Fresh wind slabs may be surprisingly sensitive to triggering as they form over old, faceted surfaces.
For areas along the northwestern border of the region with storm snow totals greater than 30 cm, refer to South Columbias forecast.
New snow, rising temperatures and strong westerly wind will likely form fresh wind slabs ontop of 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.
SUNDAY NIGHT: 5-15 cm of new snow. Freezing level 1500 m. Strong to extreme westerly ridgetop wind.
MONDAY: 5-10 cm of snow. Freezing level 1500 m. Strong west wind easing slightly.
TUESDAY: Mix of sun and cloud, light northwest wind, freezing level 1000 m.
WEDNESDAY: Sunny, light northwest wind, freezing level 1000 m.
Preliminary reports from Sunday suggest a few small natural wind slabs and loose dry avalanches running in the new snow. Cornices remain large and fragile but there have been no reports of cornice falls since last weekend.
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.
10-25 cm of new snow falls overtop of the old, faceted and/or wind affected snow that sat on the surface during the extended cold, windy drought period. At upper elevations, the new snow will be easily blown around by strong to extreme westerly winds, forming reactive slabs in lee features.
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 steep rocky areas with a shallow or thin to thick snowpack should be considered suspect trigger points.
