Watch for the formation of potentially touchy wind slabs as warm southwest flow starts to take hold of the region. Conditions are expected to deteriorate later this weekend as the warm storm pushes inland, more details in the latest forecaster blog.
A bit of new snow and increasing northwest wind may begin to form fresh wind slabs which will come to rest on weak facets and surface hoar. This combination could make them surprisingly sensitive to human triggering. Cornices may be reaching their breaking point and can act as triggers on slopes below.
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.
Southwest flow is slowly inching it’s way into the Purcells this weekend.
FRIDAY NIGHT: Freezing level at valley bottom, light west wind at most elevations with strong northwest wind at ridgetop, trace of light density snow expected.
SATURDAY: Broken cloud cover, freezing level beginning at valley bottom rising to about 800 m, light west wind at most elevations with strong west/northwest wind in the higher alpine terrain, 2 to 5 cm of snow expected during the day with another 2 to 5 cm Saturday night.
SUNDAY: Broken cloud cover, freezing level beginning near valley bottom rising to about 1500 m, moderate west wind at treeline with strong to extreme west wind in the alpine, 1 to 5 cm of snow possible at upper elevations with rain in the valleys. 5 to 15 cm of snow is expected at upper elevations Sunday night.
MONDAY: Overcast, freezing level beginning around 1500 m and holding there throughout the day, moderate southwest wind at treeline with strong west to extreme west wind in the alpine, 5 to 15 cm of snow at upper elevations with rain in the valleys.
There have been a few reports of small loose dry avalanches running in the facets this week, no other avalanche activity to report. Cornices remain large and fragile.
Last weekend, observers reported large cornice falls and several small to large (up to size 2) natural and skier-triggered wind slab avalanches on westerly aspects above 2400 m. An explosive triggered avalanche reportedly stepped down to a deeper layer.
Earlier in February, there were a few reports of large avalanches (size 2.5-3) on southeast aspects in the alpine, likely releasing on a layer of facets on a crust (see a photo here), and a large human-triggered avalanche (size 2.5) on 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 and the riding looks pretty good. A weak layer that formed in late January 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.
