Ongoing storm loading and a buried weak layer are creating tricky conditions. Watch for fresh wind slabs on Wednesday and use extra caution on south facing slopes when the sun is out. Conservative terrain selection remains critical.
Weather Forecast
Another 5-10 cm of snow is expected Tuesday overnight with moderate to strong alpine wind from the southwest. A mix of sun and cloud is expected on Wednesday with the potential for isolated snow flurries. Alpine wind is forecast to be moderate from the southwest and treeline temperatures are expected to be around -10C. Similar conditions are expected for Thursday with a mix of sun and cloud, isolated snow flurries, moderate alpine wind from the southwest, and treeline temperatures around -10C. A storm system is currently forecast to arrive Thursday evening and bring 30-40 cm of new snow by midday Friday. Alpine wind should be strong from the southwest during the storm and treeline temperatures are forecast to reach around -5C.
Avalanche Summary
On Monday, two natural size 1 wind slabs were observed on a southwest aspect in cross loaded terrain which were 20 cm thick. A skier triggered a size 1 storm slab on an east aspect at 1800 m. Explosives triggered three size 1.5 storm slabs which were 15-20 cm think. Sluffing from steep terrain features was also observed. On Sunday, a skier triggered a size 1 storm slab on a northeast aspect. The terrain was described as steep and rocky, and the slab was 50 cm thick. On Wednesday, large persistent slab avalanches remain a serious concern with the snow from the past week releasing on the February weak layers. It may still be possible for a person to directly trigger a persistent slab or a smaller avalanche could step down. Moderate to strong southwest wind Tuesday overnight may develop new wind slabs in immediately leeward terrain features which may be touchy. Old lingering wind slabs and cornices may also still be reactive.
Snowpack Summary
Another 10-15 cm Monday overnight brings the typical storm accumulation in the past week to 60-100 cm. Strong south and southwest wind during the storm redistributed much of this snow in wind-exposed terrain forming wind slabs up to 150 cm thick. This recent snow has settled into a slab that overlies the mid-February and late-February interfaces which may sit quite close together in the snowpack. The late-February interface consists of wind crust and hard sastrugi in wind-exposed terrain, and up to 20cm of facets in sheltered areas. The mid-February interface consists of a thick rain crust which extends into the lower alpine, a sun crust on steep solar aspects, surface hoar in sheltered areas, and in many places, a layer of facets overlying the crust. Recent observations suggest a poor bond between the recent snow and these weak layers. Below these layers, the mid and lower snowpack are well settled and strong.
Problems
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.
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.