The February 12th persistent weak layer continues to be a concern across the Interior Regions. Avalanches failing on this layer of buried surface hoar will be large and destructive.
Confidence
Fair - Timing or intensity of solar radiation is uncertain on Thursday
Weather Forecast
Overnight and Thursday: Mostly sunny with light Northwest winds and no precipitation. Freezing level rising to 1200 metres during the day. Alpine temperature -9.0 overnight.Friday: The ridge is expected to continue to influence most of the province bringing mostly sunny skies and light winds with freezing levels rising to about 1200 metres.Saturday: Becoming cloudy overnight with light precipitation and moderate Southwest winds.
Avalanche Summary
A few small avalanches up to size 1.0 were reported from Tuesday. Natural cornice fall continues to be a concern when the sun is shining.
Snowpack Summary
New sun crusts are reported to be forming on solar exposed aspects as the spring sun starts to pack more of a punch. The recent storm snow is settling, but storm slab releases are still possible. The weak layer of surface hoar that was buried around February 12th may be triggered by storm snow avalanches in motion, or by cornice fall and other large triggers. The strong solar radiation may trigger some cornice fall or release moist point releases in steep terrain that may step down to one of the deeper weak layers. I expect that the next couple of days will be a tricky combination of avoiding areas that slid during the warm wet weather, and gaining confidence in the areas that did not slide.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.