stronger snow resting on top, once our skis or track tip the first domino, it sets off a chain reaction in which all the dominoes (PWL) collapse. It's sorta like pulling the rug out from underneath, and the entire roof crashes down on us! Here's where it gets tricky... we don't even need to be on a steep slope, just near or connected to it (at the top, bottom, or to the side). Tip one of the dominoes over and now we're staring down the barrel of a scary avalanche!
What makes this situation tricky?
- We don't normally deal with long lasting avalanche problems like this, lingering deep into the winter. Today's avalanches act more like early and mid season slides that break on sugary snow near the ground except in this case avalanches will break in the middle of the snowpack. Avalanches may also break in areas below treeline where we don't often expect to see as many slides.
- You can trigger deep, dangerous avalanches by simply being near a steep slope but not necessarily on it.
- This weak layer is found on many slopes but not all, especially in the wind zone, where distribution is spotty and that makes stability patterns tricky to get a handle on. So... you might find weak snow on one part of a slope but not the other. This means you may see people ride a slope and not trigger a slide, but the second or third person on that same slope may be the one to trigger and avalanche.
What to do? There are two options. Ride southerly facing slopes that don't have this weak layer, but the problem is they have a much thinner snowpack. Terrain facing the north half of the compass offers a deeper snowpack and cold powder, but likely have this weak layer and are unstable. In those areas, simply ride slopes less than 30 degrees in steepness with no overhead hazard... and that means nothing steeper above you.
