Regions
Northwest Coastal.
New wind slabs are expected to have formed from recent strong outflow winds. Continually assess local conditions as you travel and use extra caution as you transition into wind affected terrain.
Confidence
Moderate - Wind effect is extremely variable
Weather Forecast
High pressure is expected to be the dominant feature for the next few days. Mostly sunny conditions are expected for Wednesday with freezing levels reaching around 500m and moderate southeast winds in the alpine. Similar conditions are expected for Thursday with mostly sunny skies, freezing levels around valley bottom, and light alpine winds. On Friday, a layer of above-freezing temperature air is forecast to move into the region and sit at around 1500m elevation.
Avalanche Summary
On Monday, three avalanches were reported north of Stewart but details were limited. A natural size 2 wind slab had a thickness of 100cm. A snowcat triggered a size 1.5 wind slab 35cm thick and a ski cut produced a size 1 wind slab 25cm thick. These were likely formed by outflow winds mean they were probably on west or southwest aspects. On Sunday, a few avalanches were reported from the north of the region: A skier accidentally triggered a size 1.5 wind slab avalanche which failed on the December 26 surface hoar layer at about 1450m on an east aspect. A few other size 1.5 explosive-triggered wind slabs were also observed in north-facing alpine terrain.
Snowpack Summary
Snowpack surfaces are highly variable. Temperature inversions and solar radiation have recently come into play and a melt-freeze crust can be found on steep, solar aspects in the alpine. On most other slopes, 25-40 cm of snow from the Boxing Day storm continues to settle and facet, and new surface hoar has been reported although its distribution is likely variable due to strong outflow winds. In areas where recent winds have been light, shaded slopes will likely provide settled powder. In areas where winds have been stronger, reactive wind slabs may exist, especially in areas where the the slab overlies surface hoar buried by the Boxing Day storm. Professionals in the region are still keeping an eye on a few other surface hoar layers that developed throughout December which are now buried in the top 100cm. Although these layers may be gaining strength, they are worth keeping an eye on, especially on steep unsupported slopes at treeline and below. In colder and shallower snowpack areas watch for weak basal facets.
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.