Problem #1: Wind Slab
Location: E-SE-S-SW aspects, and cross-loaded slopes on other aspects, at and above treeline. Strong winds out of the Northwest over the last several days have loaded lee aspects with wind slabs of varying thickness and reactivity. Any recent wind slabs will be cold and bonding may be relatively poor. Wind slab can be identified by dense, cohesive snow, cracking, and a hollow or drum like feel. Look for wind slab in top and cross loaded terrain like below ridge lines and along gullies. These avalanches could lure riders well onto the slab before failing and could be surprisingly large. Stick to wind-protected areas where the surface snow is still soft and fluffy.
Problem #2: Persistent Slab
Beneath the copious amount of new snow that fell this week lurks a layer of facets sitting above an old rain crust. This weak layer is between 80-120cm deep depending on wind loading. While triggering this deep weak layer is not likely, it will be possible, especially from areas of shallow snowpack near ridgelines and rocks. Propagation along this weak layer could be quite wide, leading to large and deadly avalanches. Heavy triggers like snowmachines and hard landings from skiers will be most likely to trigger this deep weak layer. Use extra caution until this layer has some more time to settle and bond. Make sure to keep wide spacing in your group and anticipate the worst case scenario of an avalanche breaking wider than expected.
We received some recent reports of fresh natural and human triggered wind slab avalanches to size 2 or 3, over the last few days. These were occurring on slopes that are lee to the northwest winds that blew over the last several days, where active wind loading was occurring.
Above: Human-Triggered D1.5 hard slab avalanche on the Chilkat Pass, from 12-23-2018. E aspect, crown was about 1 foot deep.
Isolated D2-D3 naturals ran during the storm last week, on wind loaded and cross loaded aspects. Crown depths were around 1 meter, running on facets above a rain crust.
Natural D3 avalanche from the big storm last week. Chilkat Pass zone, cross-loaded NE aspect at ~4500ft. 2018-12-21
A quick but strong burst of snowfall is expected Thursday, with increasing southerly winds and snow levels rising to near 1000ft by evening. 3-6" of new snow is expected. Friday will bring cooling temps and an end to the snow. Another shot of light snow is likely Saturday evening. Sunday-Tuesday will bring in a pattern change, with a strong and wet storm that could bring heavy accumulations and rising snow levels.
( *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. Come to our FREE backcountry skills workshop on January 10th (see flyer below).
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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