The Bottom Line: It has been cold and dry for most of the last 6 weeks (In fact, our least-snowy January on record). The cold weather has increased facetting in various layers in the upper 60cm of snowpack. We're getting reports in every zone of a series of wind slabs or crusts in the upper snowpack, with 2-4cm of weak suggary facets in between. We have a new buried surface hoar layer to worry about, and the usual surface wind slabs on high, wind loaded slopes.
A shot of new snow (3-6") fell Monday, with SE winds turning northerly again. In wind-loaded areas the new snow will be enough to cause increased danger. Any new wind slabs will be poorly bonded, likely sitting on widespread areas of buried surface hoar. Be cautious out there, and perform tests on the upper snowpack to look for easy shears. Expect high variability from slope to slope, and avoid convexities. Practice good risk management: limit your exposure to dangerous terrain, and plan for the worst.
Problem #1: Wind Slab
Location: Wind-loaded and cross-loaded slopes (E-SE-S-SW aspects) steeper than 30 degrees.
Strong NW winds over the last two weeks caused loading of wind slabs onto lee aspects. Because the winds were 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 tend to collect wind slabs and act as trigger points. Some of these slabs could be 30-60cm thick. They will be sliding on old weak/facetted snow and patches of surface hoar.
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 over the last 2-3 weeks and wasn't blown away by NW winds.
A new layer of surface hoar that formed over the last week is now buried under Monday's new snow. There are also areas of buried surface hoar lingering three layers down (roughly 20-60cm 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. These dangerous weak layers will persist for several weeks until they can be crushed and flushed out by lots of heavy snowfall. Be sure to dig around in wind sheltered areas to look for "the thin grey line" and clean shears. Assume this weak layer to be present in wind-sheltered areas. Use extra caution in openings around treeline, and avoid wind-sheltered rollovers in the alpine.
Reports from this weekend found more isolated recent surface slabs (D1-D2) in steep, wind loaded terrain (S-SE-E aspects). Also notable was whumphing in areas of thin snowpack in the alpine areas of the transitional zone.
Over the last two-three weeks, we've had reports of isolated surface wind slab avalanches (D1-D2), both natural and human-triggered, in top-loaded and cross-loaded terrain 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.
After 6 weeks of cold and mostly dry weather, we're finally back into a snowier pattern. Light snow will taper off Tuesday. We'll get a break Wednesday before a stronger front comes in Thursday. It looks like 5-10" is likely with this storm. The weekend is looking great for the Kat to Koot race: clear and chilly with moderate north winds.
( *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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