This is a regional backcountry avalanche forecast for the Western Chugach Mountains of Chugach State Park. It is provided pro bono by a local avalanche professional, is based on limited field data for a large area, and is meant to be used only as a general baseline for recreationists’ personal and specific assessment in the field. The Anchorage Avalanche Center and associates assume no responsibility or liability for the use of this information. Anyone using this information needs to be formally educated in regard to avalanches and backcountry travel. Please encourage statewide investment in high quality outdoor and environmental education via K-12 public schools, as this is the best way to ensure all Alaskans are “avy savvy.”
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Get outside and enjoy The Greatland! This is your weekend avalanche outlook for February 21-23, 2020.
The avalanche danger this weekend is expected to be considerable Friday, gradually decreasing to moderate through the weekend. However, this is a scary moderate and the “travel advice” for considerable is recommended through the weekend as careful snowpack evaluation, cautious route-finding, and conservative decision-making will be essential for managing complex avalanche problems.
Significant snowfall (Tuesday-Wednesday), followed by strong wind (Wednesday-Thursday), and then more snowfall (Friday) drastically increased avalanche danger this week. See the special avalanche updates for 2/18 and 2/19 to learn more. There is also a very informative observation from 2/20.
Problem 1 – Persistent Slab
Human triggered persistent slab avalanches up to D3 (large, deadly, and destructive) in size are possible above 2500′ on all aspects. While persistent slabs are likely to only release from terrain steeper than 35º, they may be remotely triggered: beware of hard slabs on lower angle (even flat) terrain that may be connected to steep slopes above you! A human triggered persistent slab may be very difficult to escape, as a person may be able to get into the middle of the slab before it releases from above with hard slab characteristics. The hard slab nature of the debris also makes trauma more likely.
Multiple persistent weak layers exist in the Chugach State Park snowpack, and stress from snowfall and wind-loading this week will make persistent slabs more prone to human triggering.
In general, the Chugach State Park snowpack has basal facets and depth hoar near the ground, extensively faceted snow that developed December-January buried mid snowpack, and more recently developed February facets buried closer to the surface. These weak layers are sandwiched between wind-packed hard slabs of varying thickness.
Snowpits and stability tests are evidencing propagation propensity. The relatively hard wind-packed layers may be able to bridge the weight of a human trigger, but if a weak spot is found (likely where the wind-packed, hard slab layer is thinner and/or less supportable) a dangerous avalanche could result.
There is no effective way to manage this problem other than simply avoiding terrain capable of producing a dangerous persistent slab avalanche, especially steep slopes that are convex or unsupported. Again, be mindful of lower angle terrain where a hard slab may be connected to a steeper slope capable of producing a dangerous persistent slab. Assessment of this problem in the specific area in which you’re traveling will require thorough snowpack investigation: digging a snowpit, analyzing the stratigraphy, and conducting stability tests like the ECT. As always, be mindful of red flags (especially recent avalanches, collapsing or “whumphing,” shooting cracks, active loading, etc.) and be very diligent about safe travel protocols.
Problem 2 – New Snow
Human triggered wind slabs up to D2 in size are possible on leeward terrain (especially west clockwise through north aspects) steeper than 35º above 2500′.
Human triggered storm slabs and loose dry avalanches (aka “sluffs“) up to D1.5 in size are possible across all aspects on terrain steeper than 35º where recent snow has been sheltered from the wind.
Weak interfaces are expected to exist where Friday’s snow has fallen on exposed, relatively low friction (and in some cases “slide-for-life”) old snow surfaces like crusts and boilerplate wind-packed. Where this setup exists new snow instabilities may be very reactive to human triggers. Weak interfaces may also exist at density changes (due to atmospheric fluctuations) in Friday’s snowfall. As with the persistent slab problem, wind and storm slab instabilities are most likely on convex or unsupported terrain features.
Thick, hard wind slabs that developed from strong winds Wednesday-Thursday now fall under the persistent slab problem. Winds are forecast to be light through the weekend, but fresh wind slabs from moderate winds in some areas shouldn’t be ruled out.
Be on the lookout for red flags of wind slab danger (recent avalanches, shooting cracks, active wind loading). Pay close attention to the appearance and texture of surface snow: this is a way to identify potentially dangerous, wind-loaded terrain. Wind-loaded snow may look pillowy, bulbous, relatively deep (“fat”), and have a buffed or etched texture. Dangerously wind-loaded snow may feel hollow and punchy, indicating a denser slab overlying looser and weaker snow.
Pole probing and quick hand pits are a great way to assess wind slab danger near the surface. Expert skiers and backcountry travelers may be able to use prudent ski cuts to manage the wind slab problem. However, wind slabs have the potential to “step-down” and trigger larger and more dangerous persistent slabs.
Storm slabs and sluffs are not expected to be large enough to be inherently dangerous, but terrain traps can make even a very small avalanche deadly. Be mindful of the consequences if you were to fall or lose control due to triggering a small storm slab or sluff. Beware of exposure, where a fall or slip could cause trauma, and channeled terrain features that might cause even a little debris to pile up deeply.
Problem 3 – Cornice Fall
Cornices have grown substantially from recent snow and wind, are very large in some areas, and dangerous human triggered cornice falls are possible on leeward terrain (primarily west clockwise through north aspects) above 2500′. Give corniced ridges a wide berth; cornices may break off further back than expected. Do not approach the edge of a snow-covered ridge, unless you’re sure it’s not corniced. A cornice fall itself is dangerous, especially if the human trigger falls onto exposed terrain. Cornice falls may also trigger slab avalanches as they “bomb” the slope they fall onto.
Danger Trend
Expect the danger to decrease through the weekend.
Mountain Weather
Expect decreasing temperatures through the weekend, potentially becoming very cold (no more than single digits) by Sunday, with generally light winds and clearing skies.
Best wishes for your weekend!
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Funding and resources for this forecast generously provided by the Post-Capitalists and Libertarian Socialists of Alaska who encourage you to #FeelTheBern, #DumpTrump, #RecallDunleavy, and tell Murkowski, Young, and Sullivan that this is their last term!