Recently formed storm slabs are expected to remain reactive on Thursday, especially in wind loaded terrain. Use extra caution on south facing slopes when the sun is out. Conservative terrain selection remains important.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Mainly cloudy conditions are expected on Thursday with snow flurries and sunny breaks both possible. Alpine wind is expected to be moderate from the southwest and freezing levels are forecast to reach around 800 m. A storm system is expected to reach the region Thursday evening. 30-40 cm of snow is forecast between Thursday evening and Friday afternoon. Alpine wind is forecast to be strong from the southwest and freezing levels should climb to around 1500 m on Friday afternoon. The next storm system is currently forecast to arrive on Saturday afternoon or evening.
Avalanche Summary
On Wednesday, explosives triggered two size 1 storm slabs which released down 10-25 cm. These were described as having minimal propagation and minimal entrainment. On Monday, a report includes whumphing and a ski cut triggering a size 2 slab which released on the crust layer. Explosives also triggered numerous storm slabs 20-30 cm thick and one stepped down to the crust layer down 40-60 cm. On Thursday, recently formed storm slabs may still be reactive to human triggering, especially in wind loaded terrain. Large persistent slab avalanches have generally become unlikely but still may be possible in isolated areas. Natural sluffing is possible from steep sun exposed slopes.
Snowpack Summary
Another 25-50 cm of snow fell in the North Shore mountains on Tuesday bringing the typical storm accumulation in the past week to 100-200 cm. Recent winds have been from a variety of directions and wind slabs should be expected on all aspects. This recent snow overlies the late-February interface that is composed of a sun crust on southerly aspects and surface hoar on shaded aspects. An observation on Wednesday suggests this layer has gained a lot of strength since the weekend. However, conservative choices are still recommended until we are sure this layer has become unreactive. We could be in a low probability, high consequence scenario where very large avalanche are still possible in isolated areas. Below this interface, the mid and lower snowpack appear to be well settled and strong.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.