Weekend Avalanche Outlook
Issued Thursday, February 9, 2017 at 1:30pm
Stubborn persistent slabs up to D3.5 in size will be a problem on steep terrain (>35*) above 2500′. These slabs are expected to be most susceptible to human triggering on more open slopes that are typically leeward near peaks and along ridges. The most likely trigger points are where the hard slab above the basal persistent weak layer is thinner and/or less supportable, and near “sweet spots” harboring faceted snow (rock outcrops and vegetation). Especially hazardous terrain also includes steep rollovers and unsupported slopes.
This problem will require vigilance: careful snowpack evaluation, cautious route-finding, conservative decision making, and effective terrain management.
The January 30-31 wind event produced the most intense natural avalanche cycle in Chugach State Park in half a decade. It revealed the extent of the persistent slab problem due to advanced facets and depth hoar at the base of the snowpack, with numerous large and widely propagating natural avalanches triggered by wind loading. While the problem is widespread throughout the park, it seems the most prolific north of the Front Range (the mountains west of Ship Creek and Indian Creek) from South Fork Eagle River to the Eklutna-Knik area. Check out the observations for some perspective on the extent of the persistent slab problem.
While an extreme inversion, with relatively warm upper elevation temperatures for several days following the wind event, helped create a more benign temperature gradient within the snowpack that helped with bonding of snow grains and stabilization; this effect was much less pronounced on northerly aspects; and all aspects still exhibit concerning snowpack structure. Check out Wednesday’s observations from Arctic Valley to better understand this phenomena.
New snow this weekend will add some stress to the snowpack. Due to such a concerning snowpack structure, a human triggered persistent slab could be very large, hard to escape, and possibly unsurvivable. This is a tricky, lower probability but very high consequence scenario. These hard slabs won’t break at your feet like the storm slabs and wind slabs (discussed below), but will allow you to get into the middle of them (inhibiting escape) before they release catastrophically.
New snow since Wednesday night has generally fallen with light winds. As there has already been a few inches of accumulation, which has fallen on a firm surface (windboard with surface hoar in areas), potentially sensitive storm slabs are possible on steep terrain (>35*) above 2500′. While storm slabs are generally expected to be small, be mindful of terrain traps and exposure which could compound their consequences. Handpits are an effective means of understanding and tracking this problem.
While winds have been light, there is now plentiful loose snow readily available for transport. If winds increase sensitive wind slabs may develop on leeward terrain, especially near peaks and along ridges and cross loaded gully sidewalls. Wind slabs that do develop are expected to be relatively soft, but they can be identified by firmer snow – especially if it feels hollow, punchy, and/or exhibits cracking. Paying close attention to snow texture, pole probing, and hand pits are quick but effective ways of understanding and tracking this problem. Be on the lookout for “fat” pockets of snow.
A few inches of new snow has fallen since Wednesday night and more is expected. It is light, low density, and came with modest wind. This new snow generally accumulated on a firm (windboard) surface, which will be prone to D1 sluffing on steep terrain (>35*). While a “sluff” isn’t likely to cause burial, keep terrain traps and exposure in mind. Practice effective “sluff management” to avoid a fall or loss of control.
The January 30-31 wind event significantly enlarged cornices in some areas. Field work in the past week has revealed many that are drooping and cracking. While the problem isn’t expected to be particularly exacerbated by recent weather, it is always prudent to give corniced ridges a wide berth; don’t approach a ridge line to look down it unless you’re sure it’s NOT overhung by a cornice. Keep in mind that cornices can break off much further back than expected and, besides the inherent danger of falling with a cornice, they can trigger a subsequent avalanche as they “bomb” the slope they fall onto.
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 rescue workshop at Turnagain Pass Saturday, February 11.
Chugach State Park now has a base for reasonable alpine touring. Keep in mind that hazards such as superficially covered rocks and vegetation exist.
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