Rapid warming is expected in the alpine, while persistent slab problems exist at treeline and below. Safe travel requires careful route selection. Steep, open or sparsely treed slopes are best avoided at this time.
Weather Forecast
Tuesday: Cold air in the valleys, warmer air up high. An above freezing layer is expected from approximately 2000-2500 m. Some valley cloud is possible, clear at upper elevations. Light westerly winds.Wednesday: basically a repeat of Tuesday, possibly slightly warmer.Thursday: Cloud developing. Lower elevations are finally expected to start to warm, although the above freezing layer is expected to remain in place. Light southwesterly winds.
Avalanche Summary
There have been a several recent reports of remote-triggered (triggered from a distance) persistent slab avalanches to size 2 on west, south and northerly aspects between 1400-1800m. Reactivity on the persistent slab is likely to increase with warmer temperatures as the upper snow becomes denser.
Snowpack Summary
30-50cm of low density new snow from last week has buried a layer of weak feathery surface hoar and/or sugary facets. This new snow is expected to start to consolidate into a slab once forecast warmer temperatures take effect, but so far the upper snow has been remained light and soft. Of greater concern is a weak layer buried mid-December that consists of surface hoar, a sun crust and/or sugary facets, and is now down approximately 50-70cm.The bond at this interface is highly variable. The most concerning areas are those that saw pronounced surface hoar development before mid-December, such as sheltered areas at and below treeline, sheltered northerly aspects in the alpine, or anywhere the surface hoar formed on top of a sun crust. Recent snowpack tests show wide ranging reactivity on this weak layer but suggest that it may be most reactive on northerly aspects. A rain crust that formed in late November is now buried 80-120cm deep. Treeline snow depths are in the region of 200 cm.
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.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.