Persistent slab avalanches on various buried weak layers have surprised riders in areas north of Nelson in recent days. Reactivity should cool down in the coming days but keep the potential for large human triggered avalanches on your radar.
Wind slabs may be easily triggered where they sit directly over a weak layer or a crust. Wind loading may be subtle so expect to find it on leeward aspects and below sharp changes in terrain shape.
Yet another layer has graduated into this category. A recently buried layer of surface hoar and/or sun crust was touchy to skier triggers north of Nelson on the weekend. Cooling temperatures and cloudy skies should help it calm down for now but once the weather becomes favorable for slab formation (warm temperatures, new snow load, wind, etc), we will have our eye on this one.
Older weak layers from February and January (mostly crusts and surface hoar) are now down 60+ cm and 100+ cm, respectively. These are primarily a concern in the northeast of the region on northerly aspects around treeline. Deep avalanches have been observed there recently but it can be difficult to tell from afar exactly which layer was the culprit.
Diligent terrain selection is required for these types of avalanche problem. Signs of instability may not be visible on the surface, and the resulting avalanches will be large. An important part of your strategy would be to avoid rocky start zones below ridgetops and convex rolls.
Monday night: Partly cloudy. Moderate northwest ridgetop wind. Treeline low around -7 °C. Freezing level dropping to valley bottom.
Tuesday: Scattered flurries bringing a trace. Light northerly wind. Treeline high around -5 °C. Freezing level 800 m.
Wednesday: Flurries bringing up to 5 cm. Light northwest wind. Treeline high around -7 °C. Freezing level valley bottom.
Thursday: Mostly sunny. Moderate northwest wind. Treeline high around -5 °C. Freezing level 800 m.
On Sunday a size 1.5 skier triggered persistent slab avalanche was reported in an treeline elevation start zone north of Nelson. Good visibility allowed observation of a 2 day old widespread avalanche cycle in the Valhallas size 2-3.5. One noteable stepped down to a weak layer 100 cm deep.
On Friday and Saturday, several accidentally triggered storm slab avalanches were reported in the northeast of the region. These avalanches were:
Similar incidents also occurred east of Slocan in the neighboring South Columbia region, resulting in a partial burial and injury. More details and photos here.
Small loose wet avalanches were also reported on solar aspects in the afternoon.
East of Kelowna, a natural wind slab avalanche and several explosive triggered wind slabs were reported up to size 1.5 on east through south aspects around treeline.
A near-surface crust may make for challenging travel conditions up to 2250 m. The crust ranges from breakable to supportive and is between 1-5 cm thick.
20-50 cm of recent storm snow sits over a variety of surfaces including a weak layer of surface hoar in wind-sheltered areas around treeline and a crust on solar aspects.
Weak layers from mid-February and late January made up of crusts and/or surface hoar crystals can be found around 60-100 cm deep. Some recent persistent slab avalanches on this layer have surprised riders, so keep this type of avalanche in mind when making terrain choices.
