Conditions are trickier than meets the eye. Warm temperatures and strong solar radiation will test the snowpack on south-facing aspects, while a buried persistent weak layer continues to surprise riders at mid-elevations on north-facing terrain.
Strong westerly winds throughout the week have created a smorgasbord of wind slabs in the alpine and exposed treeline. Wind slabs may be stubborn to human triggering but have the potential to step down to buried weak layers, creating large avalanches.
Warm temperatures compounded by strong solar radiation may make cornices weak. Cornice failures put a large load on the snowpack, and have the potential to trigger buried weak layers, resulting in large destructive avalanches.
A weak layer of surface hoar exists in the upper meter of the snowpack. This layer has been reactive to human triggering, including remote triggering, producing large consequential avalanches. The layer exists at all elevation bands and has recently been very reactive between 1600 and 2200 m. Example terrain features to particularly treat as suspect include the lee side of ridges, openings in trees, cut blocks, and burns.
A building ridge of high pressure will usher in clear skies and warm temperatures.
Thursday overnight: Mainly clear. Ridgetop winds 20-60 km/h from the northwest. Freezing level dropping to valley bottom.
Friday: Mainly clear. Ridgetop winds 10-40 km/h from the northwest. Freezing level rising to 1500 m in the afternoon.
Saturday: Mainly clear. Ridgetop winds 10-30 km/h from the northwest. Freezing level rising to 2000 m in the afternoon.
Sunday: Increasing cloud cover. Light ridgetop winds. Freezing level rising to 2200 m in the afternoon.
On Wednesday, a few skier and snowmobiler-triggered persistent slab avalanches were reported on north-facing aspects at treeline. Numerous loose wet avalanches where reported below 1800 m on all aspects from warm temperatures and rain on snow.
On Tuesday, skiers triggered several small slabs on the buried weak layer. One was triggered from a distance away. Just south of the region explosives triggered very large slab avalanches up to size 3 on the buried weak layer.
On Monday, skiers triggered several slab avalanches on the buried weak layer and avalanches also released naturally on this layer reaching up to size 2.5. The avalanches were 40-60 cm deep. Several cornice failures were reported and some triggered wind slabs on the slopes below. A deep persistent slab avalanche of size 2.5 was triggered by a machine and released on the early December facet/crust layer described in the Snowpack Summary.
Overnight, clear skies and cooling temperatures will refreeze the surface of the snowpack at lower elevations where it has been affected by rain and warm temperatures. A sun crust has formed on sun-exposed slopes at all elevations. In the alpine and exposed treeline, strong winds throughout the week have created a variety of wind-affected surfaces. Expect to find old and new wind slabs on a variety of aspects, with the most recent and reactive slabs formed in terrain features lee to the northwest.
A weak layer of surface hoar exists 40 to 90 cm deep in the snowpack, which is a prime depth for human triggering. The layer is most prominent in areas sheltered from the wind. Example terrain features to treat as suspect include the lee side of protected ridges, openings in the trees, cut blocks, and burns. The layer may not exist on steep sun-exposed slopes, where a melt-freeze crust may be found instead.
The facet/crust layer that formed in early December is buried around 100 to 240 cm. The last reported avalanche was on February 7 in the southwest of the region. Although unlikely, the layer could be triggered from a large load like a cornice fall or a shallower slab avalanche could step down to this layer. Humans are unlikely to trigger it, with perhaps the exception in thin, shallow snowpack areas. Check out the forecaster blog for more information.
