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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, November 25th, 2021 - 7:00AM
Expires
Fri, November 26th, 2021 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 2500′. We have had 15-30″ of new very light snow in the past few days falling on top of a weak snow surface. Human triggered avalanches up to 2′ deep will be likely today. Look for signs of instability like shooting cracks and collapsing or whumphing. Cautious route finding and staying on slopes less than 35° is the best way to avoid avalanches today. Pay attention to the terrain above you because this new snow could be triggered remotely from low angle terrain below, to the sides, or above steeper slopes.

A MODERATE danger exists below 2,500′. The new snow fell at treeline and below treeline elevations as well but we have had less wind at these elevations. Triggering an avalanche is still possible due to existing weak layers underneath the latest storm snow.

Special Announcements
  • Looking for holiday gifts or to finally get those new skis?? Head to Powder Hound tomorrow for White Friday – 10% of all proceeds will benefit the Chugach NF Avalanche Center!! Thank you for your long standing support Powder Hound!
  • The Turnagain Pass motorized areas will remain closed, the forest will be assessing conditions today. The new snow over the last few days is getting snow depths close to sufficient coverage, stand by for more details tomorrow morning. You can find more info here.
Thu, November 25th, 2021
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
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. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. 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.
Recent Avalanches
  • On Notch Peak in Girdwood a group reportedly remote triggered a small avalanche which appears to have failed at the interface between the new storm snow from this week and the old snow surface.

Remote triggered avalanche from Notch Peak in Girdwood 11.24.21. Photo courtsy of Heather Johnson

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

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.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Happy Thanksgiving!!!

Turnagain Pass has received 15-30” of low density snow since early Tuesday morning with strong easterly winds that ended early yesterday morning. This new snow fell on a variety of existing weak layers and is not bonding well to the old snow surface. The storm snow that has been blown into wind slabs at higher elevations will likely be easier to trigger than in wind sheltered areas. Cautious route finding and sticking to lower angle terrain below 35° will be the best way to avoid these sensitive storm slabs. Watch for shooting cracks and collapsing to identify locations where the storm snow is sensitive to triggering. It is also important to be aware of steeper slopes above you because avalanches could be triggered remotely from low angle terrain below, to the sides, or above steeper terrain.

Over the past couple of days the new snow has not been acting like a slab in areas where people have travelled because it is so light and unconsoildated. As the new snow settles into a more cohensice layer we are concerned it will start behaving more like a slab and conditions could become more reactive. In Summit Lake the snow depth from the latest storm is only 4-6″ and observations from the area have highlighted lingerly hard wind slabs (details).

Our current list of potential weak layers includes two layers of buried surface hoar as well as multiple layers of faceted snow. On Tincan yesterday these layers were buried between 20-30” (with an additional ~5″ overnight) deep and were showing the ability to propagate a slab in our stability tests. Any of these layers has the potential to cause a larger avalanche than expected due to wider propagation. This instability is looking like it may persist longer than normal because of the types of weak layers we are dealing with. 

Snowpack summary from Tincan on 11.24.21

The distribution of surface hoar and faceted weak layers in our advisory area is variable. Digging a snowpit or hand pit is the best way to determine what weak layers exist in the area you are travelling. An extended column test is a great tool to determine the most concerning weak layers and show whether the new snow will propagate. However, one snowpit only represents the snowpack structure in a single area so choose a location that is representative of the slope you intend to travel on and continue to evaluate the snowpack structure with hand pits in multiple locations.

Loose Snow Avalanches: In areas where the new snow is too loose to act like a slab sluffs (dry loose) avalanches are likely in steeper terrain.

Additional Concern
  • Glide Avalanches
    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.
More info at Avalanche.org

Glide activity seems to have slowed down in the past few days, but that doesn’t mean the hazard is gone. Glide avalanches are hard to predict, and they involve the entire snowpack so they are large and very dangerous. Be on the lookout for glide cracks, and limit the time you spend traveling below them.

Weather
Thu, November 25th, 2021

Yesterday: It looks like another 6-10″ of snow fell in the past 24 hours at Turnagain Pass with calm to light winds and temperatures in the mid-teens. Based on the weather station data Turnagain Pass received more snow than other stations in the area, but some of the sensors on the Turnagain Pass SNOTEL site have been sending faulty data the past few days so there is some uncertainty in the snowfall amounts.

Today: We should see the snowfall taper off today, with accumulations of a few inches possible throughout the day. Winds will remain light with averages in the 5-10 mph range at ridgetops with gusts up to 20 mph, wind direction will shift from easterly in the morning to westerly in the afternoon. Temperatures will remain in the low teens this morning and start to drop off toward the single digits this afternoon and evening as the weather pattern shifts.

Tomorrow: Cold temperatures are forecast to return to our area this evening ranging from 0-5 F tonight and hovering around 0 F tomorrow. Winds should remain light out of the west with a chance of trace amounts of precipitation. A break in precipiatation is forecast for the weekend.

PRECIPITATION 24-hour data (6am – 6am)

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 13 N/A* 0.8* N/A*
Summit Lake (1400′) 10 0 0 N/A*
Alyeska Mid (1700′) 12 N/A* 0.06* N/A*

* Precipitation sensors are struggling to accurately capture the snowfall totals from the past few days

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 10 NE 7 21
Seattle Ridge (2400′) 13 variable 3 12
Observations
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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.