ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, November 28th, 2021 - 7:00AM
Expires
Mon, November 29th, 2021 - 7:00AM
Forecaster
Andrew Schauer
The Bottom Line
The avalanche danger will remain CONSIDERABLE above 2500’, where it is still likely a person could trigger an avalanche 1-2’ deep on multiple weak layers that were buried by the storm earlier in the week. Be cautious with your terrain choices, and keep in mind these avalanches may be triggered remotely from above, below, or adjacent to a slope.
The danger is MODERATE below 2500’. While it is still possible to trigger a slab avalanche over a foot deep, the lack of wind at and below treeline will make it a little less likely to find a cohesive slab capable of producing an avalanche. It will still be important to keep an eye out for red flags like shooting cracks, collapsing, or any recent avalanche activity. Stick to low angle terrain if you notice any of these warning signs.
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Sun, November 28th, 2021
Alpine Above 2,500'
Considerable (3)
Treeline 1,000'-2,500'
Moderate (2)
Below Treeline Below 1,000'
Moderate (2)
Alpine Above 2,500'
Considerable (3)
Treeline 1,000'-2,500'
Moderate (2)
Below Treeline Below 1,000'
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches
Girdwood: A snowboarder remotely triggered an avalanche 2′ deep and 200′ wide on Max’s yesterday. The avalanche was on a southern aspect at 3000′. Nobody was caught or carried. More details here.
Seattle Ridge: Multiple avalanches were triggered on Seattle Ridge yesterday. This includes one small snowmachine-triggered pocket near the weather station, 3 remote-triggered slabs in Warmup Bowl, and two large remote-triggered avalanches on the front side just above the motorized lot. Some of these remote-triggers were triggered from over 100′ away from the avalanches. More details here.
View of the corner of a crown from a remote-triggered avalanche on Max’s yesterday. Photo: Rafael Pease. 11.27.2021
Remote-triggered avalanches on the front side of Seattle Ridge. These were triggered from on top of the ridge above the crowns. 11.27.2021
Avalanche Problem 1
Persistent Slabs
Almost Certain
Very Likely
Likely
Possible
Unlikely
Likelihood
Historic (D4-5)
Very Large (D3)
Large (D2)
Small (D1)
Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
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.
Our persistent weak layers that formed in the beginning in the month are still giving us cause for concern today, especially after seeing multiple avalanches in the advisory area yesterday. This includes multiple remote triggered avalanches on Seattle Ridge (details here) and a remote-triggered avalanche on Max’s (details here). While some of these avalanches were relatively small and harmless, others were large enough to carry and bury a person. A lot of this action is happening in places that have seen wind loading at some point in the past week, which means there is a stiff, cohesive slab sitting on top of weak, sugary facets. In most places this older wind slab has since been buried by 12-18″ soft snow, which makes it nearly impossible to recognize if you are not looking for it carefully.
So how can you play it safe today? Be cautious with your route finding and careful with your snowpack assessment. This means actively searching for a stiffer wind slab, possibly buried below softer snow. It doesn’t take an hour in a snowpit to do this- you can hop off your machine and notice if you feel a stiff, punchy layer under your feet. You can probe with a ski pole as you move along a skin track. It also means paying attention to red flags like shooting cracks and collapsing. In areas that haven’t been wind loaded, we have noticed the storm snow is starting to settle into a slightly stiffer layer. Be prepared to adjust your objectives if you notice signs of instability as you travel.
The most likely places to encounter a dangerous setup will be in the upper elevations near ridgelines, in gullies, and below convexities. This is exactly where we’ve seen avalanches in Tincan,Max’s, and Seattle Ridge in the past two days. Because we are dealing with persistent weak layers, it is likely a person could trigger similar avalanches today. Keep in mind we have seen multiple avalanches triggered remotely, in some cases with the nearest tracks over 100′ from the crown lines. This means it is likely a person could trigger an avalanche from above, below, or to the side of a slope. It requires extra attention with your route finding. It also demands a high level of spatial awareness- paying attention to other groups that are above you or below you.
We were really happy to see people making smart choices with terrain use yesterday on Seattle ridge. There were a ton of people out, and for the most part everyone was avoiding the steep terrain. Persistent slab problems will require some patience, and low slope angles will be the name of the game for some time.
Start zone of one remote-triggered avalanche on Seattle Ridge. This was one of multiple remote-triggered avalanches that were observed yesterday. Photo: Andy Moderow. 11.27.2021
Closeup of a crown from a small snowmachine-triggered avalanche just below the ridgeline on Seattle Ridge yesterday. The avalanche failed on facets that formed in early November, and the slab consisted of a stiff wind slab that was buried by 12-18″ soft snow. 11.27.2021.
Video describing the small snowmachine-triggered avalanche on Seattle Ridge (pictured above) and current snowpack concerns (link here):
Additional Concern
Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
We’re still keeping glide avalanches in the back of our mind, with the most recent activity on Lipps sometime in the past week. There are a lot of glide cracks in the area, and it is impossible to predict exactly when they will release. Glide avalanches are very large and destructive since the involve the entire snowpack. The only way to manage the problem is to limit the amount of time you spend traveling below them.
Weather
Sun, November 28th, 2021
Yesterday: A thick layer of valley fog broke up by late morning, giving way to mostly sunny skies. Temperatures crawled into the single digits above 0 F, and winds stayed light with variable directions. There was a trace of snow in the morning before skies cleared. Overnight lows dropped to the single digits to teens below 0 F, getting as cold as -18 F at the Summit Creek snotel station. This must be what Lou Reed was talking about when he said “It’s so cold in Alaska, It’s so cold in Alaska”.
Today: Temperatures will struggle to get above 0 F today. Luckily winds should stay light, blowing around 5 mph out of the west at ridgetops. We should get one more day of mostly sunny skies today before a system moves in tomorrow. There may be another round of valley fog this morning.
Tomorrow: It is looking like another round of active weather is moving in tomorrow. This will start with easterly winds picking up in the early morning, blowing 25-35 mph at ridgetops and 10-15 mph at lower elevations. Temperatures will continue to climb overnight and through the day tomorrow, getting back up into the low teens F during the day. There is a lot of uncertainty with precipitation predictions, but if we do get any snow we might see the rain level creep up to around 500 feet. Models are still showing different storm tracks, so be sure to stay tuned in as we track this next system.
Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff.
Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations.
Riding area questions contact: mailroom_r10_chugach@fs.fed.us
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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