The Bottom Line: Our snowpack was rocked this week with 5 inches of new precipitation (3-5 feet of new snow in the high alpine), with strong drifting and scouring from the SE. The rain line reached up to 4,000ft Wednesday, but temperatures are dropping and the snow surface is settling and freezing. You are likely to encounter recent wind slabs in the alpine, and they may be sensitive in steep, wind loaded areas. Look for recent avalanches, try to assess how deep they ran, and be sure to avoid similar slopes.

Never expose more than one person at a time to slopes 30-degrees and steeper. Don’t enter avalanche terrain without a plan for how to manage an avalanche that could break far wider and larger than you expect. If a surface wind slab were to let loose, it could possibly step down to deeper weak layers.

We should remain aware that very deep and wide destructive avalanches need to be considered and planned for accordingly. Pay attention to run-out paths and leave wide margins of safety for larger than expected slides. Terrain management is critical. An avalanche in the upper snowpack or cornice failures could step down and lead to a massive avalanche.

Think about the connectedness of a hard slab, from where you are standing. If something is triggered on a steep slope or in a thin area nearby, does that slab connect to you, or above you? What about the rest of your group?

Old Melt Layers and Crusts:

• A buried persistent weak layer about 6+ feet deep (The “Big Warmup” Layer, Formed Nov 17th) is still a concern.
• It is a thick refrozen crust with weak faceted snow above. This is an unusual and unpredictable weak layer because it formed during a thaw event and then re-froze while cold temperatures faceted the stout crystals.
• A wide safety margin is necessary. This setup could produce avalanches that break wider than expected, and are most likely to be triggered from shallow trigger points like rocks or small trees. You could ride the same slope numerous times until that right spot produces a large destructive slide.

Photos of the “Big Warmup” facet layer in the Transitional Zone at 2500ft on 1/2 with test results ECTP18 that demonstrated propagation potential on this layer. And a photo from Old Faithful on 1/12 of a 5″ thick layer of facets down 170cm.

Observations in all zones throughout December and now into January have found moderate strength and quick propagation on failures within the “Big Warmup” crust/facet layer. We know it to exist at all elevations above 1000ft — even up to the highest summits. We expect this layer (and other midpack crust-facet layers) to be an ongoing issue going forward.

Depth Hoar at the Ground (Above 3,000′):

• October snow followed by long cold snaps created depth hoar at the ground in most areas. These angular snow grains are well developed into a weak layer that behaves like brittle glass.
• As the snow gets deeper, the load over this layer increases, and it could collapse. You are most likely to human-trigger this layer from shallow spots around rocks, or trees, or have a surface avalanche step down to the ground on this layer. Any slides that break to the ground are likely to be deadly.

Photos shows 6-9mm advanced depth hoar at the ground down 180cm with ECTP 25 down 80cm on facets below a crust in the Haines Pass Zone at 4,000ft on a NE-aspect.

Below 3000ft:

Lingering water in the snowpack may keep wet slides possible in middle elevations today. You can avoid this danger by sticking to slopes less than 30 degrees, and staying out of gullies and terrain traps that funnel snow from above. Steep slopes (40 degrees and steeper) will be especially prone to large wet loose or wet slabs. This danger will linger until temperatures drop enough to freeze the upper snowpack in place.