Regions
Northwest Inland.
Rapid and sustained warming is set to undermine stability throughout the region over the coming days. The scale of the impact is uncertain, but the strength of the snowpack is about to be tested.
Weather Forecast
Monday: Mainly cloudy with isolated wet flurries and a trace of new snow. Winds strong from the southwest. Freezing level rising to 2300 metres with alpine temperatures of +3.Tuesday: Wet flurries bringing around 5 cm of new snow. Winds strong to extreme from the south. Freezing level around 2000 metres with alpine temperatures of +3.Wednesday: Flurries, possibly wet, delivering around 5 cm of new snow. Extreme south winds. Freezing level around 1900 metres with alpine temperatures of +1.
Avalanche Summary
Recent reports of avalanche activity have been limited, but one wind slab was observed running naturally from a steeper east aspect in the Howson Range on Saturday. While the slope is reported to avalanche regularly, this does serve as a reminder that wind slabs remain reactive to light triggers in specific terrain. More substantial avalanche activity was reported last weekend, including a remotely triggered Size 2 avalanche as well as naturally triggered avalanches to Size 2.5. Some of the natural avalanches involved the full depth of the snowpack.For Monday, expect recently formed wind slabs to be reactive to human triggering. Older hard slabs may be more stubborn, but also capable of much wider propagation. A storm currently impacting the north coast should bring modest new snow accumulations inland along with strong to extreme southwest winds that will promote the formation of thin, touchy new wind slabs in lee areas. Also keep in mind that a basal weakness continues to drive a low probability/high consequence problem for the region. Very large avalanches remain possible, especially in shallow snowpack areas and as forecast warming begins to take effect.
Snowpack Summary
A light snowfall over most of the region has given a thin cover to the widespread wind affected surfaces recently reported in exposed terrain. This includes scoured surfaces, sastrugi, and hard wind slabs. The new snow will contribute to wind slab formation in wind exposed areas. In areas sheltered from the wind, new snow will cover cover faceted surface snow as well as surface hoar. Below around 1400 m elevation, a rain crust can be expected beneath the new snow. Deeper in the snowpack, several buried surface hoar layers can be found between 30-70 cm deep. Recent reports suggest these layers have generally stabilized but isolated weaknesses may still exist, especially where buried surface hoar is still intact. A generally stiff mid pack sits above weak sugary snow near the ground. This basal weakness woke up during the last storm and produced numerous large avalanches which released on the ground. The layer has recently been considered dormant but it may become active once again with progressive warming over the next few days. It may also be possible for a smaller avalanche to 'step down' to this layer.
Problems
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.
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.