Regions
Northwest Coastal.
We are in an active wind slab cycle. The cold temps are prolonging availability of snow for wind transport and keeping the buried surface hoar layer reactive.
Confidence
Moderate - Wind effect is extremely variable
Weather Forecast
Thursday: Dry, some high cloud and cold. Treeline temperatures around -18 C. Winds easterly 40-50 km/h.Friday: Increasing cloud, but staying dry. Treeline temperatures around -17 C. Winds easterly 50 km/h, diminishing during the day.Saturday: Flurries. Temperatures around -18 C. Light winds.
Avalanche Summary
Several very large (size 3) avalanches were noted from Bear Pass due to wind loading late on Tuesday or early on Wednesday. Wind slab avalanches are a big concern in all areas through the forecast period, and could easily step down to the surface hoar layer below. Previously, a widespread natural avalanche cycle peaked on Friday and Saturday with avalanches noted to size 3 running on a variety of aspects and elevations from 1100-1800 m. There were a couple of close calls with people being caught out by the touchiness of the buried surface hoar layer.
Snowpack Summary
The storm from last weekend brought 30-80 cm of cold powder. This new snow has been blown around by the wind - first by southwesterly winds during the storm, then by northerly winds with the onset of arctic air, and now by easterly outflow winds. As a result, expect wind slabs in a wide variety of locations. Unless you're really certain about the recent local wind history, the safest bet is to assume wind slabs could be present on all aspects in exposed alpine, treeline, and even some below treeline locations. The recent snow buried a surface hoar layer, which now lies 60-120 cm in many parts of the region. This layer was reactive during and after the storm and is likely still reactive to human triggers. A thick rain crust exists 20-30 cm below the surface hoar layer, with isolated reports of weak facets (sugary snow) forming above the crust; however, that interface hasn't become a concern as of yet. Treeline snow depths are around 140-200 cm in the Terrace and Stewart areas, but substantially less further north. Concerns in the north are basal weaknesses that may exist in the shallower snowpack areas especially on smooth alpine features like glaciers.
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.