Avalanche Forecast

Moderate - Freezing levels are uncertain

Thursday night: Cloudy with clear periods. Light west west winds. Friday: Cloudy with isolated wet flurries bringing a trace of new snow to high elevations. Light rain below about 1500 metres. Precipitation continuing overnight. Moderate to strong southwest winds, increasing overnight. Alpine high temperatures around 0 to +1 with freezing levels to 1900 metres or higher. Lingering below freezing temperatures possible from 700-1300 metres.Saturday: Cloudy with continuing wet flurries bringing up to 5 cm of new snow to high elevations. Rain below about 1300 metres with new snow elevations dropping over the day as temperatures cool. Strong to extreme southwest winds. Alpine temperatures dropping from around 0 to -2 over the day as freezing levels decline from 1800 to 1500 metres.Sunday: Sunny. Light northwest winds. Alpine high temperatures around -7 with freezing levels reaching 700 metres.

A large (size 2.5) storm slab avalanche was triggered remotely (from 20 metres away) by a skier in the Howson Range on Tuesday. This occurred on a southwest aspect at 1700 metres and the slab had a depth of about 30 cm. It is likely that the failure plane in this slide was some variation on the surface hoar and crust combination described in our snowpack discussion. This weak layer is one of our primary concerns in the region and can be expected to produce human-triggered avalanches in areas where the overlying new snow has settled into a slab.Looking forward, increasingly warm temperatures and possible rain will promote similar avalanche activity over this surface hoar/crust combination layer. Higher elevations may experience a similar effect weakening new wind slabs or the formation of thin new wind slabs if new snow accumulates instead of rain. Loose wet avalanches are more likely at lower elevations.

New snow accumulations from the past few days are at around 30 cm. A thin crust may be found within this predominantly low density snow.At alpine and high treeline elevations, the recent snow sits on variable surface of wind-affected and faceted snow, and possibly over a weak layer of surface hoar in more protected areas. The new snow sits on a rain crust/surface hoar combination below about 1600 m and may become increasingly reactive over this weak interface as forecast warming promotes slab formation in the new snow.Under the new snow interface, the mid snowpack is generally strong. Although faceted (sugary snow) is present in this part of the snowpack, the overall dense structure here helps to overcome the weakness of these faceted grains.The bottom 30 to 50 cm of the snowpack consists of a weak layering of facets and crusts, particularly in areas where the snowpack is thin. It may be possible for large slab avalanches to step down down to ground in these thin snowpack areas, resulting in even larger avalanches.