The snowpack is complex and quite variable right now. Buried weak layers remain reactive and conservative terrain selection remains critical.Extra caution is required during the heat of the day, especially on sun exposed slopes and around cornices.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
A mix of sun and cloud is expected for Friday with moderate southwest winds in the alpine. A temperature inversion is expected with a layer of above-freezing air sitting around 2000m elevation and colder temperatures in the valleys. A weak storm pulse is expected to bring around 5cm to the region on Saturday morning. Alpine winds are expected to be moderate from the southwest on Saturday with freezing levels around 1800m. Mostly dry conditions are expected Saturday afternoon through Sunday morning before a second weak storm pulse arrives Sunday afternoon.
Avalanche Summary
On Wednesday, a natural size 3.5 slab avalanche was reported from an east aspect at 2700m which released down 100cm and was 250m wide. Three natural cornice releases size 1.5-2 were reported in the high alpine. Numerous size 1.5-2 natural loose avalanches were reported from south aspects in the alpine. Explosives triggered three slab avalanches on northern aspects which were 10-30cm thick. On Tuesday, a natural size 2 slab avalanche was reported from a gulley side wall feature on an east aspect at 2000m which released down 80cm. Also reported on Tuesday were skier triggered wind slabs, natural storm slabs, loose dry sluffing from steep sunny slopes, and large cornice releases. There is a lot of uncertainty for how reactive the snowpack will continue be on Friday with the continued warm temperatures and sun. I expect that the buried weak layer from early February will continue to produce large destructive avalanches with heavy triggers such as cornice falls or smaller avalanches stepping down, but it may also still be possible to directly trigger this layer in some areas. Sluffing can be expected from steep sun exposed slopes and cornices will become weak with warming and sun.
Snowpack Summary
A sun crust exists on south facing slopes at all elevations. Thin wind slabs are being reported in leeward features in the alpine and large cornices remain a concern, especially during the heat of the day. The surface hoar and/or crust layer from early February is now down 40-70cm and has been responsible for some large avalanches recently. This layer is expected to become most reactive during the heat of the day, especially on sun exposed slopes. The early January surface hoar/ facet layer is typically down 70-120cm. Triggering an avalanche on this layer has become unlikely but it still has isolated potential to produce very large avalanches with a heavy trigger. In general, the lower snowpack is well settled and strong, apart from some thin snowpack areas where basal facets exist.
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.
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.
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.