Deep persistent weak layer avalanches continue to be a concern in this region. Please read the Lizard-Flathead forecast for more information about recent large avalanches.
Confidence
Moderate - Wind speed and direction is uncertain
Weather Forecast
Tonight: Overcast with a chance of 3-5 cm of new snow combined with light northwest winds and freezing to valley bottoms. Thursday: Broken skies with light variable winds and -15 in the alpine. Friday: Mix of sun and cloud with light variable winds and -18 in the alpine. Saturday: Mostly sunny with moderate northwest winds and alpine temperatures close to -20.
Avalanche Summary
The deep persistent slab problem is a low probability/high consequence scenario that warrants extra caution around large open slopes, especially in shallow snowpack areas. There was a report on Tuesday of a size 3.5 avalanche at Mt Hosmer in the Lizard/Flathead region that released on or stepped down to the deep weak layer near the ground. On Wednesday we had a report from the Lizard range of another size 3.0 deep persistent avalanche on a northeast aspect in the alpine. Avalanche activity on Wednesday near Elkford was limited to loose snow up to size 1.5.
Snowpack Summary
Near Elkford on Wednesday there was 20 cm of storm snow above a melt/freeze crust that developed from the rain last weekend. The height of snow was 170 cm and there was about 100-120 cm of settled snow above the weak layer of sugary facets that developed during the cold spell in December. Snow profile tests resulted in a hard shear in this location where the facets were sitting on a hard wind crust. Recent slope testing did not show any results. Some loose snow had released naturally from steep unsupported terrain.
Problems
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.
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.