Regions
Northwest Coastal.
Confidence
Fair - Due to limited field observations
Weather Forecast
Wednesday: Moderate precipitation with 10-15mm of water equivalent and freezing levels dropping throughout the day to1000. Moderate southwesterly winds increasing throughout the day. Thursday: Continued moderate precipitation with another 10-15mm, freezing levels increasing to around 1200m and strong to extreme southwesterly winds. Friday: Precipitation and winds easing throughout the day with freezing levels around 1200m.
Avalanche Summary
Recent reports include numerous natural moist slab avalanches up to Size 2.5 and widespread wet sluffing with warm temperatures and light rain on Monday. In the eastern part of the region, extreme winds caused numerous thin wind slab avalanches in alpine areas below ridgetops, and strong solar radiation caused a few Size 2 released right to the ground. On Sunday there was a report of a size 1.5 skier triggered avalanche on a steep unsupported convex roll on a northerly aspect. This event likely released on the late December surface hoar layer approximately 60 cm deep. There are also a few reports of glide releases in steep south facing terrain.
Snowpack Summary
Recent temperature inversions and freezing rain at higher elevations has resulted in a thin surface crust in treeline and alpine areas, while surface facets and surface hoar are surviving in the cold air trapped in valley bottoms. Some higher elevations particularly in the eastern part of the region may have gotten a dusting of new snow, effectively protecting the surface hoar, before the freezing rain, which would have capped this potential future persistent weakness with a thin crust. Below this, weaknesses within the 30-60cm of well settled storm snow is only producing resistant fractures in recent snowpack tests. However, a persistent weakness of surface hoar or facetted snow buried at the end of December sits at the base of this recent storm snow and continues to react as sudden fractures in snowpack tests particularly below treeline. The mid and lower snowpack is generally well settled and strong, although well developed basal facets remain a concern in shallow snowpack areas.
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.
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.