Watch for pockets of stiff wind slab at upper elevations from recent northerly winds. These slabs have formed in atypical areas and could be particularly reactive where they overlie a slick crust.
A skier-triggered deep persistent slab in the Birkenhead on Friday suggests that this deeply buried weak layer is still in play. Avoid steep, unsupported, wind-loaded terrain where trigging this basal instability is most likely.
Northerly outflow winds have created stiff wind slabs at upper elevations. Watch for wind-loaded pockets, especially just below ridgetops and in extreme and very steep terrain. Wind slabs may be particularly reactive to where they overlie a slick crust.
A layer of large and weak facets sits near the base of the snowpack. This layer is most likely to be problematic in steep, unsupported, wind-loaded terrain or by first triggering a layer further up in the snowpack.
On Friday, a size 2.5 skier-triggered deep persistent slab occurred in the Birkenhead. This avalanche was remotely triggered, meaning the skier was a distance away from the slab when it occurred (100 m above). It occurred on a wind-loaded slope in the alpine during a period of strong solar radiation. The crown was 150 cm deep.
Looking forward to Monday, riders should be cautious around pockets of stiff wind slab at upper elevations. These slabs may fail easily where they overlie a slick crust and will be in more atypical locations, as northerly winds have created a reverse-loading pattern. Riders should also be aware of the potential for large and surprising deep persistent slab avalanches. These avalanches present a low probability, high consequence problem and are most likely to occur in rocky, shallow, wind-loaded terrain in the alpine or upper treeline.
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Northerly winds have affected open areas in the alpine and treeline, souring windward slopes and creating pockets of wind slabs in lee areas. In sheltered areas, 10-20 cm of recent storm snow is facetting with cold temperatures. A surface crust can be found below 1800 m and all elevations steep solar aspects.
In the midpack, a weak layer can be found 50-100 cm deep, as a decomposing crust, or in many places, it's more of a soft and weak section of the snowpack. There is a widespread weak layer of facets and depth hoar at the bottom of the snowpack.
Generally, the snowpack in this region is weak and shallow. Snowpack depths around treeline are highly variable and range between 150 - 230 cm.
Sunday night
Clear. Alpine temperatures drop to a low of -10 °C. Alpine temperature inversion. Light ridge wind from the north. Freezing level at valley bottom.
Monday
A mix of sun and cloud. Alpine temperatures reach a high of -5 °C. Ridge wind light from the north. Freezing level at valley bottom.
Tuesday
Cloudy with sunny periods and isolated flurries. Alpine temperatures reach a high of -8 °C. Light ridge wind from the southwest. Freezing level at valley bottom.
Wednesday
Cloudy with sunny periods and isolated flurries. Alpine temperatures reach a high of -5 °C. Ridge wind 30 km/h gusting to 60 km/h from the southwest. Freezing level at valley bottom.
More details can be found in the Mountain Weather Forecast.
