Cooler conditions should slowly improve stability but tricky conditions are still expected up high. Large slab avalanches still have the potential to release on deeply buried weak layers. Use extra caution on sunny slopes during the afternoon sun.
Confidence
Fair - Due to the number of field observations
Weather Forecast
A ridge of high pressure brings a mix of sun and cloud for Thursday and Friday. Mainly dry conditions are expected with localized convective flurries possible. Freezing levels are forecast to reach around 1500m in the afternoon and drop to around valley bottom overnight. Alpine winds are expected to be light from the NW on Thursday and light from the SW on Friday. On Friday overnight and Saturday, a weak storm system should reach the interior. Light precipitation is expected with freezing levels reaching around 1500m and moderate alpine winds from the SW.
Avalanche Summary
On Tuesday, a natural cornice failure triggered a size 2 slab avalanche with a depth of 40cm. This occurred on a NE aspect at 2200m elevation. On Monday, several natural size 2-3 avalanches were reported. Details are limited and these may have occurred on Sunday or Monday. One of the size 3 avalanches released down 100cm, most likely on the mid-Feb persistent weak layer. At below-treeline elevations, avalanches were reported to be scrubbing down to ground. Ongoing persistent slab, storm slab, and loose wet avalanches have been occurring since last Friday. Activity is now expected to taper off with the cooler temperatures. Natural avalanches are generally not expected on Thursday except possibly on steep sun-exposed slopes in the afternoon. Human-triggered storm slabs and persistent slabs are still a concern, especially on steep, unsupported slopes in the alpine. Stability is generally expected to improve with the cooling trend but this may take a few days at higher elevations and tricky conditions may still exist on Thursday.
Snowpack Summary
The upper snowpack likely contains multiple new crust layers. The most prominent of these new crusts is a thick rain crust down 20-40cm that formed last Friday when it rained into the alpine. Another crust likely exists near the surface but may be buried under 5-15cm of dry snow that fell on Tuesday after the temperatures dropped. These near surface crust layers likely extend well into the alpine. In the high alpine, strong winds have redistributed the new snow and wind slabs can be expected in leeward terrain features. Several persistent weak layers exist in the upper 1m of the snowpack and remain a concern. The mid-March crust/facet layer is down around 60cm and has been reactive to light triggers recently. Down around 80-100cm is the mid-February crust/facet/surface hoar layer which has also been reactive to heavy loads like avalanches in motion stepping down, cornice failures, or explosives. Both layers have the potential for wide propagations and very large avalanches are still possible.
Problems
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.
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.