If more than 20 cm of new snow arrives by Thursday morning expect fresh and reactive storm slabs. If you see less snow, wind slabs will remain the primary concern.
Triggering a persistent slab avalanche is most likely if a surface crust is not present at treeline or below.
New snow will be redistributed by northwest and westerly winds to create reactive wind slab in the alpine and treeline.
Dry loose sloughing may be seen from steeper slopes, especially where a firm crust exists below the new snow.
A weak layer formed in late January may be found 40 to 80 cm deep in the snowpack. This layer consists of faceted snow, a sun crust on steep solar aspect, and most notably large surface hoar crystals in sheltered terrain.
This surface hoar layer has been the culprit of several large human-triggered avalanches in the past week and remains a concern, especially in sheltered areas at treeline. Terrain features to be particularly cautious around include the lee side of ridges, openings in trees, cut blocks, and burns, especially on Northerly aspects where the layer may not be capped by a firm crust.
An unsettled weather pattern will bring light to moderate snowfall amounts and strong ridgetop wind through the weekend.
Wednesday Overnight: New snow 5-10 cm. Freezing level dropping to valley bottom. Alpine temperatures -10 and strong West to northwesterly winds.
Thursday: Snow amounts 5-10 cm. Freezing level rising to 1300 m. Alpine temperature near -5 with strong northwesterly wind.
Friday: Trace of snow. Freezing level rising to 1300 m and alpine temperatures near -5. Strong West wind.
Saturday: Snow 5-15 cm. Freezing level 1100 m and alpine temperature near -8. Moderate southwest wind.
If you see more than 20 cm of fresh snow expect to see some reactive storm slabs and deeper wind slabs on leeward slopes and terrain features.
No new avalanche activity was reported on Tuesday.
On Monday, a few persistent slab avalanches were reported up to size 3 from various aspects 1900 m to 2300 m. These slabs looked to be 24 hours old.
Last week, near-daily reports came in of human-triggered persistent slab avalanches on the January 30th weak layer, with the associated surface hoar layer taking both recreationists and professionals by surprise. Reactivity has begun to taper but this buried weak layer should not be taken lightly as it may now present a low-probability, high-consequence problem. The most reactivity on this layer has been seen between 1600-2200 m in open areas in the trees, in cut blocks and on steep convexities.
5 cm to 20 cm of new snow fell by Wednesday morning and is expected to bond poorly to the old snow surface. This old surface is comprised of near-surface faceting with some true North aspects holding soft "ish" snow down to 1500 m. Other areas have surface hoar growth overlying a melt-freeze crust at lower elevations, a sun-crust on steep solar aspects, and hard wind-affected snow in the alpine and exposed treeline.
Several weak layers exist in the upper snowpack including buried crusts, facets, and a prominent weak layer of buried surface hoar. This surface hoar has been the dominating feature in the past few weeks. It was buried in late January and exists 30 to 80 cm deep in the snowpack. The surface hoar is most prominent in sheltered openings at and below treeline.
The facet/crust layer that formed in early December is now buried around 120 to 250 cm. The last reported avalanche was on January 31 from a large explosive, and before that on January 23. Although unlikely, the layer could be triggered from a large load like a cornice fall or a shallower slab avalanche could step down to this layer.
