Problem #1: Wind Slab
Location: All aspects and elevations, but especially top-loaded and cross-loaded terrain. Slopes 28-degrees and steeper.
5-12" of new snow (highest amounts in the Lutak zone) fell Thursday onto a weak upper snowpack, and strong NW winds on Friday built up fresh wind slabs on SW-S-SE-E aspects. These slabs will be poorly bonded, and tender to human triggers. Human-triggered avalanches will be likely, and avalanche terrain should still be avoided. Some of these slabs could be 30-60cm thick in wind loaded terrain. They will be sliding on the new/old snow interface where weak/facetted snow and patches of surface hoar lurk. Take care to avoid terrain traps and gullies where small slides can pile up deep. As strong sunshine hits the freshly-loaded storm snow on steep SE-S-SW aspects, natural avalanches will be possible as well. Be sure to stay away from solar aspects when the sun is hitting them hard.
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 this week'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, and widespread whumphing Tuesday (2/19) in the Lutak 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 (including our least-snowy January on record), we've received 6-18" of new snow in the last week. The weekend looks gorgeous, with decreasing north winds, cool temperatures, and lots of sunshine. Several days of nice weather are ahead for the week.
( *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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