The avalanche danger remains MODERATE above 1000′. It is possible a person can trigger a very large avalanche on a weak layer of sugary facets buried 2.5-5′ deep. An avalanche may be triggered after there are already multiple sets of tracks on a slope, and they may be triggered remotely from the side, above, or below steeper slopes. Despite the heightened avalanche conditions, this type of problem will rarely present any of the common warning signs prior to avalanching. The only way to manage a deep slab issue is to stick to mellower terrain, avoiding spending time on or below steep slopes.
The avalanche danger is LOW below 1000′. It is unlikely a person would trigger an avalanche in the lower elevations, but it is important to be aware of the terrain above you and of other groups traveling above you. A large avalanche triggered at or above treeline may run down to valley bottoms.
Special Announcements
We are sad to share the news of the second fatal accident in the U.S. this season, down in the Big Hole mountains in eastern Idaho. Details are still coming out, but we do know the avalanche killed two people- one on a snowmachine and one skier. Our deepest condolences go out to the family and friends of those two people. You can find some preliminary details in this news article.
Sun, December 19th, 2021
Alpine
Above 2,500'
2 - Moderate
Treeline
1,000'-2,500'
2 - Moderate
Below Treeline
Below 1,000'
1 - Low
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Travel Advice
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Natural avalanches likely; human-triggered avalanches very likely.
Natural and human-triggered avalanches certain.
Avalanche Size and Distribution
Small avalanches in isolated areas or extreme terrain.
Small avalanches in specific areas; or large avalanches in isolated areas.
Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
Large avalanches in many areas; or very large avalanches in specific areas.
Very large avalanches in many areas.
Avalanche Problem 1
Deep Persistent Slabs
Certain
Very Likely
Likely
Possible
Unlikely
Likelihood
Historic (D4-5)
Very Large (D3)
Large (D2)
Small (D1)
Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing
avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and
natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question.
Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to
hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are
relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure,
or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.
With another day of quiet weather today, we are not expecting to see any big differences in the big picture stability-wise. The layer of weak faceted snow that was buried back in mid-November is still lurking below the surface, and it is possible a person could trigger a very large avalanche if they find the wrong spot on or below a slope. This layer is buried 3-5′ deep in the northern half of the advisory area, 2-3′ deep towards the southern end of Turnagain Pass and towards Johnson Pass, and 1.5-2′ deep in the Summit Lake area. The most likely place to trigger one of these monsters will be in areas with a shallower snowpack, so the likelihood of triggering an avalanche increases as you travel further south. That said, the entire advisory area (and surrounding periphery zones) appear to share the same poor structure, and you really can’t rule out a very large human-triggered avalanche given that untrustworthy setup.
As we have mentioned before, there are a few things that make deep slabs a tricky problem to deal with. They will rarely give you warning signs like shooting cracks or collapsing, or poor stability test results, even though the snowpack is capable of producing a large avalanche. They may be triggered remotely from above, below, or to the side of a slope. They can also propagate very wide, connecting multiple terrain features that may otherwise seem like they could interrupt avalanche paths (revisit the avalanche from 12/2 on Eddie’s for an example of this). Given the lack of feedback and the high consequences of being caught in a deep slab avalanche, the only way to manage this problem is to stick with the status quo of avoiding spending time on or below steep slopes for now.
Wind Slabs: There may be some isolated wind slabs that formed earlier in the week on top of a new layer of weak snow that could remain reactive today. A relatively small avalanche triggered near the surface has the potential to step down to deeper weak layers, creating a larger avalanche. Be on the lookout for lingering wind slabs near ridgelines, gullies, and convexities in the mid and upper elevations. Look for clear warning signs like shooting cracks or collapsing, and be suspect of slopes with stiff or punchy snow on the surface.
Glide Avalanches: We saw one glide avalanche release in the past week, and there were some glide cracks that look like they had been creeping recently up Lynx Creek. Glide avalanches are destructive and unpredictable, so as always, limit your time spent traveling below glide cracks.
Glide avalanche in the Lynx Creek drainage from 12/16 or 12/17, which triggered a slab avalanche on the Halloween facets on its way down. 12.18.2021
Video with a snowpack summary from Lynx Creek yesterday (12/18). Linked here.
Weather
Sun, December 19th, 2021
Yesterday: Skies were mostly cloudy in the morning, gradually breaking up with improving visibility and pockets of sun in the afternoon. Winds were light out of the west at around 5-10 mph, with gusts in the 20’s and up above 30 mph in Summit. High temperatures were in the low 20’s F at ridgetops, and in the upper 20’s F at mid and lower elevations. Some areas saw a trace of snow.
Today: Another day of quiet weather is expected today, with highs in the upper teens to low 20’s F and light westerly winds blowing around 5 mph. Skies will be mostly cloudy, with chances for some sun especially towards Summit Lakes and Seward. No precipitation is expected today.
Tomorrow: We should see some more active weather starting to pick up tomorrow. High temperatures are expected in the low to mid 20’s F, with increasing cloud cover during the day. Chances for snow pick up Monday night, with 2-4″ possible by Tuesday morning. Expect to see a bump in winds as an outflow event follows on the heels of this brief pulse late Monday night and Tuesday morning.
This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area.
This advisory does not apply to highways, railroads or operating ski areas.
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