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 this weekend as the warm storm pushes inland, more details in the latest forecaster blog.
Southwest wind is expected to pick up as we get closer to the weekend, when it arrives it is expected to form recent snow into 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 ushers in plenty of wind, moisture and a rising freezing level.
THURSDAY NIGHT: Freezing level at valley bottom, light to moderate southwest wind, trace of snow possible.
FRIDAY: Scattered cloud cover, freezing level at valley bottom, light west wind, trace of snow possible during the day with another 1 to 2 cm possible Friday night.
SATURDAY: Broken cloud cover, freezing level beginning at valley bottom rising to about 1000 m, light variable wind at most elevations, strong northwest wind at the highest peaks, 1 to 4 cm of snow possible during the day with 1 to 5 cm Saturday Night.
SUNDAY: Broken cloud cover, freezing level beginning around 500 m climbing to about 1800 m during the day, moderate to strong west/southwest wind, trace of snow at upper elevations with light rain in the valleys, trace to 10 more cm of snow expected at upper elevation Sunday night.
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.
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 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.
