Avalanche Forecast

On the 31st an observer in the Icicle Creek drainage found the snowpack to be very reactive. A supportable rain crust was found up to about 5,500ft. The snowpack above the crust at low elevations had begun to facet out, and surface hoar was observed growing on top. As soon as he got above 6,000ft, where the trees were more sparse, cracks began shooting out from his skis as he walked along the ridge. Avalanches were triggered from a distance while walking on this ridge. These were on average 18" deep, and easily up to 400ft wide. As the slabs slid downhill, light reflected off the shiny bedsurface, it was large, feathery, surface hoar. A widespread natural avalanche cycle had occurred during the tail end of the storm, but some slides may have released well after the snow stopped falling. We do not know much about the distribution of this layer across the ranges of the eastern slope. However, we need to treat our upper elevation terrain as guilty until proven innocent. 

This past storm brought 1.8” of water equivalent at the Sasse Ridge Snotel site in the Salmon la Sac drainage (which could equate to nearly two feet of snow at higher elevations), while further east Mission Ridge picked up just an inch or two of new snow, but with winds of over 100mph. This is likely creating a pronounced difference in avalanche danger between the western and eastern portion of the ranges. 

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.