Substantial warming and sun exposure on Tuesday will destabilize the upper snowpack. Extra caution is required around and below cornices, in recently wind loaded areas, and on steep sun exposed slopes.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
A blocking ridge of high pressure is the dominant feature for the week. Dry and clear conditions are expected for Tuesday through Thursday. Freezing levels are expected to climb to as high as 3000m on Tuesday afternoon and should be above 3000m on both Wednesday and Thursday afternoon, with cooling during the overnight periods. Strong outflow winds are expected on Tuesday in exposed terrain. Light to moderate outflow winds are forecast for Wednesday and Thursday is forecast to be calm.
Avalanche Summary
On Sunday, a natural size 1 storm slab was reported from a north aspect 40 degree slope at 2000m elevation. Skiers were triggering size 1 soft slabs in wind loaded northeast aspect slopes at 1900-2000m. Explosives triggered storm slabs up to size 2. On Saturday, a natural cornice failure triggered a size 2 storm slab on a northeast aspect at 2000m. Another natural cornice failure triggered a size 2.5 storm slab on a northwest aspect at 2100m. Explosives also triggered several size 2 cornice releases. One of these cornice releases triggered a 15-40cm thick storm slab on the slope below. Loose wet sluffing was also reported on steep south facing slopes. As things heat up on Tuesday, natural cornice releases and solar triggered sluffing are both expected. Cornices have the potential to trigger lingering storm slabs on the slopes below, especially on north aspects. Wet slabs may be possible on steep south facing terrain.
Snowpack Summary
Up to 15cm of new snow accumulated over the weekend in sheltered areas. Over the weekend, southerly winds were loading north aspect leeward slopes. However, outflow winds on Monday are reported to be loading south aspects in the alpine while exposed north aspect slopes are being wind pressed and scoured. Large cornices are lingering and are expected to become very weak this week. Below the surface problems, the mid snowpack is generally well-settled throughout with no notable persistent weaknesses. On Monday, a sun crust is expected to have formed on south aspects. Melt-freeze conditions are expected to be widespread this week with very high freezing levels.
Problems
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.
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.
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.