Winter is loosening its grip as sunshine and warm temperatures team up to test the snowpack in the coming days. Check out the Forecasters' Blog for more details: http://www.avalanche.ca/blogs/2018-03-09-first-big-warm-up
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
Saturday: A mix of sun and cloud, clearing over the day. Light south winds. Freezing level rising to 1500 metres with alpine high temperatures around -4.Sunday: A mix of sun and cloud, becoming cloudier over the day. Light southwest winds. Freezing level rising to 1900 metres with alpine high temperatures of -2.Monday: A mix of sun and cloud. Light south winds. Freezing level rising to 2000 metres with alpine high temperatures of 0.
Avalanche Summary
Several more loose dry avalanches to size 2 were observed on Thursday. A natural cycle of size 1.5-2.5 loose wet avalanches was reported on south-facing slopes on Wednesday. A couple of size 2-2.5 persistent slabs were also observed on south aspects. These most likely failed on a crust buried in February. Some large cornice chunks fell down too. Avalanche activity is expected to increase as we move into a period of intense sun and prolonged warming. Predictable loose wet avalanches from steep, sunny terrain should be easy to manage. The trickier beast will be when warmth penetrates into the snowpack and starts tickling deeper persistent weak layers. The next few days will be a time to avoid exposure to large sun-exposed features and keep well away from corniced slopes.
Snowpack Summary
Around 10-15 cm new snow has formed storm slabs that overlie a sun crusts on solar aspects. A couple of layers buried in mid-late February (down around 40-100 cm) are variably reactive, but both have the potential to create surprisingly large avalanches if triggered. Initially, these interfaces were most reactive on solar aspects, where they present as buried sun crusts. However, persistent slabs have been triggered on shady aspects too, where surface hoar and/or facets exist.Deeper persistent weak layers from January and December are generally considered dormant, but could wake up with forecast warming, a surface avalanche stepping down, cornice fall, or a human trigger in a shallow or variable-depth snowpack area. These layers consist of sun crust, surface hoar and/or facets. Facets also linger at the base of the snowpack.
Problems
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.
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.