It's still winter at treeline and alpine elevations!
Confidence
Moderate - Due to the number of field observations
Weather Forecast
TUESDAY: Wake up to 5 to 10 cm of new snow, but only a trace more during the day. Mix of sun and clouds. Treeline temperatures a few degrees either side of zero and alpine temperatures a few degrees below zero. Freezing level around 1600 m. Light southeast winds.WEDNESDAY: Mix of sun and clouds. Possibility of traces of new snow in flurries. Freezing level around 1500m with temperatures within a few degrees of zero at treeline and alpine elevations. Moderate southeast winds.THURSDAY: Cloudy. Flurries with around 5 cm of new snow. Freezing level near 1400 m with alpine temperatures up to -2C. Light southerly wind.
Avalanche Summary
Shallow wind slabs in alpine terrain is representative of the ongoing wind slab concern: localized pockets, lee and cross-loaded features, around 20 to 40 cm thick. Saturday's fatal avalanche accident on Mt. Harvey, although not in the Sea To Sky region, does highlight several of the risks posed by cornices: multi-ton snow boulders serve as large triggers potentially releasing large avalanches on the slopes below, they can break well back making for tricky travel along ridge crests. It's all the more tricky when visibility is obscured in fog or heavy snow, if the easiest pathway (flat, hard snow, open straight line) is within the danger zone, or when they're so big that you need to be 10 or 15 or more metres back from the edge to remain safe. There were two reports of size 3 Cornice triggered avalanches from the Whistler backcountry on Sunday as well as a widespread avalanche cycle to size 3 in the Tanatalus Range, likely releasing on Friday and Saturday during the previous storm.
Snowpack Summary
Recent snow in the last few days built wind slabs, primarily on northerly aspects at alpine and treeline elevations. They should stabilize quickly with warm temperatures but isolated wind slabs behind ridges and similar terrain may linger. Expect multiple crusts in the upper snowpack, especially on southerly facing (sunny) slopes. Thin crusts with facets above are possible making southerly aspects complex and harder to forecast. Cornices are large and remain a concern: yesterday's fatal accident near Lions Bay illustrates the danger of them breaking off, and the large avalanches they can trigger. Sunshine and solar radiation can quickly weaken snow on solar facing slopes.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.