New snow and shifting wind directions may create wind slabs on a variety of aspects, including slopes that are generally windward.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
THURSDAY: 5-10 mm of rain or 5-10 cm of snow above roughly 1100m. Light Southwesterly ridgetop winds. Freezing level around 1200m.FRIDAY: Mostly cloudy with isolated flurries. Light Southwesterly ridgetop winds. Freezing level around 1200 m.SATURDAY: Sunny. Light southwesterly ridgetop winds. Freezing level around 1500m.
Avalanche Summary
On Thursday the main concern will be wind slabs below alpine ridgetops. At tree line expect small loose wet avalanches in steep terrain if the sun appears for any length of time. Saturday's fatal avalanche on Mt Harvey (near Lion's Bay) was the result of people being on a cornice that failed. A large avalanche was subsequently triggered by the multi-ton snow boulders hitting the slope below.
Snowpack Summary
15-25 mm of precipitation is forecast to fall as snow above roughly 1200 m and as rain below on Wednesday. This means snow in the alpine, rain/snow at treeline, and rain below treeline. Strong southeast winds are expected to accompany the storm and create fresh wind slabs on lee features in the alpine which may be reactive to human triggers. The mix of rain and snow at treeline elevations may create the possibility of loose wet avalanches. Below treeline, rain has saturated an already wet, isothermic snowpack. Cornices are large in some areas and could trigger large avalanches when they fail. Additionally, cornices present a falling hazard and should be given a wide berth while traveling on ridgetops.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.