Avalanche Danger Forecast

Issued Monday, December 14, 2020 at 2pm for the greater Anchorage area Western Chugach Mountains (i.e. Chugach State Park):

Tuesday-Thursday:

Avalanche danger is expected to remain at a steady moderate rating through this forecast period due to modest snowfall and localized wind loading.

While continuing to be a relatively low probability concern, human triggering an isolated and dangerous persistent slab avalanche on steep terrain is not out of the question.

Additionally, localized winds may build fresh wind slabs that are reactive to human triggers on steep leeward terrain.

Considering the unsettled weather forecast with generally cloudy skies and periods of snow, combined with the generally flat light this time of year due to low sun angle, be mindful of how poor visibility and darkness increases the difficulty of mountain travel, consequences of an accident, and complicates rescue.

Avalanche Problems:

 

 

 

 

 

Isolated persistent slabs up to D2.5 in size will remain possible through this forecast period on all aspects above 2500′ where the terrain is steeper than 35º. Some terrain features may harbor a hard slab with persistent weak layer reactive to additional stress from a human trigger.

While persistent slabs are low probability, they are relatively unpredictable and have the potential to be high consequence (due to hard slab characteristics).

Although there has been a lot of backcountry traffic in Chugach State Park and recreationists have been exploring steeper terrain in recent days without reported incident, diverse and widespread persistent weak layers exist in the snowpack. Faceted snow exists above and below crusts in some areas, sandwiched between wind packed layers in many areas, and a basal weak layer of advanced facets and depth hoar is widespread.

As they’re currently expected to be relatively dormant, red flags of persistent slab danger like recent avalanches, shooting cracks, and collapsing (aka “whumphing“) may not be present. Understanding how this problem is manifesting where you intend to travel is likely to require digging a snowpit and conducting instability tests.

Terrain management is simply the best way to avoid this high consequence and unpredictable avalanche problem: don’t expose yourself to terrain capable of producing a dangerous persistent slab.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Localized wind slabs up to D2 in size will remain possible through this forecast period on leeward aspects above 2500′ where the terrain is steeper than 35º. Potentially dangerous wind slabs are most likely to exist below corniced areas and along the lee sides of ridges and cross-loaded features like gullies (see graphics above).

Recent avalanches, shooting cracks, hollow and/or punchy feeling snow (denser wind-packed snow overlying looser and weaker snow), and pockets of deeper snow (especially with a bulbous appearance) are all indicators of potential wind slab danger.

Pole probing and hand pits are a quick and effective means of assessing this problem as you travel. Use pole probing to quickly feel out areas of denser, wind-packed snow overlying looser and weaker snow. Use hand pits to quickly assess how near-surface layers of snow are bonded.

Digging a snowpit and conducting a compression test and/or extended column test will provide an even better assessment of bonding and instability before you travel on terrain of consequence.

You can also assess wind slab reactivity via safer “test slopes” that are representative of higher consequence terrain.

While an unlikely scenario, wind slabs do have the potential to “step down” and trigger larger and more dangerous persistent slab avalanches.

 

 

 

 

 

Dry, loose snow avalanches are possible on terrain above 3000′ where the terrain is steeper than 38º. These may be in the form of naturally triggered wind-induced spindrift or human triggered sluffs where unconsolidated snow has been sheltered from the wind.

While these avalanches will be relatively small (D1), they do have the potential to cause a fall or loss of control and build up mass that “steps down” and triggers a larger slab avalanche.

Don’t let a small loose snow avalanche catch you off guard, especially if you’re on exposed terrain. Here’s a recently released 30-minute film in which a very experienced mountain guide tells the story of losing the lives of two close friends while climbing steep terrain to ski a classic line in the Tetons:

Before traveling on, through, or under terrain that has the potential to avalanche; think about the consequences and have a plan (to escape the avalanche, for re-grouping, and rescue).

Be mindful of terrains traps!

Click the hyperlinks and icons to learn more.

Food for thought: Have you ever thought about the distinction between “natural” and human triggered avalanches? While it conveys a necessary distinction, it implies humans are other than natural. The notion that humans are not part of “nature,” and not enmeshed thoroughly in the natural world, is part of the psychosis of the modern and industrial era. Might there be a better way (that doesn’t contribute to the lingual psychosis of humans as separate from “nature”) to convey the distinction between avalanches triggered by wind, snow, rain, species other than Homo sapiens, etc. versus those triggered by the human animal?