Confidence
Fair - Freezing levels are uncertain on Monday
Weather Forecast
A firmly embedded ridge of high pressure will continue to dominate the region bringing clear skies and light southerly winds throughout the forecast period. An inversion with above freezing alpine temperatures will persist on Sunday, but will give way to a gradual cooling pattern forecast for Monday and Tuesday.
Avalanche Summary
A significant avalanche cycle occurred in the region on Friday as many large loose snow and slab avalanches (size 2-4) were observed. For the most part, the cycle was specific to southeast to southwest aspects in the alpine, with some slides running down to mature forest. Cornice fall was also an issue as many large cornices failed naturally, in some cases triggering a slab on the slope below. There were also a few isolated deep slab avalanches (to size 3.5) on north aspects that failed without cornice fall. It is thought that some of these released at a crust that was formed in October. Expect ongoing avalanche activity with forecast warm alpine temperatures on Sunday.
Snowpack Summary
January was a snowy month in the South Columbia region and average snowpack depths at 1700m are now between 2 and 3m. For the most part, the storm snow is well settled on shaded aspects and riders are gaining increased confidence on steeper terrain. There are, however, several storm interfaces within the top 150cm that operators have been watching, most notably the January 13th interface which had spotty surface hoar. At the bottom of the mid-pack is a surface hoar layer that formed in mid-December. Until Friday these layers were showing very little in the way of reactivity. That changed on Friday as the region experienced above freezing temperatures at alpine elevations due to an inversion and direct solar radiation. As a result, a significant avalanche cycle took place waking up a number of these deeper layers as sun exposed slopes and cornices became moist and weak. Reactivity of deeper layers was not limited to south aspects as some deep failures were observed on north aspects without cornice fall as a trigger. Expect continued weakening of the snowpack with forecast weather on Sunday.
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.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.