Strong solar radiation may result in large natural avalanches releasing down to a deeply buried weak layer.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
Record warming on Monday should cool slightly on Tuesday as the ridge of high pressure begins to break down. Expect high overcast skies with periods of sunshine on Tuesday combined with moderate Southwest winds and freezing levels slowly descending from 3000 metres to around 2500 metres. High overcast is forecast for Wednesday with light winds and freezing levels around 1600 metres. The freezing levels should drop down to valley bottoms by Thursday morning. Expect valley cloud and a high overcast layer during the day on Thursday.
Avalanche Summary
In the Hosmer area on Monday several slab avalanches size 2.0-3.0 were observed on South aspects in the alpine in a shallow snowpack area, one of these had a wide propagation of approximately 2 kilometres. Recent natural wind-triggered slabs of size 1-2 were observed in the Elk Valley on Saturday on lee and cross-loaded slopes. Cornices were also building. In the SE of the region, a natural size 2 slab failed on the Dec crust at 1800 m on a south aspect. Loose wet natural avalanches to size 1.5 were also observed.Nearby in the Lizard region, there have been several concerning human-triggered avalanches on the mid-Dec layer and two very large avalanches on the Nov persistent weak layer. Check the Lizard/Flathead bulletin for more info. Until we have more observations from the South Rockies to build our confidence, assume that further deep persistent slab avalanches are possible, especially while experiencing unseasonably warm temperatures.
Snowpack Summary
Record warm temperatures may have resulted in moist snow and an isothermal snowpack at lower elevations on Monday. High freezing levels (above mountain tops) are rapidly settling 10-20 cm of recent snow which overlies a rain crust to 1900m, facets, and/or surface hoar. The mid-December crust/facet layer is down 40-80cm. Tests indicate that this layer is still reactive in some areas. Isolated large avalanches continue to release on this interface. In shallow snowpack areas, the mid-December crust is breaking down through the faceting process. Closer to the ground a crust/facet interface that formed in November is generally dormant but may still be reactive in isolated areas. In some places the basal snowpack consists entirely of sugary facets. Below treeline, the snowpack is shallow and weak in many places.
Problems
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.