New snow and wind on Tuesday will add to an already complex and tricky snowpack. Choose conservative terrain and minimize your exposure to overhead hazard. Large avalanches may run long distances.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
TUESDAY: Snow. Accumulation 10-15 cm. Ridge wind moderate to strong, southwest. Temperature -5. Freezing level 800 m.WEDNESDAY: Mostly cloudy, light flurries. Accumulation trace. Ridge wind light, west. Temperature -8. Freezing level 500 m.THURSDAY: Mix of sun and cloud. Ridge wind light, southwest. Temperature -10. Freezing level valley bottom.
Avalanche Summary
On Sunday a few small natural (size 1-1.5) storm slab releases were observed on northerly aspects above 2000 m.A very large, widespread avalanche cycle was observed on Thursday, with a few events continuing on Friday. Slopes of all aspects and elevations ran full path and reached valley floor. In some cases, mature timber was smashed. These failed on all of the persistent weak layers discussed in the snowpack section. Avalanches involving only the recent storm snow were also very large. Explosives continued to produce large persistent slabs on Saturday, and a few wind-triggered slabs were also reported. The natural cycle seems to have diminished however, human-triggering remains a real concern.
Snowpack Summary
Winds have shifted 30-40 cm recent storm snow into wind slabs on lee slopes. Alpine and open treeline areas have seen variable wind-affect while a sun crust exists on solar aspects, and a rain crust is present below 1600 m.Below the snow surface several persistent weak layers make up a troublesome snowpack. In the top 1.5-2.5 m of the snowpack, two surface hoar/ crust layers buried in January can be found. Expect to find one or the other of these on all aspects and elevations.Deeper in the snowpack (around 200 cm deep) is a facet/crust/surface hoar layer from December, most prevalent at and below treeline.Near the base of the snowpack is a crust/facet combo, most likely to be triggered from thin spots in the alpine.All of these layers have produced large avalanches recently. The wide distribution and ongoing reactivity of these layers suggests that avoidance through choosing simple terrain is the best strategy.
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.