The storm marches on! With more warm, wet, and, windy weather in the forecast, dangerous avalanche conditions will persist on Monday.
Weather Forecast
Sunday night: Heavy snowfall bringing 30-40 cm of new snow. Rain below about 1100 metres. Strong to extreme south winds.Monday: Continuing snowfall bringing 20-30 cm of new snow. Rain below about 1100 metres. Strong to extreme southwest winds. Freezing level to about 1500 metres with alpine high temperatures around -2.Tuesday: Cloudy with continuing flurries bringing approximately 5 cm of new snow. Moderate southwest winds. Freezing level to 900 metres with alpine high temperatures of -6.Wednesday: Mainly cloudy with isolated flurries and a trace of new snow. Light west winds. Freezing level to 700 metres with alpine high temperatures of -7.
Avalanche Summary
Reports from Saturday showed explosives control in the Whistler area producing numerous storm slab results from Size 1.5-2, as well as several Size 1.5 cornice releases.Throughout this region there has been evidence of a natural avalanche cycle up to size 3 that occurred within the storm snow on Wednesday. More notable are the numerous explosive triggered, very large avalanches up to size 3.5 that where reported near Whistler on Thursday. Two additional explosive triggered 3.5's were reported Friday. These avalanches started in the storm snow and then stepped down to the early January and even the mid-December crusts which resulted in very large avalanches running full path. They occurred on northwesterly aspects below rocky ridgetops between 1950-2200 m. Although these layers are over 200 cm beneath the snow surface, a large trigger such as a falling cornice or a smaller human triggered avalanche has the potential to "step-down" to these layers, which would result in a large and destructive avalanche.
Snowpack Summary
Another 10-20 cm of new snow on Sunday morning brings recent storm totals to around 90-120 cm. This snow continues to be redistributed by strong southeasterly winds, creating dense storm slabs in lee and cross-loaded features. Impressive cornice growth has also been observed on the lee side of ridgetops. A crust that was buried in early January is buried 200-250cm below the surface at all elevations. Below this there is another crust which was buried in mid-December. Recent explosive control work has triggered storm slab avalanches that have "stepped down" to both of these crusts, resulting in very large avalanches running full path. Although these layers are over 200 cm beneath the snow surface, continued potential exists for large triggers such as a falling cornice or a smaller human triggered avalanche to "step-down" to these layers. This would result in a very large and destructive avalanche.
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.