A lot more snow than expected fell in the Coquihalla. Variable snowpack conditions through the region.
Confidence
Fair - Due to variable snowpack conditions
Weather Forecast
Synopsis: Northwest flow gives way to a frontal system that impacts the region on Tuesday afternoon and into Wednesday.Monday: Patchy convective snowfall. Some areas may see nothing, others 2-5 cm. Sunny breaks in the cloud giving daytime warming. Afternoon freezing level around 700 m. Winds gusty from the northwest.Tuesday: Day starting clear, but clouding over then moderate snowfall starting in the afternoon. 10-15 cm overnight. Freezing level around 1500 m. Southwest winds to 60 km/h at ridgetop.Wednesday: 10-15 cm new snow. Freezing level around 1600 m. Southwest winds around 30 km/h.
Avalanche Summary
On Sunday, slab avalanches were easily human-triggered up to size 2 in the Coquihalla on south aspects with 40-60 cm storm snow failing on a crust. On Friday, a size 2.5 natural loose wet avalanche was reported from the Coquihalla. Along the Duffey, small skier-triggered avalanches could be triggered on north aspect slopes at treeline, with crowns of 30 cm in the recent storm snow.
Snowpack Summary
Heavy snowfall affected the Coquihalla on Saturday Night with around 40 cm new snow by Sunday morning. On the Duffey, more like 10 cm fell and at Allison Pass there was around 8 cm. This new snow sits on a crust along the Duffey at elevations below 1900 m. In the Coquihalla region, this crust was not reported to have formed due to the insulating effect of the new snow. Instead, the new snow here sits on wet snow from the previous storm. For both sub-regions, expect wind slabs and cornices in the alpine. Below the recent storm snow lies one or two weak layers buried about 60 cm below the surface (more like 80cm in the Coquihalla). These comprise faceted snow, surface hoar and/or a crust. The snowpack structure is complicated and highly variable at this time. It may change dramatically with only a subtle change in elevation, aspect, wind or sun exposure. With further cooling, the upper snowpack should start to bond better. However, with the amount of moisture and the insulating effect of the new snow, expect this to take longer than usual. The mid-pack is strong and well settled, although the entire snowpack will remain volatile below 1500m until it has a chance to freeze.
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.
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.