It is likely that avalanche danger will start to ramp up this week as new snow settles into a cohesive slab and overloads several weaknesses lurking below.
Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain
Weather Forecast
In general we should see unsettled conditions as a series of disturbances move across the province this week. Monday: Light to locally moderate snowfall - 5-10cm. Freezing level (FL) 800-1000m. Winds light to moderate from the southwest. Tuesday: Light to locally moderate snowfall - 10-15cm. FL rising to 1500m. Winds increasing to moderate from the W-SW. Wednesday: The strongest pulse should arrive late on Tuesday and bring moderate to locally heavy precipitation into Wednesday. 15-30cm is possible. FL lowering to 1000m. Winds moderate from the SW.
Avalanche Summary
Many areas have reported natural and easily triggered soft slab avalanches up to size 2. Most of these avalanches were 20-40cm deep and released on the Feb. 16 surface hoar layers, with some releasing on the Feb. 8 surface hoar/ facet/ sun crust interface. The size and likelihood of avalanches should continue to increase with continued incremental loading and particularly with any increase in winds.
Snowpack Summary
The upper snowpack is quite complex and we could see enough precipitation and wind early this week to tip the scales to more widespread and larger avalanches. Up to 40cm of low density new snow now sits on the February 16th Surface Hoar layer. The February 8th weakness is now down 50-60cm. Feb. 8th is used to describe a combination of weak layers: In most locations it marks a widespread surface hoar layer. On steep south & west facing aspects a sun crust that formed during the drought was also buried on Feb. 8th. This crust likely has a bit of faceting below it too, meaning that most terrain in the region has a weak layer 40-60cm under the snow surface. Below this problematic interface a well settled mid pack is in place. The exception is the eastern and southern parts of the region where deeper weaknesses and basal facets may still be a factor. In these areas, a low-probability/high-consequence condition exists. It should be noted that large cornices remain widespread throughout the region.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.