Distribution: Specific — wind loaded slopes. Winds started out of the NW with this storm, and then turned SE, so there will be a lot variability with the loading patterns and any aspect could be wind loaded with new snow. Anywhere this new snow is building up, it will form a wind slab that will be sitting over some very weak old surfaces.  Human triggering of these fresh new wind slabs will be likely today on slopes 30 degrees and steeper. Continuing snowfall is slowly increasing the load on recent weak layers. We expect increasing avalanche danger through the weekend.

The potential danger of any given slope will depend on how big it is and how much new snow it is holding. The consequences of a slope (including terrain traps, cliffs, and trees) may be a big factor. In addition, there is the potential for surface slabs to step down to deeper persistent weak layers. See the Persistent Slab section below.

Examples of top loading and cross loading (click image to read more).

Confidence: High. Distribution: Specific to Widespread.

Weak old snow surface conditions will promote a bed surface for new snow to build up on and run over. These now buried persistent weak layers (PWL‘s) include surface hoar, near surface facets and melt-freeze crusts. In other areas, the snowpack above the MLK rain crust (about 45cm deep) has been scoured by wind and cold resulting in sastrugi and hard wind slab.  Due to slow, steady new loading from this weekend’s new snowfall and increasing temps, the danger of triggering these mid-pack weak layers is increasing.

Keep an eye out for pregnant, recently wind loaded slopes, or anywhere new snow has formed a cohesive slab, especially below terrain features (see photo in Avalanche Activity section below).  Avoid slopes greater than 30 degrees, dig and look for layers to make specific hazard assessments. Remember that variability is high right now, and your snowpit is just a point observation. Pole probing and other quick tests will help you map out wind slabs and potential weak layers under the snow.

 New Persistent Layers and Snowfall Timeline

1.  Feb. 3 – 10cm of very low density snowfall
2. Jan. 30-31 – Surface Hoar event, north winds Jan. 30 – Feb. 2
3. Jan 29 – 10cm of snowfall
4. Jan 26-28 – Surface Hoar event, north winds Jan. 26-27
5. Jan. 24 – 10cm of snowfall
6. Jan. 20-23 – Surface Hoar event, north winds Jan. 24

Old Persistent Layers

1. Jan. 18 – MLK Day Crust/Facets 45-65cm down in elevations up to ~3000′
2. Jan. 10 – Melt-freeze crust 10 down 160-180cm in elevations up to ~3000′
3.  Jan 31 – New-Years Surface Hoar and Near-Surface-Facets interface ~100-150cm down in protected areas near tree-line, steep openings and alpine bowls.

(Surface hoar near-treeline in the Transitional Zone 1/31. Once buried by wind slab or new snow these feathery crystals become a dangerous persistent weak layer that will require extremely careful assessment and overall immediate avoidance in avalanche terrain. Photo: Jeff Moskowitz)