The snowpack is complex with numerous concerning avalanche problems. As we leave an active stormy period, conservative terrain choices are recommended to handle these tricky conditions.
Strong to extreme wind from the west coupled with moderate amounts of new snow has left unstable wind slabs at upper elevations. Wind slab avalanches in motion could step down to buried weak layers and release very large persistent or deep persistent slab avalanches.
30 to 60 cm of settled snow rests above a widespread layer of surface hoar that has produced numerous avalanches over the past week. These slabs have been most sensitive to triggering at treeline, which is a setup that can catch us off guard.
Recent loading from new snow and wind has aggravated the 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.
Stormy and windy on Thursday then clearing weather for the weekend...
WEDNESDAY NIGHT: Frontal system crosses the region overnight delivering 5-15 cm of new snow, strong south wind, alpine temperatures drop to -12 C.
THURSDAY: Scattered flurries with another 5 cm of snow, strong south wind, alpine temperatures around -10 C.
FRIDAY: Sunny with a few clouds, light south wind, alpine high temperatures around -8 C.
SATURDAY: Sunny, light wind, alpine high temperatures around -8 C.
Over the past few days there have been reports of wind slab, persistent slab (failing on the February 19 surface hoar layer), and deep persistent slab (failing on basal facets) avalanches. This pattern highlights how current stormy weather is aggravating multiple weak layers in the snowpack.
Reports from Tuesday suggest wind slab avalanches occurred naturally in alpine terrain, while at treeline elevations several size 2-2.5 persistent slab avalanches released naturally and with artificial triggers. These slabs failed on the Feb 19 surface hoar layer on north and east aspects.
Size 1 human triggered avalanches on the Feb 19 surface hoar layer have been reported almost daily since last Thursday. These have typically been 30 cm thick on a range of aspects.
Cornice falls have been a common trigger for wind slabs on the slopes below, and last Friday a cornice fall triggered a size 3.5 deep persistent slab on a northeast facing slope at 1800 m.
Extreme wind from the southwest, west, and northwest has hammered open terrain, scouring some slopes and forming wind slabs on other slopes. Another 5-15 cm of snow is expected to form fresh wind slabs on Thursday.
Loading from new snow and wind has made several persistent weak layers problematic over the past week. A combination of surface hoar and sun crust layers were buried on February 19 and currently sit 30-60 cm below the surface that 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.
