Strong solar inputs will destabilize the snowpack in the afternoon. Loose wet sluffing, cornice failures, and solar triggered slab avalanches are all possible. A tricky weak layer is expected to be most reactive on high elevation north aspect slopes.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Warm, sunny conditions dominate the weather for the forecast period. On Friday, a mix of sun and cloud is expected with moderate winds from the SW to NW and freezing levels reaching around 2500m or higher. A weak storm front is forecast to bring increased cloud cover for Friday evening and light scattered flurries are possible in the north of the region. On Saturday, mostly sunny conditions are expected with light NW winds in the alpine. Freezing levels are expected to be around 1500m in the morning and around 2300m in the afternoon. A mix of sun and cloud is expected for Sunday with light alpine winds.
Avalanche Summary
On Wednesday, a couple natural storm slab avalanches size 2-2.5 were reported from high elevation south facing terrain. Skiers and explosives triggered size 1-2 storm slab avalanches. On Tuesday, a natural cornice failure triggered a size 2 slab avalanche on the slope below. Three size 1-1.5 skier triggered avalanches were also reported. Two of these were triggered remotely with the furthest being triggered from 100m away. Several remotely triggered avalanches have been reported recently as well as whumphing and wide propagations. Over the weekend, natural, human-triggered, and explosive-triggered storm slab avalanches up to size 2.5 were ongoing. Solar triggered slab avalanches, cornice releases, and loose wet sluffing are all expected to have occurred on Thursday with the big warm-up. On Friday, the same types of solar related activity are expected. If a supportive crust forms Thursday night, stability will begin to improve. Human-triggering remains a major concern at higher elevations, especially north facing terrain where the storm slab has been unaffected by the sun.
Snowpack Summary
Warm temperatures and strong solar are rapidly changing the upper snowpack. A moist snow surface is expected to exist to well over 2000m elevation on north aspects and to ridgetop on solar aspects. Overnight refreeze on Thursday night is expected to form a widespread surface crust which is expected to be supportive and capping at lower elevations. Down 20-60cm is a weak layer that was buried last Friday and has been very reactive recently. This weak layer typically consists of surface hoar and facets overlying a melt-freeze crust that exists everywhere except high elevation north-facing terrain. In exposed alpine terrain, recent strong SW winds had formed wind slabs in leeward features. Large cornices exist in the alpine and may become weak with daytime warming. There are three dormant persistent weak layers that we are continuing to track. The late-March crust is down 50-70cm and was reactive last week during the warm period. The mid-March and mid-February layers are typically down between 70 and 100cm and have been dormant for several weeks. These layers have the potential to wake up with sustained warming, a significant rain event, and/or a big cornice fall.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.