A remotely triggered wind slab under Chair Peak on 12/23. Observer Jeremy Allyn elected to forego the descent to Snow Lake. Photo by Kurt Hicks
This post focuses on wind slabs, a particularly tricky avalanche problem. Several parties have reported unintentionally triggering wind slabs in the region recently (and we commend those people for sharing their experiences on our observations page: check out reports from Silver Peak (12/23), Humpback Mountain (12/23), and the Cache couloir above Snow Lake (12/24). Wind slab was implicated in two fatalities in Washington last year, on Red Mountain near Snoqualmie Pass and in Silver basin near Crystal. We wanted to get people thinking about what wind slabs are and why they are so challenging.
Wind slabs, in concept, are deceptively simple. The National Avalanche Center defines a wind slab avalanche as a: “release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the windward side of terrain features and deposits snow on the lee side. Winds slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard.” It’s tempting to use this concept, combined with a knowledge of wind direction, to predict their location. But the statistics and recent events in our mountains confirm what savvy backcountry travelers know: it’s hard to predict where wind slabs will form, and easy to push it too far as we seek to confirm their presence.
This large wind slab was triggered by two snowboarders on December 5th on a west-facing slope near treeline on Panorama Point, not far from Paradise in Mount Rainier National Park. Hazard was forecast as Moderate in the area that day. Photo courtesy MRNP Climbing Ranger Seth Waterfall
The trap may be this: we’ve all experienced snow that has been stiffened by wind effects, but isn't a slab–it’s just a dense layer. And we tend to believe we should be able to tell one from the other on a slope scale. In his incident report after getting caught and carried by a wind slab last year, Ian Nicholson describes stepping onto some hard, hollow-sounding snow in an opening in the trees and investigating it with a hand pit. “Due to the conditions encountered, we had already been toning down our objectives for the day and had already ruled out our first two options because of much more widespread wind slab development than we had originally anticipated. We started to discuss turning around at that moment and I decided to push down on the edge of the skin track. I heard a crack and felt the slope begin to slide…” Even with a plan for avoidance, discerning wind effects from wind slab can be a dangerous business.
Below, we offer a review of some wind slab fundamentals. This isn’t a comprehensive primer or a replacement for experience, but we hope it will help readers grow their understanding.
A relatively small human triggered wind slab release near the bottom of Chair Peak's north slope on 12/23. The next day day, in the nearby Cache Couloir, skiers triggered a D2 hard slab, whose crown varied from about 1 to 3 feet in thickness. Photo by Kurt Hicks
Wind Slab Formation
- Wind slabs form from snow light enough to be moved by the wind. Take note of times when there is “snow available for transport.” In the PNW, these times not only occur during storm cycles, but also during periods of high pressure when loose snow remains intact on the surface. A deep layer of low density snow is ripe for transport and a period of wind slab problems will often persist for a week or more until there’s nothing left to blow around.
- Critical slab formation typically occurs during moderate-strong wind events. These are defined as periods with average wind speeds from 15-45 mph. Small trees begin to sway, then larger ones. As winds crank up during a storm, slabs can thicken rapidly, often at rates 10 times that of the prevailing rate of snowfall.
- Stronger winds can produce wind slabs much lower in the terrain than we might expect–they can form well below treeline.
- Following storms and during periods of high pressure, winds can shift from southerly to northerly (and frequently easterly in the passes), reversing our typical wind loading patterns. Easterly winds likely produced the wind slab involved in the fatal Granite Mountain accident in 2015.
Avoiding Wind Slabs
- Obvious signs that wind slabs are forming can often be observed from the parking lot or on the valley bottom: blowing snow on ridge tops or on the slopes; variability in snow surface texture; scouring and loading. This is a good time to check in with your partners on your travel plans.
- Practice avoidance, realizing that the difficulty of predicting wind slab location means giving yourself a bigger physical buffer; “threading the needle” is not a good idea with wind slab. To give yourself that buffer, rule out larger areas of terrain: whole elevation bands and aspects are a good place to start.
- Using the NWAC Aspect/Elevation diagram can be helpful, but it’s important to realize how dramatically terrain can influence wind direction and thus the location of wind slabs. A west wind does not mean you will not find any wind slabs on west facing terrain! Local area knowledge is perhaps more helpful with wind slab than with any other avalanche problem, since a given valley will tend to channel wind in similar ways in successive weather events.
- In the terrain, remain vigilant for signs that wind slab may be present where you have chosen to travel, even if you were aiming for avoidance. These obvious signs of wind slab instability include: a hollow or drum-like snow surface, shooting cracks, or the collapsing of a weak layer beneath the snow surface.
- Hard wind slabs deserve special respect as they can lure you far out onto them before they release. For this reason, ski cutting is not advised when dealing with any type of wind slab.
- Wind slabs deposited over persistent grain types (surface hoar, facets or depth hoar) demand extra caution and may be termed persistent slabs, or turn into persistent slabs. As the name suggests, persistent slabs remain sensitive to triggering for longer; they can be remotely triggered, and avalanche size is frequently much larger.
This weekend the avalanche danger will evolve quickly as heavy precipitation gives way to dry weather; changing freezing levels and shifting winds will produce varied conditions across the forecast region, and wind slab will likely be an issue. Meanwhile, read the forecast, follow us on Instagram, and please share your observations. Enjoy the new snow, and travel with care.