Expect to see clear skies and rising freezing levels through the weekend into next week. This will initially impact the surface snow especially on solar aspects.
Weather Forecast
SATURDAY: Mix of sun and cloud / light south wind / Alpine temperature -3 / Freezing level 1400m SUNDAY: Mix of sun and cloud / Light to moderate south wind / Alpine temperature 1 / Freezing level 2000m MONDAY: Mix of sun and cloud / Light south wind / Alpine temperature 5 / Freezing level 2500m
Avalanche Summary
On Friday there were reports of a several explosives triggered storm slab avalanches size 1.5-2 that entrained the storm snow only on a variety of aspects. There were also explosives controlled cornice releases to size 1.5. Reports from Tuesday included a small wind slab on a northwest aspect in the McBride Range stepped down and triggered a much larger persistent slab avalanche when it reached 1900 m. The result was a size 3 avalanche (100 cm thick) that propagated across the entire bowl and ran all the way to the bottom of the valley. A few smaller size 1 natural slabs were also observed at and below treeline.
Snowpack Summary
Recent new snow has been redistributed by by strong west and south winds. This new snow sits on a sun crust on steep south-facing slopes, cold dry snow on northerly aspects, and wind affected snow at upper elevations.Last week's storms buried a weak layer composed of soft facets, surface hoar, and/or crust that is roughly 50-100 cm below the surface. This layer has produced whumpfing, sudden results in snowpack tests, and some remotely triggered avalanches. Lots of the activity has been on buried surface hoar on north aspects at upper treeline elevations. The snowpack is well settled and strong beneath this interface. Variable winds in the past month have produced cornices on many ridgelines. They will become touchier as they grow in size, as temperatures rise, and when they are subject to the strong late-winter sun on clear days.
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.
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.