The bottom line: The avalanche danger is dropping, but you could still trigger avalanches on wind loaded slopes at upper elevations or on the eastern edge of the zone. If you plan on traveling in these areas, carefully choose terrain that minimizes your exposure to avalanches and use small test slopes to evaluate the snow. In adjacent zones to the north and east, observers have triggered dangerous avalanches on a layer of buried surface hoar near and above treeline.
Regional Synopsis
Happy New Year!
Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.
December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.
The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.
A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.
It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.
Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.
Avalanche Problems
Wind Slabs
You may still be able to trigger wind slab avalanches above treeline. Use caution where you find slopes thickly blanketed with snow below ridges or wind sculpted features. You'll likely find this scenario on northeast through south aspects. Steer around convex rolls and unsupported terrain features steeper than 35 degrees. When in doubt, minimize your exposure to avalanche terrain by traveling on ridges and slopes under 35 degrees.
Snow and strong wind out of the west and north formed wind slabs near and above treeline on the 29th and 30th. By the 30th observers reported natural, human, and explosive triggered avalanches. Some of these wind slab avalanches ran on a slick crust. Now it’s more difficult to trigger these loaded slopes, though not impossible. There’s still uncertainty above treeline and on the eastern edge of the zone where slabs could be poorly bonded to underlying layers. Using slope and snowpack tests on small, inconsequential slopes can help you gather more information about the layers in the top 2-3 feet of the snowpack.
Wind features on a ridge above a human triggered avalanche (1/1/2019)
