Although backcountry ski conditions have been phenomenal in Tahoe the past few days the persistent weak layer (PWL) that continues to plague our snowpack remains on the minds of all. As Tahoe prepares for the storm two prominent questions continue to be asked over and over:
1. How will the new snow and precipitation influence our greater snowpack?
2. How will the new snow and precipitation react with the PWL?
As mentioned from day one in crafting the State of the Tahoe Backcountry, and echoed by just about every reputable backcountry outlet in Tahoe, the Sierra Avalanche Center (SAC) is our definitive local source for everyday reports on the constant evolution of the Tahoe area snowpack and backcountry ski conditions.
With that in mind I contacted SAC to get some thoughts based on those two above mentioned questions. Taking time out of an already busy day, here’s what Andy Anderson told me- some quality food for thought as we enter into a serious Sierra storm train.
They will make the snowpack deeper which is a good thing for recreation and hopefully for stability eventually. As far as how this will affect the snow stability, I have a few concerns. The first round of precip (yesterday through Thurs) started as snow and then is supposed to switch to rain up to around 8000. Rain on snow is bad for stability so that could cause some issues. It will also melt snow and cause us to lose depth which we don’t really want to happen. Luckily it is possible that this rain could percolate down to the PWL and could help make it stronger when it refreezes. We will have to see how much rain we get and how far it penetrates in the snowpack. Of course this will only work where it rains enough. Above the rain level we will likely see an upside down snowpack with a heavy layer from today on top of lighter snow from yesterday and last night.
Once the second wave hits this weekend with rain turning to snow (alot of it), I expect that several weaknesses will form within the new snow during the storm. These will tend to be the more common Tahoe weaknesses that should become less active faster than the PWL (as in days rather than months). However this new load will also stress the PWL much more making slides that can step down into older snow a real threat once again. If we get enough snow to put the PWL so far down that we can no longer transmit force to it as skier, snowboarders, snowmobilers, etc, then it may go dormant again assuming that the new snow becomes strong enough to hold the snowpack in place above the PWL. This will create conditions that will make triggering slides more difficult but if they go they could still go big.
So I expect us to see a period of avalanche activity during the storm that will gradually become more difficult to trigger as the weather stabilizes. Over time if the snowpack gets deep enough we could also see the PWL start to slowly gain strength and start to heal itself. Again this will take some time. All in all the answer is it depends on how much and what kind of snow we get. Regardless it should be interesting. We will certainly continue to publish all of our observations and current thinking on the avy hazard on the site and that will be a good place for people to check for info.
Per the forecasters request, if you, yes you backountry users want to send in your personal observations, that can help SAC by giving them extra data to write even more informed forecasts. You can always post in the comments section on State of the Backcountry to help further the dialogue with our Unofficial readers, or click here to submit observations right to the SAC website.
Tuesday’s snowpit analysis near Castle Peak (Andesite Ridge):
A final note, and this is only meant to inspire cautious, safe travel, but while speaking with a friend and avalanche forecaster yesterday he mentioned at the end of our exchange that he was actually in Carson City, at a scuba shop, filling the canisters in his airbag. Just remember you make the decision to go out in the backcountry, and assess where and what you plan to ascend and descend. Never neglect the human factor, and make sure whatever you do, through this storm cycle and beyond, you come home at the end of the day to let us know how good it was!