Confidence
Fair - Freezing levels are uncertain on Monday
Weather Forecast
High cloud moved across the region on Sunday afternoon that limited the extent of solar warming, but trapped warm air beneath the cloud layer that raised the alpine temperatures to near zero. The freezing level is expected to drop down to near valley bottom by Monday morning. Unsettled weather producing cloud and variable southerly winds during the day Monday. The wind is expected to increase Monday evening and become strong from the southwest by Tuesday evening. The freezing level is forecast to rise to about 1400 metres on Monday and only drop down to about 1200 metres Tuesday morning. Precipitation is expected to begin Tuesday afternoon and become heavy overnight into Wednesday morning.
Avalanche Summary
Some snowballing and sluffing on solar aspects in the afternoon on Saturday. One report of a remotely triggered large avalanche on a north aspect of Mt Tupper in Glacier National Park. The group of ski tourers had a large settlement on a ridge, and the adjacent slope failed down about 150 cm. Large remotely triggered avalanches have become less frequent, but they are occurring about every second day.
Snowpack Summary
Wind slabs in the alpine and at treeline are becoming less reactive. A melt-freeze crust exists from valley floor into the alpine on solar aspects. A key concern is a persistent weak layer of buried surface hoar from mid-February that is buried 1-2 m deep. Snowpack tests give sudden "pops" results on this layer in some areas, indicating the ongoing potential for very large avalanches. Because the layer is so deeply buried, it's unlikely to fail without a large trigger (e.g. cornice fall or explosive). However, there's always the chance of someone stumbling across a sweet spot, particularly in shallow snowpack areas. Solar warming could also destabilize the overlying snowpack, with the potential for step-downs or cornice triggering on this layer.
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.
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.
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.