Warm temps and strong solar radiation input are likely to trigger a round of natural avalanche activity this weekend. Use caution and be thinking about the sun and potential for full depth large avalanches.
Confidence
High - Due to the quality of field observations
Weather Forecast
The weather this weekend is looking to be clear with relatively light winds and warm temps. Freezing levels are expected to climb up to around 2000-2200m on Saturday and higher on Sunday so this will be the first big pulse of spring heat into the snowpack. This heat will likely wake up the basal instabilities in a few places and trigger some larger avalanches so use caution.
Avalanche Summary
There was a recent sz 3 off of Mt Nestor on Friday that was on a NE aspect around 2900m likely triggerred by a cornice collapse. There were also a few more sz 2 avalanches on solar aspects around 2400m that appeared to be failing on one of the recent march crusts. In addition to this, there was also numerous loose dry avalanches up to sz 1 on all aspects.
Snowpack Summary
Very little wind affect is being noticed throughout the forecast region. Windslabs are isolated in both treeline and alpine areas confined to ridgelines, gullied features and crossloaded slopes. The snowpack differs depending upon which aspect you are on. On solar aspects, there are a series of crusts in the top 100cm of the snowpack that are producing moderate sheers. On more polar aspects, the snowpack is more settled with only the basal weaknesses such as facets and depth hoar being the main concern. Forecasters were noticing whumpfing from thin shallow snowpack areas while travelling into alpine terrain today. Be especially cautious around these thinner snowpack areas as these are likely trigger points for a skier. Cornices are still very large at this time and we have seen numerous collapses over the past few days causing large destructive avalanches. Solar radiation that is likely to occur over the weekend is only going to decrease the stability making these problems more prominent. Use caution and be thinking about the sun and full depth, full path avalanches.
Problems
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.