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Issued
Sun, December 3rd, 2023 - 7:00AM
Expires
Mon, December 4th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 2500′. Today’s weather is calm, but with last night’s moderate easterly winds you may be able to trigger an avalanche on steep, wind-loaded slopes today. If you are trying to get into steep terrain, be sure to take the time to identify and avoid slopes that have fresh slabs of stiffer windblown snow on the surface. The danger is LOW below 2500′.

Special Announcements

New for this season, we are now posting Weekend Avalanche Outlook products for the Chugach State Park, Summit Lake, and Seward zones. The first outlooks are up, so be sure to check them out if you plan on getting out in any of these areas.

Sun, December 3rd, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Mon, December 4th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Mon, December 4th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

We received one photo this morning of a skier-triggered avalanche on the south side of Tincan Ridge yesterday. The avalanche appears to have failed on a wind-loaded convex rollover. With clear skies for the past two days, we have also seen the aftermath of many natural avalanches failing within the storm snow, with some deeper activity possible in the Placer Valley.

Skier-triggered avalanche on the south side of Tincan yesterday. Photo submitted anonymously, 12.02.2023

Mid-storm avalanche on the back side of Seattle Ridge near Main Bowl, just left of Widowmaker. Photo: Graham Predeger, 12.02.2023

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

It’s looking like we’ve got another day of quiet weather on the way today, and although we are keeping an eye on some deeper issues in the snowpack (see the Additional Concerns section below for more on this), our main concern for now is the possibility of triggering an avalanche in steeper terrain that has been recently wind loaded. The winds picked up for a few hours yesterday evening, and with 2-3′ of snow on the ground after last week’s storm, there is plenty of material available for building wind slabs.

Don’t let the quiet weather today catch you off guard- be sure to take the time to identify and avoid features that have fresh windblown snow on the surface. The most likely places you will run into trouble will be in steep terrain just below ridgelines, on steep convex rolls, and in cross-loaded gullies. Fresh wind slabs will feel more dense than snow that has not been blown around, and may give you warning signs like cracks shooting from your skis or snowmachine. Keep in mind, you will not always get warning signs like that even when the snowpack is capable of producing an avalanche. This is the kind of problem where you may be able to gain some insight by using small, low-consequence terrain features to test the snowpack before committing to bigger terrain.

The wind has done some work along the ridgelines, but there is still plenty of soft snow out there. Photo looking south into Warmup Bowl on the back side of Seattle Ridge. 12.02.2023

It’s still dark up there, but the Sunburst webcam is back! Big thanks to Claire Bicknell, Ben Cross, and Eeva Latosuo for volunteering time to get the camera back up and running.

 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

In the highest elevations in our area, there may still be isolated slopes with weak snow on the ground. This layer only exists at elevations above 3500′, and is now buried 6-8′ deep or deeper, That makes it hard to trigger and even harder to identify. The only known avalanche that has failed on this layer was three weeks ago (more details here). At this point it is very unlikely you will encounter this problem, but it is big enough that it is worth keeping in mind. The most likely place to find it will be in steep, rocky terrain, where total snow depth tapers to  a thinner snowpack.

Weather
Sun, December 3rd, 2023

Yesterday: We had another beautiful winter day yesterday with partly cloudy skies and high temperatures in the low to mid 20’s F, with lows in the low to mid 20’s F. Winds were light out of the east, picking up to around 10-15 mph for a few hours last night with gusts of 20-25 mph. We did not see any precipitation across the advisory area.

Today: We should see another day of quiet weather today, with some high clouds and light west to northwest winds. High temperatures will be in the high teens to mid 20’s F, with lows in the high teens to low 20’s F.

Tomorrow: A low pressure system passing well to the south should bring increasing clouds and a trace of snow during the afternoon. The rain line should stay down around 500 feet. Easterly winds should increase tonight at 15-25 mph, with high temperatures in the low 20’s to 30 F, and lows tomorrow night in the upper teens to mid 20’s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 0 0 50
Summit Lake (1400′) 16 0 0 39
Alyeska Mid (1700′) 27 0 0 36
Bear Valley – Portage (132′) 25 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 E 8 25
Seattle Ridge (2400′) 25 SE 6 14
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.