Merry Christmas! Looks like we've been well behaved, and Santa might deliver us some new snow starting tonight. The worrying part about this is it is falling on a good crop of surface hoar, so be leery of sheltered terrain after that snow falls. We currently have isolated wind slabs where the recent snow loaded below ridges on the north and east aspects. Winds will continue to be from the south, so these will continue to build with the cold snow coming. The lower pack has started to heal from last week's storm, but there are still persistent weak layers lurking below.
The recent, light density snow and winds from our typical southwest direction have formed wind slabs in the leeward terrain. Look for chalky, hollow sounding snow, that has a pillow appearance. Give these a wide berth if you are approaching the tops of ridges. They have had a few days to stabilize, but new ones will start to form tonight as we get the new storm rolling in.
As we know from last week's advisory and observations that came in last weekend and earlier in the week, the persistent weak layers we've been watching came alive in the last storm. The rain injected a lot of heat in the snowpack and destabilized them for the duration of that storm. But, as it cooled down, the rain helped lock up the upper snowpack, which is bridging over these layers in most locations. There are many facet/crust combos that are buried, varying from location to location, but they are showing to be much less reactive than earlier in the week. As usual with any buried persistent weak layer, they are guilty until proven innocent. Really scrutinize your riding area, as they are spatially variable. In one location they may not show any reactivity in pit tests, but in others they might. The only way you will know if they are there is to dig down and look at what's going on under the surface of the snow.
We ventured out yesterday to the Moon Pass area and found a nicely healing snowpack. Similar reports from Burke yesterday that stability tests weren't getting any results on persistent weak layers. But there is a lot of spatial variability across our region and those two data points are enough to make me commit to bigger terrain. Shallower snowpack areas may see a rise in reactivity as they get loaded with new snow.
The best thing about yesterday might also be the worst thing about yesterday in terms of future avalanche problems. The glorious sunshine!! Even if the skiing and riding were mediocre, you can't beat days like that out in our mountains. Christmas Day today looks to be a pretty good repeat for visibility and sunshine until the storm rolls in. What we were seeing that is worrisome was very well-developed surface hoar, up to 2cm in size. I expect that grew even more last night in the cold, clear, calm air. This was primarily on the north sides, as the sun was warm enough yesterday to knock it down on the south aspect. The winds at the beginning of the incoming storm are supposed to be light then increasing. This surface hoar could be buried intact on our northern sheltered aspects. Protected areas include gullies and wide-open trees. Be wary on Saturday before stepping out into bigger terrain. Quick hand pits in a variety of locations should give you some information on if this will be a problem in your riding location or not. Storm totals through Sunday could bring up to 13 new inches at the highest elevations, which is definitely more than enough snow to bury you or sweep you off a cliff.
Stay safe, send in observations, and have a very Merry Christmas!
