Another storm hits the region tonight into Tuesday. Strong winds and new snow has increased the avalanche danger.
Confidence
Fair - Intensity of incoming weather is uncertain on Tuesday
Weather Forecast
Monday overnight brings 8-15 cm of new snow. This is accompanied by strong South winds, and freezing levels near valley bottom. Tuesday: Snow amounts 15-25 cm. Ridgetop winds strong from the SW. Treeline temperatures near -5. Freezing levels near 500 m. Wednesday: Due to the convective weather pattern, scattered light-moderate snow accumulations are expected. Ridgetop winds moderate from the SW. Treeline temperatures near -8. Freezing levels valley bottom. Thursday: Cloudy in the morning, broken skies in the afternoon. Light afternoon flurries near 5 cm. Freezing levels valley bottom in the morning, rising to 700 m in the afternoon.
Avalanche Summary
Last week was very active with natural avalanche activity up to size 3.5. Additionally we had several reports of rider involvements and close calls. Some were failing on the mid-February persistent weakness. The size and frequency of avalanches seems to have decreased slightly over the weekend as storm slabs and wind slabs slowly gained some strength. On Sunday a skier accidently triggered a size 1.5 from a NE aspect, in steep rocky terrain. Avalanches were also triggered naturally and by explosives in steep, rocky terrain to size 2. Avalanche activity may increase with additional snow and wind on Tuesday. The danger ratings will rise.
Snowpack Summary
Last week, 100-180 cm storm snow fell, accompanied by strong south-easterly to south-westerly winds. Storm slabs and wind slabs now appear to be slowly gaining strength. Cornices are large and threaten slopes below. A persistent weakness, formed in mid-February, continues to produce hard, sudden planar results in snowpack tests. The likelihood of triggering this layer has gone down, but very large avalanches remain possible, which could be triggered by a shallower avalanche or cornice fall. The average snowpack depth at treeline is 350 cm. With the arrival of Tuesday's storm, new storm slabs and wind slabs will develop. It may take several days for the new storm snow to settle out and start to stabilize.
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.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.