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 28 to March 1, 2020.
The avalanche danger this weekend is expected to be moderate through daylight hours Saturday. Avalanche danger is expected to increase to considerable Saturday night into Sunday. Even while avalanche danger is moderate Saturday, it’s 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.
Recent weather has been relatively quiet. The cold and clear conditions heading into the weekend significantly faceted surface snow, and a textbook layer of (standing, intact) surface hoar blanketed all aspects of the Arctic Valley backcountry Thursday. The surface hoar and facets are expected to become another buried persistent weak layer with up to several inches of mountain snowfall forecast Saturday night into Sunday.
Wind is forecast to increase significantly with the incoming storm Saturday night into Sunday. While the forecast strong wind will increase avalanche danger due to wind loading, it could also dismantle the surface hoar and round surface facets before new snow arrives. This would be ideal. Regardless, with a few to several inches of new snow expected in the mountains by daybreak Sunday, it will be important to dig into the snow in the specific area in which you’re traveling to assess bonding of new snow to old and determine to what degree surface hoar and facets were preserved as a buried persistent weak layer. If you conduct this investigation, PLEASE let us know what you find by sending an email, Facebook or Instagram message, or submitting an observation!
New snow and wind loading this weekend will also stress the now longstanding persistent slab problem.
Problem 1 – Persistent Slab
During daylight hours Saturday (prior to the forecast storm), human triggered persistent slab avalanches are expected to be the primary avalanche problem.
By Sunday, fresh wind slabs are expected to be the most likely avalanche problem but the persistent slab danger is expected to increase and be a higher consequence problem.
There is significant loose snow available for transport by strong winds Saturday night into Sunday, and new snow with wind loading is expected to increase stress on the longstanding persistent slab problem.
Human triggered persistent slabs up to D3 (large, deadly, and destructive) in size are possible above 2500′ on all aspects. For a period of time Saturday night into Sunday, naturally triggered persistent slabs (from new snow and wind) are also possible.
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 Saturday night into Sunday will make human triggered persistent slabs more likely.
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 continue to evidence 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 – Wind Slab
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′.
Wind slab danger will increase significantly Saturday night into Sunday!
Fresh wind slabs that are expected to develop this weekend (from the ample old, loose snow currently available for transport as well as new snow forecast Saturday night into Sunday) may be very reactive to human triggers Sunday. Saturday night into Sunday, fresh wind slabs may even occur naturally.
Weak interfaces are expected to exist where fresh wind slabs form on relatively low friction (and in some cases “slide-for-life”) old snow surfaces like crusts and boilerplate wind-packed. As with the persistent slab problem, wind slabs are most likely on convex or unsupported terrain features.
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.
Remember that 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 just a small slab. Beware of exposure where a fall or slip could cause trauma, as well as channeled terrain features that might cause even a little debris to pile up deeply.
Problem 3 – Cornice Fall
Cornices over leeward slopes (primarily west clockwise through north aspects above 2500′) are large in many areas of Chugach State Park. Such cornices are expected to grow and become more prone to falling via natural or human triggers Saturday night into Sunday considering the forecast wind and snowfall. 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 increase significantly Saturday night into Sunday.
Mountain Weather
Sunny to partly sunny skies are expected Saturday, with mountain temperatures in the teens and winds increasing from light to moderate through the day. Strong wind and a few to several inches of mountain snowfall are expected Saturday night into Sunday. Cloudy skies, lingering snow, mountain temps in the low 20s, and decreasing winds are expected Sunday.
Best wishes for your weekend!
Please let us know what you’re seeing by tagging us on Instagram @anchorage_avalanche_center, submitting an observation, sending an email to info@anchorageavalanchecenter.org, or via FaceBook message. All observations help us provide the public with a better forecast product – no matter how basic. We are more than willing to keep observations confidential, and only use the information to inform forecast products.
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! Our democracy and planet are at stake, stand up and fight the power!