Avalanche danger will increase to CONSIDERABLE today and eventually to HIGH danger late this afternoon or evening near and above treeline on NW-N-NE-E-SE aspects on slopes 35 degrees and steeper. For all other areas avalanche danger is MODERATE on slopes 35 degrees and steeper.
Following the onset of high intensity snowfall today, new wind slabs will rapidly build near treeline and above treeline on NW-N-NE-E-SE aspects. Human triggered avalanches will become possible to likely today with natural avalanches becoming possible by late this afternoon or this evening. New wind slabs 2+ feet thick are expected to have formed by the late afternoon or evening hours.
Periods of high intensity snowfall hold good potential to create unstable storm slabs in below treeline areas on all aspects today. This avalanche problem will be highly dependent on what weak layers form within the storm snow as the day progresses. Human triggered shooting cracks in wind protected areas below treeline are an excellent indication of the presence of this avalanche problem.
Near the end of this storm cycle following the accumulation of 1 to 2 feet of new snow, unstable persistent slabs may form on NW-N-NE aspects above 8,000'. The weak layer of near crust facets that exists just below the melt-freeze crust at the base of the recent storm snow has the potential to become over loaded by new snowfall. This weak layer has been observed in many locations around the forecast area and is most well developed in near treeline and below treeline areas above 8,000' on NW-N-NE aspects. Any avalanches that fail on this weak layer will be large, involving all of the snow accumulated since March 25th.
Observations made yesterday on Andesite Ridge (Donner Summit area) and on Powderhouse Peak (Luther Pass area) both gave some indications of a potential developing persistent slab avalanche problem on NW-N-NE aspects above 8,000'. Near crust facets just below the melt-freeze crust at the base of the recent storm snow are a weak layer that in some areas have shown the ability to propagate collapse in formal snowpit tests. This weak layer can currently support the snow load above it, but that may change as significant new snow accumulation occurs over the next few storm cycles.
On Andesite Ridge, wind slabs that had formed near and above treeline over the past few days appeared to have gained strength with no signs of instability observed.
Overall, the snowpack below 8,000' is in good condition to handle new snow loading. The snowpack above 8,000' on NW-N-NE aspects is in poor conditions to handle new snow loading due to the presence of near crust facets that has been observed as a weak layer in some areas.
