Weather Forecast
Synopsis: A frontal system will cross the South Coast tonight bringing moderate to heavy precipitation, strong to extreme winds, and rising freezing levels. Drier but unsettled conditions are expected in the wake of the system on Thursday and into Friday. Tonight and Wednesday: 15-20 cm in the Duffey, and 25-40 cm in the Coquihalla. The freezing level will climb as high as 1400 m before dropping sharply in the evening. Winds are strong to extreme from the southwest. Thursday: Mainly cloudy with a chance of flurries. The freezing level drops to valley bottom and winds ease to moderate from the southwest. Friday: A mix of sun and cloud with a slight chance of flurries. Temperatures remain cool with the freezing level near valley bottom.
Avalanche Summary
On Monday, a cornice fall triggered a size 3 slab avalanche on a NE aspect with a crown depth up to 100 cm. There is limited information on an accidentally triggered slab avalanche on Sunday in the Duffey Lake area that resulted in 2 people being buried. Slab avalanches were also easily human-triggered up to size 2 in the Coquihalla on south aspects with 40-60 cm storm snow failing on a crust.
Snowpack Summary
Heavy snowfall has continued in the Coquihalla region, with recent storm totals over the last four days of 80 cm. During the same period, around 20 cm fell along the Duffey and around 30 cm in Manning Park. 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 all areas, expect extensive wind slabs and cornices in the alpine. A deeper weak layer comprising surface hoar and/or a crust is buried around 50-70 cm on the Duffey/ Manning Park, and more like a metre on the Coquihalla. Natural avalanche activity has subsided on this layer. However, I wouldn't discount the possibility of an avalanche stepping down to this layer, especially in steep alpine terrain.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
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.