Confidence
Poor - Freezing levels are uncertain on Monday
Weather Forecast
A ridge of high pressure is expected to continue to provide drier and cooler temperatures to the interior on Sunday. The temperature should rise on Monday causing the freezing level to rise dramatically to near 3000 metres. Monday evening the ridge is expected to weaken, and light flurries are expected by Tuesday.
Avalanche Summary
Control work done on Tuesday produced a size 2 avalanche running to ground from a northeast aspect. There is still very limited observations from the region, so you will have to do the detective work. I suspect these conditions will continue through the forecast period, and it's likely that avalanches are susceptible to human/ sled triggering. With clear skies ahead this is a good opportunity to make snowpack, and avalanche observations from your surrounding mountains before jumping in to deep. I bet you'll be able to see evidence of older, large avalanches that occurred on your local mountains. I do suspect that these slopes will be ripe for human, and/or sled triggering especially on northerly aspects where avalanche activity has not occurred. Remember these avalanches are failing on weak facetted (sugary) snow at the bottom of the snowpack, producing full depth avalanches.
Snowpack Summary
Snow depths are quite variable through the region. In the alpine there is 170- 250cm. At treeline there is anywhere from 130-150cm. Sunday's warm, moist storm combined with strong southwesterly winds created stiff wind slabs on north-east slopes. Changing winds through the weekend at ridgetop and treeline will redistribute snow, forming new wind slabs on southerly aspects. The upper snowpack is tightening up forming a stronger mid-pack. A highlighted concern for the Purcells was the mid-November storms that created a heavy slab over a weak basal faceted base. The Purcells saw a significant cycle of large avalanches running full depth as a result. (The Lizard Range also saw very similar results all failing on the weak basal layers.) Conditions are ripe for human triggered avalanches, starting as a wind slab that could step down to weak basal facets (sugary snow) producing large destructive avalanches with high consequence.
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.