The snowpack in our region is becoming complex, with new snow and wind creating fresh slabs, and increasing the load over both recent and deeply buried weak layers. Avoid limiting your concerns to the depth of new snow while multiple layers are in play.
Around 30 to 60 cm of snow overlies a weak layer of surface hoar. This layer has been most problematic on northwest to northeast aspects between 1600 m and 2300 m, but it may exist on all aspects and at all elevations. Assess the layer prior to committing to avalanche terrain or adopt a conservative mindset to avoid the problem.
Up to 15 cm of new snow is expected to accumulate above an array of surface hoar and sun crust throughout the region by Wednesday morning, with possible enhanced accumulations in localized areas. Aside from forming a thin new storm slab problem, the new snow adds to the load overlying another active storm interface as well as that over a more deeply buried persistent weak layer.
Tuesday night: Cloudy with continuing flurries bringing 5-10 cm of new snow. Moderate to strong southwest or west winds.
Wednesday: Decreasing cloud with final flurries leaving up to 5 cm of new snow. Light to moderate northwest winds. Alpine high temperatures around -9.
Thursday: Mainly sunny. Light west winds. Alpine high temperatures around -9.
Friday: Cloudy with flurries bringing about 5 cm of new snow. Moderate east winds. Alpine temperatures dropping to about -15.
Two large (size 2 and 2.5) persistent slab avalanches were triggered by a skier and intentionally with a large snow machine on Monday. Both ran on the February surface hoar layer described in our Snowpack Summary and occurred on north and northeast aspects around treeline and below.
Over the past two weeks, small to large (size 1 to 2) avalanches have released on this surface hoar layer. They have most often released between 1800 and 2300 m on northwest to east aspects. This trend is suspected to continue as this layer slowly gains strength.
Reports from Saturday and Sunday showed numerous natural, skier triggered and explosives controlled storm slab avalanches reaching size 1 to 2 on all aspects between 1900 and 2100 m. A gradual trend toward artificial triggers is evident in reports, as well as improving visibility allowing observation of earlier releases. These avalanches were generally limited to the depth of new snow, reaching up to 40 cm deep.
10-20 cm of new snow is expected to accumulate in the region by Wednesday morning. The new snow has buried a thin new weak layer of surface hoar on shaded aspects as well as new sun crusts on more sun-exposed slopes. It is unlikely to bond well with either of these surfaces over the near term.
The new snow adds to 10-15 cm of snow in the west of the region and 20 to 35 cm in the east of the region from the past weekend that has been gradually settling into a storm slab. As a result, both the new snow interface and this deeper storm slab interface may now be active failure planes for storm slab and wind slab releases.
Collectively, these recent snowfalls have been loading a layer of feathery surface hoar crystals from late February that is currently found 30 to 60 cm deep. This layer is most commonly found in open trees (e.g. in this MIN) or sheltered terrain around the upper part of below treeline, treeline, and lower alpine elevations. Check out the latest forecaster blog that offers a deeper dive into these conditions.
The remainder of the snowpack is strong and well-settled.
