Problem #1: Wind Slab
For Thursday-Sunday: Human-triggered avalanches are likely on wind-loaded slopes (E-SE-S-SW aspects) steeper than 28 degrees.
Strong NW winds of 30-50mph are stripping recent snowfall from windward aspects and heavily loading wind slabs onto lee aspects. Because the winds are so strong, loading patterns are likely to be unusual, so look out for wind slabs that get thicker/sketchier mid-slope, further down below ridgelines than usual. Convexities will tend to collect wind slabs and act as easy trigger points. Some of these slabs could be 30-60cm thick. They will be sliding on the new/old snow interface where weak/facetted snow and patches of surface hoar are lingering.
Problem #2: Persistent Slab
Location: ALL elevations, clearings in the trees, and specific slopes above treeline (mainly slopes sheltered from NW winds) where surface hoar formed last week and wasn't blown away by NW winds.
There are patches of buried surface hoar lingering underneath last week's layer of new snow (roughly 15-45cm deep). We observed areas of natural wind slab avalanches running in low-angle terrain as mellow as 25-degrees, indicating how slick this weak layer is. It's distribution is sporadic and depends on how sheltered a slope was to last week's NW winds. This dangerous weak layer will persist for several weeks or until it can be crushed and flushed out by lots of heavy snowfall. Keep this layer in mind over the next few weeks, and be sure to dig around in wind sheltered areas to look for it. Surface hoar is sneaky because it's hard to map out, but easy to trigger. Use extra caution in openings around treeline, and avoid wind-sheltered rollovers in the alpine.
Photo: human-triggered persistent slab on low-angle terrain in the Chilkat Pass zone. 2019-1-27
Observations from the last week found areas of D1-D2 natural windslab and storm slab avalanches on specific terrain that was wind loaded or cross-loaded, between 2500-5000ft. Some wind slabs were sliding on low-angle slopes as low as 25 degrees, and appeared to be sliding on a buried surface hoar layer 10-30cm deep. Distribution of these avalanches was limited to wind loaded areas that had been protected from strong N/NW winds.
Clear skies, strong NW winds and frigid temperatures have hit the area, and should continue this weekend. Snow is being stripped from W-NW-N aspects and loaded onto leeward aspects.
( *star means meteorological estimate )
If you get out riding, please send in an observation!
Do a rescue practice with your partners. Always carry a beacon, shovel, and probe, and KNOW HOW TO USE THEM.
Practice good risk management, which means only expose one person at a time to slopes 30 degrees and steeper, make group communication and unanimous decision making a priority, and choose your terrain wisely: eliminating unnecessary exposure and planning out your safe zones and escape routes.
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