Lingering problems are maintaining heightened avalanche conditions. Our consistently active storm slabs have graduated to 'persistent slab' status and still need to be managed with conservative terrain selection.
Weather Forecast
Tuesday: A mix of sun and cloud. Light to moderate southwest winds. Freezing level to 1100 metres with alpine high temperatures around -9.Wednesday: A mix of sun and cloud. Light west winds. Freezing level to 800 metres with alpine high temperatures around -10Thursday: Mainly cloudy with flurries beginning in the afternoon and increasing overnight. Light east winds. Freezing level to valley bottom with alpine high temperatures around -11.
Avalanche Summary
Reports from Sunday included observations of several more large (size 2) recent natural storm slab releases. These were noted on south aspects from 2000 to 2300 metres, with failure planes at the late-march layer mentioned in our snowpack discussion. This avalanche problem is now being labeled a 'persistent slab'.Saturday's reports included two more natural slab avalanches. A size 1.5 storm slab failed on a south aspect at 2300 metres while another deeper size 2.5 slab released from a very steep north aspect at 2700 metres. Their respective crown fracture depths were 80 cm and 200 cm and the failure plane of the deeper release is not certain.Reports from last week showed evidence of a natural avalanche cycle during and following the storm, with Storm slabs in the size 2-3 range were reported on all aspects between 1700 and 2500 m. Southerly aspects were the most reactive with numerous large and very large (size 2.5-3.5) avalanches running on the buried sun crust. Cornice falls and skier-triggered wind slabs also featured in reports. More recent reports have shown a gradual decrease in activity.
Snowpack Summary
Last week's storms brought totals of 60-100 cm of new snow to the region. The snowfall was initially accompanied by strong west wind, forming reactive slabs at higher elevations. More recent north winds reached extreme, so a mix of old and stubborn and newer, more reactive wind slabs can now likely be found on a range of aspects. In sheltered areas, the storm snow has been settling into a slab above a persistent weak layer buried in late-March that consists of crusts at low elevations and on south aspects, and surface hoar on shaded aspects at higher elevations. Although this problem is now being labeled a persistent slab, our recent storm slab releases have been running on this layer.Other persistent weak layers from early January and mid-December are still being reported by local operators, but are generally considered dormant.
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.