Regions
Northwest Inland.
Sustained warming is set to undermine stability over the coming days. The scale of the impact is uncertain, but the strength of the snowpack is being tested.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Tuesday: Flurries bringing 5-10 cm of new snow. Winds strong to extreme from the south. Freezing level to 1900 metres with alpine temperatures of +2.Wednesday: Mainly cloudy with scattered flurries and up to 5 cm of new snow. Winds moderate to strong from the southwest. Freezing level to 1600 metres with alpine temperatures around 0.Thursday: Mainly cloudy. Winds light to moderate from the southeast. Freezing level to 1100 metres with alpine temperatures of -2.
Avalanche Summary
A MIN report from Sunday includes observations of recent Size 2 wind slab in the Hudson Bay area that is suspected to have run naturally on an east aspect. Perhaps more striking is the fact that the party also experienced several whumphs, suspected to be failures at one of our more shallow persistent weak layers. For Tuesday, 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 focused on the north coast has brought modest new snow accumulations inland along with strong to extreme southwest winds that have promoted the formation of 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 warm temperatures begin to penetrate deeper into the snowpack.
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 as well as a rain crust below about 1400 m. The new snow will contribute to wind slab formation in wind exposed areas while covering faceted surface snow as well as surface hoar in more sheltered areas. 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 sustained warming over the coming 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.