Hazard is expected to slowly drop with the cooling over the weekend but avalanche problems are expected to linger for a while longer. Extra caution is required around and below cornices, and on steep sun exposed slopes during the heat of the day.
Weather Forecast
A mix of sun and cloud is expected for Saturday with freezing levels dropping to around 2000m and moderate southwesterly winds in the alpine. Mostly cloudy conditions are forecast for Sunday with light southwest winds in the alpine and afternoon freezing levels around 2000m. A storm system is forecast to reach the south coast on Sunday evening. 5-15mm of precipitation is currently expected by Monday evening. Freezing levels are forecast to start around 2000m at the beginning of the storm and drop to around 1500m by the end of the storm on Monday.
Avalanche Summary
On Thursday, a natural cornice release triggered a size 2.5 deep slab release in the Duffey area. A few size 2 wet slabs were also reported in the Duffey area. Lots of loose wet sluffing up to size 2 was observed. A size 2 deep persistent slab was also reported from a southwest aspect at 1900m elevation. On Wednesday, a skier triggered a size 1.5 avalanche on a south aspect at 2200m which was 10-40cm deep with the most recent snow sliding on a crust. Loose wet avalanches up to size 2.5 and cornice releases up to size 2.5 were also reported. Cooling temperatures on Saturday will mean that natural sluffing and cornice releases will become less likely, but with all the heat in the snowpack, it will still be possible for human-triggering of cornices and sluffing. It may still be possible for a cornice release to trigger a deep weakness in the north of the region. Once the snow surface develops a widespread supportive crust layer, it will become unlikely to trigger any deep weaknesses.
Snowpack Summary
A moist/wet snow surface is reported to be widespread to mountaintops except for shaded true north aspects above 2600m. The last few nights have seen some limited overnight crust formation which has quickly broken down the following morning. With freezing levels dropping on Saturday, a surface crust is expected to form at higher elevations and not completely break down during the day. Large cornices are lingering and are expected to remain weak until there is a substantial period of cooling. Below the snow surface, there is a layer of concern in the northern half of the region. Professionals are tracking a thick layer of facets down over 1m which likely resulted in a couple deep persistent avalanches which were reported on Thursday. Deep persistent weaknesses will remain a concern until there is a substantial period of cooling.
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.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.