Persistent slab avalanches on various buried weak layers have surprised riders in areas north of Nelson in recent days. Reactivity has cooled down along with the temperatures for now 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 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.
Tuesday night: Localized flurries in the southeast corner bringing 5-10 cm. Light northerly wind. Treeline low around -14 °C. Freezing level valley bottom.
Wednesday: Mostly sunny. Light northwest wind. Treeline high around -7 °C. Freezing level valley bottom.
Thursday: Sunny. Light to moderate northwest wind. Treeline high around -5 °C. Freezing level 700 m.
Friday: Sunny. Light to moderate west wind. Treeline high around -3°C. Freezing level 1300 m.
No new avalanche activity reported on Monday. The current cooling trend has helped calm down avalanche activity on buried weak layers, old and new. But the many reports of avalanches over the warm sunny period on the weekend indicate that our various weak layers are still triggerable under the right conditions.
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 notable stepped down to a weak layer 100 cm deep.
Storm slabs sitting on a weak layer were touchy Friday and Saturday, with several riders getting surprised in the northeast of the region. These avalanches were:
South of Nelson, around 10 cm of lightly wind-affected snow sits over a near-surface crust up to 8 cm thick which may make for challenging travel conditions as high as 2250 m. Further north where the crust does not exist, more surface snow is available for wind transport so we anticipate more substantial wind slab development at upper elevations.
A recently buried weak layer sits 20-50 cm deep. It consists of surface hoar crystals in shady, wind-sheltered areas around treeline and a crust on solar aspects. This layer was reactive to human triggers over the weekend in areas where it was not bridged by the above-mentioned crust.
Weak layers from mid-February and late January made up of crusts and/or surface hoar crystals can be found around 60+ cm and 100+ cm deep, respectively. Some recent persistent slab avalanches on this layer have surprised riders, so keep this type of avalanche in mind when making terrain choices.
