A weak layer of facets sitting on a firm crust is creating a persistent slab problem which has resulted in numerous human-triggered avalanches over the past few days. Check out the new forecaster blog for more details.
Watch for new wind slab formation on Saturday. Periods of moderate wind from the north may be enough to develop reactive new slabs in exposed terrain at higher elevations.
Lingering cornices may become weak during the heat of the afternoon. Cornices are most likely on north and east aspects in the alpine.
A persistent weak layer of facets on a crust exists 30-70 cm below the surface and has been touchy to human triggers, especially on northerly aspects around treeline. Recent avalanche activity has primarily occurred between 1600 and 2000 m and includes some occurrences of remote triggering (from a distance away). Numerous natural and human-triggered avalanches occurred on this interface on Wednesday and Thursday.
With each subsequent day of melt-freeze conditions, loose wet avalanches will become less likely. However, extra caution is still recommended on steep sun-exposed slopes during the heat of the afternoon. Where the crust is breaking down or non-existent, loose wet avalanches will be more likely.
A ridge of high pressure brings dry and sunny conditions for the weekend.
Friday Night: Clear, light N wind, freezing levels around 800 m.
Saturday: Sunny, light to moderate N wind, freezing levels reaching around 1500 m.
Sunday: Mainly sunny, moderate N wind, freezing levels potentially reaching around 2000 m in the late afternoon.
Monday: A mix of sun and cloud, moderate NW wind, freezing levels reaching around 1500 m.
On Thursday, several persistent slab avalanches were reported up to size 2.5. Four of these were natural and the rest were human-triggered including one which was remotely triggered. Two of the naturals were on south aspects which were triggered by loose wet avalanches stepping down. Most of the rest of the activity was on northerly aspects. The majority of the activity appears to have occurred between 1600 and 2000 m elevation. Slab thickness was typically 30-70 cm and the failure plane was a combination of the mid-February and late-February weak layers which may be acting as more of a single thick weak layer.
In addition to the persistent slab activity, a natural wet slab was observed on a south aspect at 1750 m and several natural wind slabs were observed. Numerous loose avalanches were observed from steep sun-exposed slopes. Explosives triggered one size 3 cornice.
On Wednesday, a natural avalanche cycle up to size 3 was reported from the recent storm. A few skier controlled and remotely triggered (from as far away as 40 m) avalanches were also reported. These persistent slabs failed on the mid-February facet/crust interface buried approximately 50 cm down. They caught people by surprise, but no involvements occurred.
30-50 cm of wind-affected storm snow from earlier in the week sits over a variety of surfaces including sun crusts on solar aspects, a thick rain crust in windswept terrain and facets in shady, sheltered terrain. The recent storm snow has a decent bond in some areas, but a poor bond exists on northerly slopes (NW, N, NE, E) at treeline and in low alpine elevations (1600-2000 m) where the mid-February facet/ crust persistent weak layer has been touchy to human triggers. Whumpfing and remotely-triggered avalanches have been failing on a 3-5 cm thick layer of facets sliding on a hard melt-freeze crust. Some surfaces in windward alpine terrain have been scoured down to this firm crust.
Large and looming cornices have grown during the recent storm, primarily on north and east aspects. Periods of moderate northerly wind on Saturday may develop new wind slabs.
