Dangerous conditions are expected this weekend. New snow and warming will contribute to the storm slab and deep slab problems. Conservative terrain choices are essential.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
SATURDAY: Clear skies in the morning with increased cloud in the afternoon and light flurries, 30-50 km/h southwest winds, alpine temperature around -6 C.SUNDAY: Overnight snowfall of 5-10 cm easing off throughout the day, 30-40 km/h southwest winds, alpine temperatures around -4 CMONDAY: Chinook conditions, cloudy with sunny breaks, 50-70 km/h winds, freezing level spiking to 2000 m.
Avalanche Summary
Reports from Friday include several size 2 natural storm slabs in steep northerly terrain and numerous ski cut storm slabs in the size 1-1.5 range (and one size 2 skier triggered avalanches). Storm slabs will likely remain reactive to human triggers over the weekend.On Thursday, explosive avalanche control along the White River (in the northwest corner of the region) produced over 10 deep persistent slab avalanches releasing on basal facets. Most of the avalanches were size 3, included several than ran full path to valley bottom taking mature forest and blocking the river with up to 8 m of debris. An older size 2 natural avalanche failing on basal facets was also reported in the Crossing Creek area. Looking ahead, dramatic warming will likely wake up the deep persistent slab problem and create the potential for very large avalanches.
Snowpack Summary
Friday's storm delivered about 15-30 cm of heavy snow that settled into a touchy slab thanks to mild temperatures and moderate winds. Strong southwest winds are expected to shift these accumulations into much deeper deposits in exposed lee terrain. The overall storm total since the start of the month now stands at roughly 50-90 cm. In some areas, the resulting slab may have a poor bond to the old snow interface which consists of stiff wind slabs, crusts, and facets. The mid-pack in this region is generally strong, but the bottom third of the snowpack is composed of weak facets (sugary snow crystals). Approximately 100-120 cm of settled snow sits above this weak layer of sugary, rotten facets that developed in December. This remains a concern and should be on your radar, especially with the recent loading and upcoming warming.
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.