Regions
Northwest Coastal.
Solar radiation and significant warming could tip off a natural avalanche cycle, primarily in areas close to the coast. Outflow winds are also adding to the current challenges. Conservative decision making remains crucial.
Weather Forecast
Monday: Mainly sunny. The freezing level is at valley bottom. Ridge winds are light to moderate from the north with strong outflow winds in coastal inlets and valleys. Tuesday: Mainly sunny. The freezing level jumps to 2300 m along the coast with a strong temperature inversion. Treeline temperatures should be near +5. Temps may remain cool further inland. Winds are light from the SW. Wednesday: A mix of sun and cloud. The freezing level on the coast remains near 2300 m and near surface inland. Winds are light and variable.
Avalanche Summary
A few large remotely triggered slab avalanches were reported in the Shames backcountry on Saturday. These were primarily from open alpine and treeline terrain and probably released on the February persistent weakness. This problem could become more touchy due to wind loading from outflow winds and/or from solar radiation. In addition, a size 2.5 accidentally triggered slab avalanche was reported in the northern part of the region. This slide released on basal facets in steep rocky alpine terrain.
Snowpack Summary
Northerly outflow winds and picked up on Saturday resulting in reverse loading and cross loading in exposed terrain at all elevations and have scoured windward slopes. The prominent mid February persistent weakness (surface hoar, facets, crust and/or wind pressed surfaces) is down 60-100 cm in the north and 120-160 cm in the south. Widespread whumpfing, cracking, natural avalanche activity and remote triggering at all elevations are a strong indication of poor bond between the new snow and these old surfaces. In addition, snowpack tests give consistent easy to moderate "pops or drops" shears on the mid Feb layer. Basal facets and/or depth hoar remain a concern in shallow snowpack areas in the northern part of the region. Cornices are also large and may become weak if northerly winds start to undercut them.
Problems
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.
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.