Tricky conditions are expected on Wednesday with warm temperatures continuing, strong winds in the alpine, and new snow/rain. Use a conservative approach and avoid avalanche terrain during periods of rainfall.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A warm storm system reaches the interior on Wednesday morning. 2-4 mm of precipitation is expected during the day on Wednesday with freezing levels around 2500 m in the morning falling to 2000 m in the afternoon. Alpine wind is forecast to be moderate to strong from the southwest. Another 5-10 mm of precipitation is forecast for Wednesday overnight through Thursday afternoon. Freezing levels are forecast to remain around 2000 m on Thursday and alpine wind is forecast to remain moderate to strong from the southwest. Light precipitation is currently forecast to continue Thursday overnight and taper off Friday morning. Friday is forecast to be mostly dry and sunny with freezing levels falling to around 1500 m.
Avalanche Summary
On Monday, natural loose wet avalanches up to size 1.5 were reported from steep sun exposed slopes. Ski cutting triggered a size 1 wind slab on a north aspect at 2200 m. Explosives triggered a couple cornices and several size 2-3.5 slabs. A few of these appear to have released on old weak layers down 120-150 cm in the snowpack. On Sunday, a few natural size 1.5-2.5 wind slabs were observed. One of the wind slabs stepped down to a deeper layer. A ski cut on a northeast aspect at 1300 m elevation triggered a size 1.5 deep persistent slab which failed on an old layer near the ground. Explosives triggered numerous storm slabs up to size 2.5 as well as two deep persistent slabs up to size 3.5 which released down 200 cm, most likely on the November crust. On Wednesday, rain at lower elevations is expected to destabilize the upper snowpack resulting loose wet sluffing and potentially increasing the sensitivity of triggering persistent slab avalanches. In the alpine, new snow and strong southwest winds will likely form touchy new wind slabs. Wind slab avalanches have the potential to step down and trigger persistent slab avalanches.
Snowpack Summary
The storm snow from last week has settling rapidly due to the recent mild temperatures. The snow surface is expected to have undergoing melting on Tuesday afternoon on sun exposed slopes. Recent strong winds from the south and west had redistributed the recent storm snow in exposed terrain forming wind slabs. The early February weak layer is typically down 60-80 cm and includes a sun crust on steep sun-exposed slopes, faceted snow, as well as surface hoar on sheltered open slopes. This layer has remained reactive recently and has been the main sliding layer for most of the recent slab avalanche activity. A persistent weakness buried mid January is now down 80-150 cm and the November crust is down around 200 cm. These deep persistent weaknesses appear to be waking up with the warmer temperatures and several avalanches have recently released up to 2 m deep.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.