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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, March 15th, 2024 - 7:00AM
Expires
Sat, March 16th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 1000′. Many human triggered avalanches 1-3′ deep occurred yesterday, which is the number one sign of dangerous conditions. Today human triggered avalanches up to 3′ deep in the new snow from this week remain likely, especially in areas that saw strong winds overnight. In addition there are a variety of different buried weak layers 3-5′ deep that could cause larger and more destructive avalanches. We recommend conservative decision-making today and sticking to low angle slopes to avoid the potential for triggering a large avalanche.

Below 1000′ the avalanche danger is MODERATE. There is an icy crust up to about 700′ from wet snow and rain that fell on Tuesday afternoon. Avalanches in the new snow above that crust are possible and awareness of overhead runout zones is recommended, especially in areas with active wind loading.

Special Announcements

Turnagain Pass Avalanche Awareness Day – Saturday, March 23!
Swing by the Turnagain motorized parking lot between noon and 4pm to grab a hotdog, practice your beacons skills, chat with the forecast team, and possibly test out a demo snowmachine provided by local dealers.

Fri, March 15th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sat, March 16th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Sat, March 16th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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

There were a lot of human triggered avalanches yesterday, thankfully as far as we know nobody was buried or injured by any of the avalanche involvements. Human triggered avalanches were reported on Sunburst, Tincan, the front side of Seattle Ridge, the back bowls of Seattle Ridge, and Explorer glacier. Natural avalanches were also observed across the forecast area from the aftermath of the past 4 stormy days.

On Sunburst a group of skiers triggered an avalanche on the SW face at about 3100′ that was roughly 150-200′ wide and 2′ deep. The skier who triggered the avalanche was the 5th skier on slope and the avalanche took out several existing ski tracks when it released. Over on Seattle Ridge there were several smaller human triggered avalanches in main bowl, triangle bowl and some unknown locations. On Explorer glacier an avalanche was triggered about 18″ deep by 30′ wide by 2 skiers descending steep terrain.

Thank you, to everyone who submitted observations yesterday!!!

Skier triggered avalanche in motion with skier standing above crown for scale. Photo from Billy Finley 3.14.24

One of several avalanches triggered on the back side of Seattle Ridge yesterday. Photo from Anonymous 3.14.24

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • 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.

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

The results are in, and based on the 8 or more human triggered avalanches yesterday the verdict is that the new snow is not bonding well to the old snow surfaces. This is not too surprising given that Turnagain Pass and Placer Valley received 3-5′ of new snow this week. That is a huge amount of new weight added to the snowpack, and it takes some time for the snowpack to adjust to such a big loading event. Hopefully the new snow will become more stable relatively quickly, but for today we recommend continuing to take a cautious approach and stick to lower angle slopes. If you decide to push into steeper terrain we recommend being very selective about the terrain you choose and avoiding any slopes where terrain traps in the runout could cause avalanches to bury a person more deeply or carry a person into trees or other undesirable locations. The size of storm slabs and wind slabs that could be triggered today are much larger than normal, easily reaching 2-3′ deep just within the layer of new snow. In addition we have concerns about buried weak layers 3-5′ deep that could create much larger avalanches (more in problem 2).

Overnight the winds pickup up a bit, with averages of 10-15 mph and gusts up to 30 mph out of the east. There is plenty of soft snow on the surface right now to be redistributed into fresh wind slabs 1-2′ deep. These fresh wind slabs that formed overnight could be easily triggered by the weight of a skier or rider. Keep an eye out for active wind loading along ridgelines, areas with hollow feeling snow on the surface, and use small test slopes to check for shooting cracks or small avalanches before committing to larger terrain.

Loose snow avalanches are very likely on steep terrain features. On northerly aspects dry loose avalanches or sluffs could be quite large and run quickly due to the clear skies yesterday drying out the snow on the surface. On southerly aspects there will likely be a crust today from the strong sunshine yesterday. If it ends up being sunnier than expected today those crusts could melt in the afternoon and create prime conditions for wet loose avalanches.

Skier triggered avalanche on Explorer glacier that likely failed within the new storm snow from this week, about 1.5′ deep by 30-50′ wide. Photo from Anonymous 3.14.24

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

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

Underneath all that new snow from this week there are a variety of different buried weak layers about 3-5′ deep that seem to be causing human triggered avalanches. On southerly aspects there is a sun crust that formed last week which has developed some facets around it and is likely the culprit for the large human triggered avalanche on the S face of Sunburst yesterday. In addition there was a widespread layer of surface hoar on the surface this weekend that was buried by the new snow and could be responsible for some of the reports of remotely triggered avalanches that came in from Seattle Ridge yesterday. Buried surface hoar is a notorious weak layer that can cause avalanches on low angle slopes, be triggered from adjacent terrain above, to the sides, or below the slope, and tends to be difficult to identify in a snowpit.  We saw this layer on the surface from valley bottoms to ridge tops prior to the storm, so even if we are not finding it in snowpits we can assume it is preserved in some locations and has the potential to cause large avalanche with no warning signs.

Despite the current avalanche conditions being complex due to the distribution of different weak layers, the advice is simple: stick to low angle terrain and avoid areas with terrain traps below that could cause avalanches to be more dangerous.

Remotely triggered avalanche in Main Bowl on the back side of Seattle Ridge that may have been caused by a layer of buried surface hoar. Photo from Anonymous 3.15.24

Weather
Fri, March 15th, 2024

Yesterday: Mostly sunny skies with calm to light winds during the day averaging 0-10 mph out of the WNW with gusts to 15 mph. Overnight the wind direction changed to ENE and the wind speed increased to averages of 10-15 mph and gusts up to 28 mph. Temperatures were in the teens to low 20s F at upper elevations and low 20s F to low 30s F at mid elevations. The sun was strong, and had a significant impact on melting the snow surface on southern aspects.

Today: Another day of mild weather is expected on Friday before the next storm system enters the area on Saturday morning. Winds are expected to remain in the 10-15 mph range out of the east with gusts of 20-30 mph through the afternoon and then decrease to averages of 0-10 mph on Friday evening. Temperatures will start out in the teen to low 20s F and rise to the high 20s or low 30s this afternoon. Snow flurries are possible, but there is no real accumulation of new snow expected. Cloud cover is the biggest question mark today, we are on the edge of a low pressure system and could either see mostly sunny skies or mostly cloudy skies, depending on how that low pressure system shifts throughout the day.

Tomorrow: Snowfall is expected to start again on Saturday morning, with rain line initially starting out near sea level and then rising to about 1000′ by Saturday evening. Snowfall amount are expected to between 6-12″ throughout the day on Saturday and into Saturday night. Continued snowfall is expected on Sunday with rain line increasing to about 1400′. Winds are expected to increase as the snowfall starts on Saturday morning, with averages of 20-30 mph out of the east and gusts of 40-50 mph. Temperatures are expected to reach the low 30s F at upper elevations and mid 30s F at low elevations on Saturday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 0 0 109
Summit Lake (1400′) 19 0 0 46
Alyeska Mid (1700′) 24 0 0 109
Bear Valley – Portage (132′) 20 0 0
Grouse Ck (700′) 23 0 0 74

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 ENE 6 28
Seattle Ridge (2400′) 18 SE 5 13
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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.