Weekend Avalanche Outlook
Issued Friday, February 17, 2017 at 3:15pm:
*Heightened Avalanche Conditions*
Human triggered avalanches are possible. Natural avalanches are unlikely. Complex avalanche conditions will necessitate careful snowpack evaluation, cautious route-finding, conservative decision-making, and effective terrain management.
The snowpack has experienced several days of stressful weather; approach the mountains with caution. You will need effective avalanche hazard assessment, rescue, and backcountry travel skills to enjoy recreating safely in avalanche terrain this weekend.
Don’t forget your (fully operational) beacon, shovel, probe, and (most importantly) BRAIN!
Generally cloudy skies, with a chance for both some clearing and snow showers at times, are expected this weekend. Winds are expected to be light to moderate, with alpine temperatures in the teens. Keep in mind that reduced visibility in the alpine due to flat light can complicate safe travel and avalanche avoidance.
Stubborn persistent slabs up to D3.5 in size are possible on steep (>35*) upper elevation (above 2500′) terrain on multiple aspects. Relative to other avalanche problems this weekend, these persistent slabs are lower probability but much higher consequence. These hard slabs may release unpredictably, allowing a human trigger to get will onto the slab before it releases from above and around making escape difficult. Large persistent slabs could make for deep burial, and their hard slab characteristics will create more risk for casualty from trauma.
Persistent slabs are expected to be the most problematic on typically leeward aspects (which vary across the park) where there’s more snow, but shallow or unsupported “sweet spots” exist as trigger points. Steep terrain along leeward ridges and near peaks with sparsely protruding rocks (facet gardens), thinner snowpack areas (more human “stress bulb” penetration), or steep convexities (rollovers on slopes or cross loaded gully sidewalls with less compressive support) are expected to be problematic trigger points.
Harsh weather and dangerous avalanche conditions this week have drastically limited recent upper elevation observations. However, we do have some information about the persistent slab problem from extensive observations last weekend and can extrapolate on the nature of the problem this weekend based on recent weather. Last weekend, from the Front Range to the South and North Fork Eagle River areas, snowpits revealed that persistent slabs failing at the ground on basal facets and depth hoar still exhibit propagation propensity and energy (ECT’s with sudden collapse propagation). Weather this week (rain even at upper elevations, several inches of relatively heavy new snow, strong winds, and warm temperatures) has stressed the snowpack.
A newer persistent weak layer created by well-preserved surface hoar, buried by the February 9-10 snowfall that accumulated with light wind, was also observed in a few areas. As of last weekend, this layer was not of much concern due to the absence of a consolidated slab overlying it. However, as it was buried and preserved by ~10cm of snow, it may still exist (in areas where it was not scoured out by this week’s strong winds) and now have a significantly thick slab on top of it. While a persistent slab release on this layer may not be as catastrophic as a deeper release on the basal facets and depth hoar, it could still produce a D2-D2.5 avalanche…and possibly “step down” to trigger a deeper slab on the basal weak layer as it descends.
The persistent slab problem demands careful snowpack evaluation, cautious route-finding, conservative decision-making, and effective terrain management. As always, practice “safe travel protocols.” Having an escape plan in mind will be especially prudent.
Touchy to stubborn D2 wind slabs are possible on steep (>35*) upper elevation (above 2500′) terrain on leeward aspects (which vary across the park, but are expected to be the most problematic on westerly and northerly aspects from this week’s strong, predominantly SE, wind). The most prone areas to wind slab avalanche are expected to be wind loaded terrain near peaks and along ridges and cross loaded gullies. Multiple layers of wind slab of varying depth are possible due to multiple bouts of strong wind and new snow accumulation this week.
Wind slabs are more predictable than the persistent slab problem. Pay close attention to the snow surface as you travel: notice how the density, texture, and depth changes. Wind slabs can be identified by areas of deeper and firmer snow, especially if it is “fat” or “pillowy” in appearance and/or feels “hollow” or “punchy” (denser wind slab overlying looser, weaker snow). Check out this observational video from last weekend that has a short segment on identifying wind slabs. Quick snowpack assessment via pole-probing and handpits are effective means of grasping the wind slab problem and potential distribution.
While wind slabs this weekend could be large enough to bury, injure, or kill a person; it’s important to note that even a fall or loss of control caused by a small wind slab could result in serious consequences. Keep terrain traps and exposure in mind. If you’re hiking along a ridge, don’t take shortcuts: do what it takes to avoid pockets of potentially sensitive wind slab.
Sustained strong winds this week have affected the mountains of Chugach State Park. They have left their mark with extensive scouring, sastrugi, cornice development, and anti-tracks as low as 1500′ elevation in unprotected terrain. Cornices in the upper elevations are expected to have grown significantly, and may be especially unstable. As always, don’t approach a snowy ridge line to look down slope unless you’re sure it’s not corniced. Cornice falls are inherently dangerous, and they can also trigger an avalanche as the “bomb” the slope they fall onto. Give corniced ridges a wide berth; they have the potential to break further back than may be expected.
A few to several inches of new snow that has fallen late this week with lighter wind may make for D1 human triggered “sluffing” on steep (>35*), wind-sheltered, upper elevation terrain. While these loose snow avalanches will be small and aren’t inherently dangerous, practice effective “sluff management” and beware of terrain traps and exposure that could compound the consequences of a fall or loss of control.
Many avalanche accidents that have happened in Anchorage’s backyard of Chugach State Park could have been prevented by basic avalanche awareness. If you don’t have this level of awareness, here are some online resources to help you start the learning process. There are also numerous options for getting a real avalanche education locally. Many of these learning opportunities are even FREE, like this avalanche awareness class in Anchorage on the evening of Wednesday, February 22.
*click avalanche problem icons and hyperlinks for further info