Warming temperatures have settled the recent storm snow into a cohesive slab sitting on weak facets (sugary snow) that is ripe for human triggers; intense solar radiation will increase the likelihood of natural and human triggered avalanches.
Weather Forecast
SATURDAY NIGHT: Mostly clear / Light, northwesterly winds / Alpine low -5 / Freezing level valley bottom.NOTE: Saturday night is the last night of the forecast period that freezing levels are expected to drop at night. This will significantly increase the impact of day time warming on the snowpack.SUNDAY: Sunny / Light, northerly winds / Alpine high 0 / Freezing level rising to 2100 m.MONDAY: Sunny / Light, northeasterly winds / Alpine high 5 / Freezing level rising to 2600 m.TUESDAY: Sunny / Light, southeasterly winds / Alpine high 7 / Freezing level rising to 2800 m.
Avalanche Summary
On Friday, numerous natural loose wet and persistent slab avalanches up to size 2 were reported on solar aspects at all elevations. A skier triggered, persistent slab, size 2 avalanche was reported on a south aspect at 2000 m. Human triggered persistent slab avalanche activity is expected to increase with the forecast sunshine and rising freezing levels.On Thursday, two rider triggered persistent slab avalanches up to size 2 were reported on northerly aspects at treeline and below treeline. Additionally, a cornice failure triggered a size 2.5 persistent slab on a northeast facing treeline ridgetop.On Wednesday, numerous natural storm slab avalanches up to size 3 and rider triggered up to size 2 were reported on primarily solar aspects at treeline and above. Some of the avalanches were triggered remotely from lower angle terrain by skiers 50 m. away from the slope that avalanched. This suggests that recent storm snow is still very sensitive to human triggers in specific locations.
Snowpack Summary
There is 30-60 cm. of recent storm snow is sitting on a pile of facets (sugary snow), as well as a crust on sun exposed slopes. The recent storm snow has settled into a cohesive slab and is ripe for human triggering. This persistent slab is currently our primary concern.At lower elevations below treeline, a weak layer of surface hoar (weak, feathery crystals) buried in mid-January can be found approximately 60-120 cm. deep and may be combined with a crust on south facing slopes. This layer has recently been unreactive, however, the forecast warming event may awaken this layer, resulting in large, destructive avalanches. Steep cutblocks and large open glades at lower elevations are the most likely places to trigger this layer.The lower snowpack is generally considered to be strong, except for shallow, rocky areas in the alpine.
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.
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.
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.