A push of cold arctic air and light snowfall will be forming new slabs over several active snowpack layers on Friday. This weather pattern brings low elevation winds and reverse loading, so expect new slabs to form in counterintuitive places.
Light snowfall with shifting and increasing winds are expected to form touchy new wind slabs over the day on Friday, with lower elevations potentially seeing the greatest wind effect. Wind loading and any resulting surface avalanches will test recently buried storm interfaces and a deeper persistent weak layer, bringing potential for step-down avalanche activity.
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.
Thursday night: Mainly cloudy with possible clear periods. Light northwest winds shifting northeast and increasing.
Friday: Cloudy with flurries bringing 10-15 cm of low density new snow, continuing overnight. Moderate to strong east winds increasing over the day. Alpine temperatures dropping to about -15.
Saturday: Mainly cloudy with isolated flurries and a further trace of low density new snow. New snow totals of 15-20 cm. Moderate northeast winds easing over the day. Alpine high temperatures around -18.
Sunday: Sunny. Light northeast winds. Alpine high temperatures around -8.
Many more storm slab and wind slabs were triggered with ski cutting, explosives and with skier traffic in the region on Wednesday, with no new natural releases reported. Sizes generally ranged from 1-2 with crown fractures from 15-50 cm deep, again suggesting both wind redistribution and the potential for multiple storm slab failure planes in the upper snowpack. Only one small release was observed running on the February 22 surface hoar layer described in our Snowpack Summary. Northeast through northwest aspects were well represented in reports.
Numerous small avalanches were triggered naturally and with skier traffic during the peak of Tuesday's storm, with crown depths between 5-30 cm deep.
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 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.
10-20 cm of new snow accumulated in the region during Tuesday's storm. The new snow buried a thin new weak layer of surface hoar on shaded aspects as well as new sun crusts on more sun-exposed slopes.
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. In some cases, the interface below this older storm snow may act as a secondary failure plane in the upper snowpack.
Collectively, these recent snowfalls have been loading a layer of feathery surface hoar crystals from late February that is now found 50 to 80 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.
