Wind slabs are forming in new places with reverse-loading. Stick to sheltered areas, avoid exposure to overhead hazards, and investigate buried weak layers on Saturday.
Winds associated with an arctic front are expected to drift the recent snow in a reverse-loading pattern. This may form wind slabs that are possible to human trigger, especially in areas where surface hoar sits at the interface. These conditions may also bring cornices to their breaking point.
40 to 90 cm of settled snow rests above several layers of buried surface hoar that have produced avalanches over the past week. These slabs have been most sensitive to triggering in sheltered areas at treeline.
Recent loading from new snow and wind has aggravated the region's deep persistent slab problem. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine. Cornice falls are a likely trigger for these slabs.
Friday night: Clear, moderate east winds with strong gusts at ridge-tops, alpine temperature -28 C.
Saturday: Clear, moderate east winds with strong gusts at ridge-tops, alpine high temperature -12 C.
Sunday: Clear, light northeast winds, alpine high temperature -7 C, freezing level 600 m.
Monday: Clear, light variable winds, alpine high temperature -5 C, freezing level rising to 2500 m in the southern half of the region.
Sustained strong winds from the northeast are expected to continue to build wind slabs in lee terrain features.
Over the past several days, large (size 2) human-triggered avalanches have been reported releasing in this drifted snow/weak interface combination and breaking 20-80 cm deep. These avalanches primarily occurred on leeward features above 1400 m.
Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below.
Last week, there were reports of large (size 2-2.5) human-triggered avalanches failing on the March 1st surface hoar layer. Over the past two weeks, avalanches have been reported on an earlier surface hoar layer from February 19th as well as a deep persistent slab failing on basal facets. This pattern highlights how shallow avalanches in the surface snow have the potential to strain multiple weak layers in the snowpack and release larger avalanches.
Strong northeast winds are drifting the recent snow into touchy wind slabs, and there is ample snow for transport with this reverse-loading pattern. 5-10 cm of new snow on Thursday added to the 10-20 cm of snow from the storm earlier in the week. Wind slabs will likely be more reactive where this snow rests on a weak layer of surface hoar and/or a crust on solar aspects. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below.
Loading from new snow and wind has made several deeper weak layers problematic over the past week. A surface hoar layer from March 1st may be found 40-80 cm deep, and another combination of surface hoar and sun crust from February 19 may sit 60-90 cm deep. These layers seem to be most sensitive to human triggering at treeline elevations.
A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface. Below that, an early season crust/facet layer lurks at the base of the snowpack.
