February 22, 2018

Avalanche Danger Update

Dangerous avalanche conditions in Chugach State Park:

Avalanche danger has increased in Chugach State Park, and is expected to continue to increase through Thursday due to several inches of new snow and moderate to strong winds in the alpine.

The Chugach State Park snowpack is generally very weak, with poor structure, and seems to have residual energy that is continuing to produce concerning collapses (whumphs). There are two widespread persistent weak layers of concern: large depth hoar at the ground and a thin layer of facets (potentially with buried surface hoar) sandwiched between wind-packed layers in the upper 1/2-1/3 of the snowpack. Collapses were still occurring in these persistent weak layers after a significant period of quiet weather that did not add any stress to the snowpack (i.e. these weak layers seem VERY persistent).

Collapses were the most pronounced where there was a stout wind-packed layer (slab) near the surface (generally under the looser snow from the Valentine’s week snowfall). Otherwise, where there was no stout wind-packed layer, the snowpack was generally too weak, faceted, and without a supportable slab to collapse the persistent weak layers. Now, avalanche problems will be complicated and compounded by the new snow that is being deposited with moderate to strong wind.

New snow and wind will stress persistent slab instabilities deeper in the snowpack, as well as create fresh wind slab instabilities nearer the surface. Three specific avalanche problems are expected to exist (listed from most to least likely – which also corresponds to most to least predictable):

1. Fresh, sensitive, and widespread wind slabs up to D2 in size (could bury, injure, or kill a person) above ~2000′ (and possibly lower in areas like Turnagain Arm) are likely to be triggered by humans (and may occur naturally) nearer the surface: recent avalanches, shooting cracks, and leeward terrain and catchment features with deeper snow (especially if it appears “fat” or bulbous) are red flags and obvious clues. Such wind slabs will be most dangerous where the snow feels hollow and/or punchy (with denser snow overlying looser snow). Keep in mind that the diverse terrain of Chugach State Park is characterized by very different wind patterns. Currently (late Thursday morning), easterly winds are impacting the Turnagain Arm area while more southerly winds are impacting areas further north like the Hillside and Arctic Valley. The National Weather Service is forecasting southerly and westerly winds, becoming more northerly Thursday night into Friday as the snow-producing storm system clears out.

2. Persistent slabs up to D2.5 in size are possible in specific areas above 2500′ on terrain steeper than ~35*. Expect these to be most problematic where they’ve been stressed by a significant load from new snow and wind: primarily upper elevation leeward terrain features especially along ridges and near peaks. Big, steep, and open slopes should also be suspect. Such slopes produced significant whumphing recently in South Fork Eagle River areas (e.g. North Bowl, Harp, Two Bowls, Gordon Lyon).

3. Isolated, deeper persistent slabs breaking near the ground on the extensive and well-developed depth hoar and producing avalanches up to D3 in size are possible on terrain steeper than 35* above 2500′, but the least likely and most unpredictable avalanche problem (lower probability, higher consequence).

Keep in mind that persistent slab avalanche problems 2-3 will behave unpredictably with hard slab characteristics. Contrasted to fresh wind slabs (avalanche problem 1), which are likely to behave more characteristically of soft slabs (releasing near a human’s feet) making them more predictable and escapable, persistent slabs may be remotely triggered or allow a human trigger to get into the middle of the slab before releasing above and around the trigger: making escape difficult.

Sunny, beautiful weather is expected for Friday with chances for snow again Saturday and another clearing with sunny skies Sunday. It will be important to pay attention to the winds, and consider their impact on the snow and stability. It will be prudent to approach avalanche terrain with caution and conduct thorough stability assessment before exposing oneself to potentially dangerous avalanche terrain.

Keep in mind that avalanches naturally triggered in the upper elevations have the potential to run down into lower elevation terrain. Such avalanche paths exist along the Powerline trail from Glen Alps, up Falls Creek, and in the bowls leading to the Penguin Ridge trail. Avalanches may also be remotely triggered from below.

The sun is also starting to have a significant impact on stability. As Friday and Sunday are expected to be sunny, and potentially with fresh snow not yet exposed to solar radiation, dangerous natural avalanches may be triggered on solar aspects (primarily south to west).

As always, make sure you’re properly prepared and equipped for a day in Alaska’s backcountry: know how to recognize potentially dangerous avalanche terrain and assess stability, have avalanche rescue gear (beacon, shovel, probe) and know how to use it, and dress and pack appropriately for your chosen adventure and any unforeseen circumstances that may arise. Leave a trip plan with someone staying in town, in case an unexpected situation arises.

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