The recent storm snow has been particularly reactive to human triggers at higher elevations. Riding preserved powder in sheltered trees is a good option for the weekend.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
We're looking at clear and cold from Sunday onwards, with overnight lows near -25 Celsius in some locations. Sunday: Clearing in the afternoon but cold. Tree line temperatures around -15 Celsius. Winds light to moderate northeasterly.Monday: Sunny with cloudy periods. Tree line temperature around -16 Celsius. Winds light northerly.Tuesday: Sunny with cloudy periods. Tree line temperature around -14 Celsius. Light winds becoming moderate northerly.
Avalanche Summary
On Saturday, explosives control work produced several storm slab avalanches to size 2.5 on a wide range of aspects near tree line. On Friday, touchy storm slabs to size 2 were reported in the Monashees, thanks to locally intense flurries. At least three small (size 1) human-triggered avalanches were reported on Wednesday, that ran in recent storm snow with crowns approximately 30 cm deep on north, east and south-facing slopes at around 2000 m. There were also two large (size 2.5) natural avalanches in alpine terrain that initiated as wind slabs but stepped down to the mid-January weak layer. In Rogers Pass on Monday, skiers remotely triggered a size 3.5 persistent slab avalanche from 40 m away on a south-facing, cross-loaded slope at 2600 m. Although deeper, persistent avalanche activity is becoming less frequent, there is still potential for these layers to react.
Snowpack Summary
35cm of recent storm snow is settling into a slab in the upper snowpack. Strong winds, most recently from the north / east, shifted these accumulations into deeper, reactive slabs in wind-exposed terrain. The more recent snow sits on a mix of older wind slabs in exposed higher elevation terrain, a sun crust on steep solar aspects, a rain crust below 1600m, and surface hoar on sheltered slopes.Below the snow surface, several persistent weak layers make up a troublesome snowpack. In the top 1.5-2.5 m of the snowpack, two surface hoar layers buried in January can be found. Expect to find one or other of these on all aspects and elevations.Deeper in the snowpack (around 200 cm deep) is a facet/crust/surface hoar layer from December, most prevalent at and below treeline.Near the base of the snowpack is a crust/facet combo, most likely to be triggered from thin spots in the alpine.All of these layers have produced large avalanches recently. The wide distribution and ongoing reactivity of these layers suggests that avoidance through choosing simple terrain is the best strategy.
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.