Watch for wind slabs in unusual places. These slabs have potential to break wider than expected and may be slow to bond. Read about managing these conditions in this forecaster blog.
Winds from all compass directions in the past week have formed a complex pattern of wind slabs at upper elevations that may be possible to trigger. Given the potential for cross-loading and reverse-loading, treat all aspects as suspect. Cornices may be reaching their breaking point.
There is a lingering possibility of triggering persistent slab avalanches in isolated steep terrain features. The primary layer of concern is layer of surface hoar, facets, and/or crusts buried 30-60 cm deep throughout the region.
MONDAY NIGHT: Partly cloudy, isolated flurries with a trace of snow, light northwest wind, alpine temperatures around -14 C.
TUESDAY: Mainly cloudy, isolated flurries with a trace of snow, light northwest wind with moderate gusts at ridgetop, alpine temperatures around -11 C.
WEDNESDAY: Mainly cloudy, isolated flurries with a trace of snow, light northwest wind, alpine temperatures around -12 C.
THURSDAY: Mostly clear, light south wind with strong gusts in the afternoon, alpine temperatures around -11 C.
Over the weekend, observers reported large cornice falls and several small to large (up to size 2) natural and skier-triggered wind slab avalanches on slopes facing northwest, west, and southwest above 2400 m. An explosive triggered avalanche reportedly stepped down to a deeper layer. A few point releases from rocks on solar aspects were also observed, although they did not run far or gather mass.
Last weekend there were a few reports of large avalanches (size 2.5-3) on southeast aspects in the alpine. These avalanches released naturally, possibly due to the influence of solar radiation or cornice falls. See a photo here.
Earlier in February, a large human-triggered avalanche (size 2.5) released on a persistent weak layer of buried surface hoar near Quartz Creek.
Winds have varied in strength and direction over the past week, loading cornices and forming shallow wind slabs at upper elevations that may be possible to trigger. Watch for these wind slabs in unusual places as complicated cross-loading and reverse-loading patterns are prevalent. Wind slabs may overlie weak, sugary, faceted snow, meaning that they could break wider than expected and will likely be slow to bond.
In wind-sheltered areas, the upper snowpack is becoming soft and faceted with the cold temperatures. A weak layer formed in late January and is now 30-60 cm deep. In the northern Purcells this layer has been reported as a surface hoar layer at treeline and below treeline elevations, but a combination of facets on crusts could exist at all elevations throughout the region.
The northern Purcells also have an older surface hoar layer that can still be found 60-100 cm deep at treeline. Additional weak layers may exist near the bottom of the snowpack and should be considered in steep rocky areas with a shallow or thin to thick snowpack.
