We are on the verge of another sharp transition from cold and dry to wet and mild. Avalanche danger is expected to start to rise on Sunday.
Confidence
Fair - Freezing levels are uncertain on Monday
Weather Forecast
Synopsis: On Saturday we should see a brief ‘calm before the storm’ with mainly cloudy skies and a chance of flurries. The freezing level is at valley bottom and ridge winds are generally light. A frontal system reaches the South Coast by Sunday morning dumping 10-20 cm of snow throughout the day. The freezing level remains at valley bottom and winds increase to moderate or strong from the SW. Moderate to heavy precipitation continues on Monday but temperatures should start to rise, possibly to 2000 m by the end of the day or overnight.
Avalanche Summary
Avalanche activity over the past couple days has mostly been loose wet sluffs from solar radiation and warm alpine temperatures. Earlier in the week there were numerous reports of natural and rider triggered wind slabs up to size 2.5 from southerly aspects as a result of outflow winds.
Snowpack Summary
The current snow surface varies significantly and includes wind affected surfaces in exposed alpine and treeline terrain, a sun crust on steep solar aspects, dry faceted powder in sheltered shady terrain, and pockets of surface hoar (mainly below treeline). Older wind slabs are still lurking on south facing slopes near ridge tops from last weeks outflow winds. The mid-December SH layer may be present in specific areas (sheltered, shady, near open water sources) down 40-70 cm. On Thursday one observer found this layer down 55 cm on a N aspect at 1700 m. Snowpack tests gave easy or moderate "drops" results. Below 2000 m and buried 60-80 cm deep you might find a crust with facets or mixed forms above or below.
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.