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 surfaces.  Human triggering of these fresh 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.

Because the new snow is relatively thin so far, we don’t expect many large slabs to have developed. But 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 bigger factor than the slab itself. That said, 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: Isolated to Specific.

The snowpack above the MLK rain crust (about 45cm deep) has been punished by wind and cold. This leads to weakening over time and fresh wind slabs that can cause surprises. Due to slow, steady new loading from this weekend’s new snowfall, the danger of triggering these mid-pack weak layers is increasing.

Strong northerly outflow raked higher elevations the past two weeks, and deposited wind slabs on E-SE-S-SW-W aspects.  Specific terrain features lower down on slopes and below treeline are areas that wind slab, or recent deposits, may have built over persistent weak layers. These sheltered, or protected areas may be tempting for riders looking for soft surface conditions, but where triggering a persistent slab is the most possible.

Keep an eye out for pregnant or wind loaded slopes, 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)