Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
Until yesterday the weather story was all about snow with the last storm starting warm and finishing cold. Monday overnight a cold front came through. Looking forward the story for Wed & Thurs is sunshine - warm temperatures and SOLAR WARMING. Freezing overnight. Moderate NW winds. Friday becoming cloudy, maybe flurries ahead of the next forecast system.
Avalanche Summary
Widespread natural avalanche cycle Monday with storm, wind, and warm temperatures; size 3 avalanches common, some size 4 likely. Timber fell. Observations limited by clouds, fog, and snow in the air so we won't know the full extent of the cycle until Tuesday's reports are tallied. One remotely triggered avalanche triggered from 300m away caught my eye - it speaks to the propagation potential of the surface hoar now deeply buried. If this layer is triggered the avalanche will be big; this layer can be triggered from below so be mindful of overhead hazard. This layer will not go away in the next few days. Also, a sledder triggered a size 3 avalanche in the Quartz Creek area and while the machine is reportedly totaled, the rider is okay.There are two great observations from professionals working in the region recently. The first one can be found on the MCR, it talks about the widespread & sensitive nature of the SH in the Northern Purcells. You can start here and work your way into the March observations: https://www.acmg.ca/mcr/archives.asp The other is in the form of a youtube video which also deals with the surface hoar, highlighting the potential for remote triggering and large propagations when this layer fails. Check out the shooting cracks in the video. Wild!: https://www.youtube.com/watch?v=UmSJkS3SSbA&feature=youtu.be
Snowpack Summary
See avalanche observations below. 30 cm of storm snow fell across the region during daylight hours Monday accompanied by winds out of the W/NW & W/SW at speeds capable of wind slab formation. Another 15cm arrived overnight before the cold front arrived. A cohesive slab continues to settle over the mid-February surface hoar, which is now down 70 to 150 cm. Recent reports include whumpfing and cracking, and moderate but sudden compression test results. This persistent weakness is susceptible to remote triggering and has the ability to propagate in low angled terrain, and the overlying slab structure creates the potential for step-down avalanches. Basal facets have only been reactive on steep, shallow, and rocky slopes, but operators continue to monitor this layer in tests. Triggering this deep persistent weak layer is unlikely, but shallow snowpack areas or shallow weak areas adjacent to deeper wind loaded slopes are the most suspect locations.
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.
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.
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.
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.