Fresh storm snow will be touchy in wind affected terrain on Friday, while another wet storm is lined up to increase the danger this weekend.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Saturday
Weather Forecast
THURSDAY NIGHT: 10-15 cm of new snow with strong southwest winds.FRIDAY: Isolated flurries before the next storm arrives Friday evening, strong southwest winds, alpine temperatures around -8 C.SATURDAY: Storm starts Friday night and delivers about 30 cm, strong southwest winds, freezing level around 1000 m with alpine temperatures up to -5 C.SUNDAY: Clearing skies following the storm as alpine temperatures drop to around -10 C, moderate west winds.
Avalanche Summary
On Wednesday, evidence of a recent widespread natural avalanche cycle was reported, including numerous size 2-3 storm slabs and cornice failures. A few avalanches likely stepped down to deeper weak layers including the mid-February crust. Initial reports of explosive avalanche control on Thursday include up to size 2 cornice releases and several size 1.5 storm slabs. Ski cutting also produced smaller size 1 slabs. These avalanches failed at the interface between the new snow and rain crust and were up to 50 cm deep on wind loaded features.With more snow and wind in the forecast, expect fresh storms slabs to be reactive to human triggers on Friday. It has become difficult to trigger the mid-February crust/facet layer but there is still a chance that small storm slab avalanches or cornices could step down and release a persistent slab avalanche.
Snowpack Summary
By Friday morning expect 20-30 cm of recent snow sitting above a widespread layer of wet snow up to about 2200 m. The wet snow layer is likely turning into a crust and may become a weak layers as storm snow accumulates. Strong south winds are likely forming thicker slabs in the lee of exposed terrain. The mid-February crust/facet layer is now typically down 80-120 cm and in wind loaded terrain, it may be as deep as 200 cm. With a thick layer of weak faceted snow above it, this crust layer was producing easy, sudden results in snowpack tests prior to the warming event. The layer is expected to gain strength with the cooler temperatures but there is a lot of uncertainty as to how long this layer will remain reactive.
Problems
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.
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.