Prolonged reactivity of storm slabs has earned them a persistent slab label, but the avoidance strategy hasn't changed. Keep it in mind as you navigate around the more obvious wind effect.
Weather Forecast
Tuesday: Mainly cloudy with isolated flurries and a trace of new snow. Light southwest winds. Freezing level to 1300 metres with alpine high temperatures around -8.Wednesday: Mainly cloudy. Light southwest winds. Freezing level to 1100 metres with alpine high temperatures around -9.Thursday: Cloudy with flurries bringing 6-10 cm of new snow, continuing overnight. Light southeast winds. Freezing level to 1300 metres with alpine high temperatures around -8.
Avalanche Summary
Reports from Saturday included numerous observations of storm slabs releasing naturally as well as with skier traffic and explosives control. Sizes ranged from 2-3, with crown fracture depths varying from 30-100 cm. This activity occurred on all aspects but was focused at alpine elevations and this problem is now being identified as a 'persistent slab'. Wind slab releases made up a large number of observations, noted predominantly on southeast aspects.Widespread avalanche activity occurred in the top 30 cm of new snow on Friday. Numerous natural and skier triggered size 1 avalanches were reported, with the most reactive conditions on wind-affected slopes at treeline on north and east aspects.On Thursday, several size 1 skier triggered storm slab avalanches were reported. They were 30-50 cm thick and occurred on north and east aspects above treeline. One of the slabs subsequently triggered a larger size 2 avalanches that ran on the late-March crust. Some natural size 2-3 storm slabs avalanches were also reported in alpine terrain and a cornice fall on a north aspect in the Selkirks triggered a size 2.5 avalanche on an unidentified 2 m deep weak layer.
Snowpack Summary
A variable 5-20 cm of new snow over Sunday night has brought storm snow totals from the past week to a wide-ranging 50-90 cm. with amounts that taper with elevation. The snowfall was initially accompanied by strong west wind and formed reactive slabs at higher elevations. More recent north winds reached extreme, so a mix of stubborn old and newer, more reactive wind slabs can now likely be found on a range of aspects. Below the wind effect at the surface, a break between storm pulses allowed for the formation of yet another layer of sun crust (south aspects) and surface hoar (shaded aspects) that is now buried by 20-30 cm of new snow. The full depth of recent storm snow sits on a persistent weak layer buried in late-March that consist of crusts below 1900 m and on south aspects, and surface hoar on shaded aspects at higher elevations.Pockets of surface hoar (buried mid-March) have been reported on shaded aspects at higher elevations and may be found approximately 70-100 cm below the surface.Deeper persistent weak layers from January and December are still being reported by professional observers, 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.