Avalanche Forecast

High -

SATURDAY: Early morning snowfall, accumulation 2-5 cm, then mostly cloudy with possible valley cloud, light to moderate westerly winds, alpine temperature near -2 C, freezing level near 1000 m.SUNDAY: Partly cloudy with valley cloud, light to moderate westerly winds, alpine temperature near 0 C, inversion conditions with cold valley air and above-freezing level between about 2000 m and 2500 m.MONDAY: Partly cloudy with valley cloud, light southerly winds, alpine temperature near 0 C, freezing level below valley bottom with inversion conditions.

On Friday, several storm slab and persistent slab avalanches were triggered with heavy triggers and remotely by a skier, generally at treeline and below treeline elevations with depths between 40 and 150 cm on northerly aspects.  On Tuesday to Thursday, a widespread avalanche cycle was observed. Numerous large avalanches (size 3 to 3.5) were triggered in the alpine and ran to near the valley floor and numerous size 2 avalanches started at and below treeline. Operators also reported numerous large human, vehicle, and remote-triggered avalanches.  Some of these avalanches ran on relatively shallow slope angles (around 30 degrees) and in fairly dense trees.  Expect continuing avalanche activity because of the touchy snowpack.

The snowpack reached its tipping point on Tuesday and Wednesday as warming and precipitation combined to overload buried persistent weak layers in the snowpack and create a large natural avalanche cycle.Over 100 cm of snow fell within the past week.  The snow fell relatively warm with moderate winds, which formed storm slabs.  The snow also formed large cornices.  This storm snow sits on variable feathery surface hoar in the region.  Deeper in the snowpack, an unstable weak layer from mid-December (predominantly feathery surface hoar crystals and/or a sun crust) is found at treeline and below treeline elevations.  Below, a rain crust that developed late-November with associated sugary facets are also being stressed. Snowpack test results show sudden fracture characters and high propagation potential for all of these buried layers, indicating that they can be triggered and could propagate into large, destructive avalanches. This has been the case, as shown in the Avalanche Summary.