Persistent weaknesses deeper in snowpack still can't be totally trusted. Approach big open slopes with caution, especially in shallow snowpack areas.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
We've had a bit of a shift from the very cold conditions, and temperatures have warmed by roughly 5 degrees Celcius. Only some isolated flurries are possible until late Sunday, when a small amount (5-10 cms) of light dry snow is forecast.SATURDAY: Sunny breaks with increasing clouds overnight. Flurries late in the day with little accumulation. Winds light southerly. Alpine highs to -13 Celcius.SUNDAY: Cloudy with flurries in the afternoon. Wind light southerly. Alpine temperatures between -12 and -16 Celcius; 5-10 cms light dry snow overnight.MONDAY: An additional 5-10 cms light dry snow are possible during the day. Winds light - moderate westerly. Alpine temperatures steady near -12 Celcius.
Avalanche Summary
Recent avalanche control work showed no signs of triggering slabs; however, don't let that give you a false sense of security.... normal caution is advised. Small, predictable skier-triggered avalanches (thin windslabs) were reported on Wednesday. Reports from earlier in the week were limited to a few Size 1-2 wind slab avalanches triggered naturally and by explosives. Wind slabs may remain reactive to human triggers in specific areas, such as the immediately lee of ridges and sun-exposed slopes.
Snowpack Summary
Surface hoar is growing and the upper snowpack is faceting. In exposed areas at all elevations, recent winds have resulted in scouring, hard wind slabs, and thicker reactive wind slabs in unusual places as the winds shifted from west to northeast. Continued moderate variable winds have been keeping wind-slabs fresh and touchy in some areas and sun-exposure is likely making the wind slabs extra touchy on south-facing slopes. Facets and/or surface hoar buried mid-December has been giving hard and broken or no results in snowpack tests where it is down close to a metre. However, recent snowpack tests in a shallower area, gave moderate sudden planar results on this persistent weakness where it was found as surface hoar down 20-30 cm. This suggests that the primary concern for persistent slab avalanches is in shallow snowpack areas; however, the potential for step-down avalanches remains where it is deeper. The lower snowpack is well bonded and features a thick rain crust near the ground.
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.