Regions
Kootenay Boundary.
The high elevation rain event is creating very dangerous avalanche conditions. Very large natural avalanches are expected on Wednesday. Now is an appropriate time to avoid all avalanche terrain including the run out zones of large avalanches paths.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
Another 10-20 mm of precipitation is expected Tuesday overnight with strong southwest wind in the alpine and freezing levels remaining high. On Wednesday, 10-20 mm of precipitation is expected with freezing levels remaining around 2000 m and strong alpine wind from the southwest. Thursday is forecast to be mainly dry with sunny breaks. Freezing levels are expected to drop to around 1500 m and alpine wind is forecast to be light from the southwest. The next storm system is currently forecast to arrive midday Friday and persist through Saturday.
Avalanche Summary
An early report on Tuesday suggests widespread natural avalanche activity north of Nelson. A natural size 2.5 persistent slab avalanche was also observed on an east and southeast aspect slope at 2200 m in the area south of Nelson. On Monday, natural size 1 avalanches were observed and isolated to the most recent storm. Ski cutting produced numerous size 1-1.5 storm slabs and explosives triggered two size 2 storm slabs. These storm slabs were typically 10-30 cm thick. A remotely triggered wind slab was also reported and triggered from 5 m away. On Sunday, explosives triggered numerous size 1-2.5 storm slab avalanches which were 20-30 cm thick. One natural size 2 storm slab avalanche was reported south of Nelson. On Wednesday, wet slabs and loose wet avalanches are possible at lower elevations where rain is saturating the upper snowpack. At the highest elevations, storm slabs will be very touchy, especially in wind loaded terrain. The warm conditions and rain are expected to wake up any lingering weaknesses within the snowpack and persistent slab avalanches are a serious concern.
Snowpack Summary
On Tuesday, freezing levels reached 2200 m or higher. The snow surface was reported to be moist above 2000 m and wet below 2000 m. High elevation rain is expected to continue on Wednesday and will continue to destabilize the snowpack. Prolonged high elevation rain has the potential to wake up deeply buried weak layers in the snowpack. The late-February interface is now down 70-120 cm. This layer is composed of sun crust, surface hoar, and weak faceted snow and continues to produce easy, sudden results in snowpack tests. The thick mid-February crust layer is now down 90-135 cm and exists on solar aspects and at lower elevations. This layer had gone dormant but will be tested during this storm. In most deeper snowpack areas, the snowpack is generally well settled and stable below this crust layer. However, some areas have a lingering basal weakness at the bottom of the snowpack. This is especially prevalent in shallow snowpack areas of the region.
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.
Wet Slabs
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.