This weekend's weather forecast includes higher freezing levels and rain to mid-mountain elevations. This will have a destabilizing effect on the snowpack with avalanches increasing in size and frequency.
Confidence
Fair - Freezing levels are uncertain on Friday
Weather Forecast
Thursday Night: Up to 10cm of snow / Moderate westerly winds / Freezing level at surfaceFriday: Light snowfall / Moderate to strong southwest winds / Freezing level at 1000m climbing to about 1800m in the eveningSaturday: 10-15cm of snow at higher elevations / Moderate to strong southwest winds / Freezing level at about 2000mSunday: Up to 30cm of snow at higher elevations / Moderate to strong southwest winds / Freezing level up to about 2000m
Avalanche Summary
Several natural storm slab avalanches to size 2.5 were reported on a variety of aspects and elevations on Wednesday. Numerous size 3 storm slab avalanches were also reported on south facing alpine terrain. These avalanches formed in response to new snow and wind. I would expect more of this with the potential to step down to deeply buried persistent weaknesses with forecast precipitation and rising freezing levels over the weekend.
Snowpack Summary
Up to 80cm of recent storm snow overlies small surface hoar in sheltered areas, a sun crust on solar aspects and wind slabs at higher elevations. In wind exposed terrain these new accumulations have been pushed into much deeper deposits by generally moderate southwest winds. Forecast snowfall, wind and warming will add to the reactivity and destructive potential of this developing storm slab.There is ongoing concern for a mix of weak surfaces which were buried on February 10th. This persistent interface lies between 80 and 180cm below the surface, and includes surface hoar, well developed facets and a mix of hard surfaces which remain widespread at all aspects and elevations. Although natural avalanche activity has tapered-off at this interface, it remains sensitive to remote triggers in isolated terrain and may see a significant "wake-up", particularly with the increased load of the new snow and forecast rain.For the most part, the mid and lower snowpack are strong and well consolidated.
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.