Weekend Avalanche Outlook
Issued Friday, March 31, 2017 at 8:45pm. This is a general backcountry (recreational) avalanche advisory for Chugach State Park with the Front Range and South Fork Eagle River areas as the core advisory zones.
*Dangerous Avalanche Conditions*
Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Natural avalanches possible; human triggered avalanches likely. Small avalanches in many areas; large avalanches in specific areas, very large avalanches in isolated areas.
Heavy snow and wind this week has significantly increased avalanche danger from the relatively stable conditions of the past month.
Recent avalanches, collapsing (“whumphing”), and shooting cracks are the mountains’ way of screaming “avalanche danger!” If you are traveling around the mountains this weekend keep in mind that avalanches triggered in the upper elevations have the potential to travel down into lower elevation, even flat, terrain. Make sure you know how to identify avalanche terrain, and dangerous avalanche paths that can threaten commonly traveled routes like summer trails (e.g. Crow Pass, Penguin Ridge, Falls Creek, Powerline, Flattop, O’Malley Gully).
As always (and especially important this weekend) practice safe travel protocols, carry necessary personal safety and rescue gear, and leave a trip plan (with emergency response) with someone that is staying in town.
Wind slabs up to D2.5 in size are possible on terrain steeper than 30* above 2750′. These wind slabs are expected to be the most problematic on leeward (primarily west to north aspect) upper elevation terrain, especially near peaks and along ridges and cross-loaded features like gully sidewalls.
Upper elevation human triggered wind slabs were reported as early as Tuesday afternoon on Peak 3, following the Monday night to Tuesday morning minor snowfall. A significant accumulation of snow followed Tuesday night through Wednesday with over a foot accumulation in many areas of Chugach State Park. This snowfall was followed by strong (predominantly southeast) wind, which has likely created dangerous wind slabs in many areas. The recent snow has been relatively moist and heavy making active wind loading less apparent, but wind loading has undoubtedly occurred. Collapsing (“whumphing”) and shooting cracks were reported on Peak 2 Thursday afternoon.
The recent snow fell on a variety of weak and slick surfaces, prone to creating especially concerning avalanche problems, due to the roughly month long dry spell. The snow surface prior to this week’s storm(s) was generally a firm wind slab of varying hardness capped by a heavily faceted layer of snow of varying thickness. In many areas a very firm wind slab was capped with a thin layer of loose facets and/or surface hoar: an ideal recipe for human triggered avalanches.
A variety of firm and slick surfaces (sun crust, boilerplate, etc.), creating weak interfaces, exist in other areas. Continued winds capable of redistributing snow and loading leeward terrain, along with minor snow accumulation through the weekend, will keep the danger elevated…and could even increase the danger further.
Persistent slabs up to D3 in size are a lower probability but very high consequence avalanche problem that will be possible this weekend in isolated areas (primarily leeward terrain above 3000′ that’s steeper than 35*).
A pronounced layer of very weak and uncohesive basal facets and depth hoar is widespread throughout Chugach State Park. There is also the potential for lingering pockets of buried surface hoar that could become reactivated by this week’s stress from new snow and wind loading. Persistent slabs are likely to behave unpredictably with hard slab characteristics, and could let a human trigger get well on to the slab before releasing above and around – making escape difficult.
Especially problematic areas are expected to be steep convexities or rollovers (where the snow lacks compressive support), areas where these weak layers exist but the snowpack is thinner (allowing a human trigger’s stress to more easily penetrate and initiate fracture), and areas where there are sparsely protruding or thinly covered rocks (harboring faceted snow and/or depth hoar closer to the surface).
Both wet and dry loose snow avalanches will be possible this weekend. Warming temperatures may naturally trigger loose wet avalanches on steep terrain, especially if there’s any sun on solar aspects. These wet loose avalanches are likely to be relatively slow moving, but will be heavy and could carry a lot of volume. They will have the potential to injure a person due to their wet-concrete-like nature, and could definitely push someone over dangerous terrain and even bury a person in a terrain trap.
Dry loose avalanches are possible primarily on sheltered northerly terrain, but could also be triggered on upper elevation steep (35*+) terrain where there’s drier snow. These will likely be faster moving, but lower volume, than a wet loose. These could prove problematic given terrain traps and exposure. While they may not typically involve enough mass to bury a person, they could definitely cause a fall or loss of control.
A significant amount of new snow and strong wind (predominantly SE) this week has likely grown cornices and made them more susceptible to falling. Cornices are most likely to have grown over leeward west to north aspects and cross loaded terrain features like gully sidewalls.
As always, don’t approach a snowy ridge to look down slope unless you’re sure it’s not corniced. Remember that cornice falls pose an inherent hazard, as well as their ability to trigger an avalanche as they “bomb” the slope they fall onto. Give corniced ridges a wide berth; they can break back further than expected.
Expect cloudy skies with possible snow showers, alpine temperatures in the mid 20s to mid 30s, and moderate SE wind.
Many avalanche accidents that have happened in Anchorage’s backyard of Chugach State Park could have been prevented by basic avalanche awareness. This level of awareness can typically be gained through a free or low-cost class. Unfortunately, such opportunities are few and far between in the spring. If you missed the many offerings earlier this season, make sure you take advantage of the numerous free and low-cost courses that will be offered in the fall. You can start learning with some free online resources here. Getting avalanche education is a big part of recreating safely on your vast and wondrous public land in Alaska, and being able to enjoy the mountains during the snow season will greatly enhance your life.
Here are links to further information on fatal avalanche accidents in Chugach State Park. You can learn from others’ mishaps.
*click avalanche problem icons and hyperlinks for further info