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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 15th, 2024 - 7:00AM
Expires
Fri, February 16th, 2024 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE as this week’s wind storm continues to make larger natural and human-triggered avalanches likely where sensitive wind slabs are forming. We are also concerned with a weak layer of snow buried 2 to 3′ deep that will make even bigger avalanches failing deeper in the snowpack possible, even in sheltered terrain. The continued warm temperatures and sunny skies are the wild card, which may increase the likelihood of these avalanches, as well as making wet loose avalanches possible.

Special Announcements

Summit Lake Avalanche Accident:  An avalanche was triggered on Tuesday afternoon by a group of three backcountry skiers on John Mtn. Two of the three people involved sustained injuries and the third did not survive. Our deepest condolences go out to the friends and family of the deceased. We have posted a preliminary accident report here and will publish a full report by the end of next week.

 Chugach State Park:  We are also expecting dangerous avalanche conditions to continue in the front range because of these strong winds and warming temperatures.

Thu, February 15th, 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
Fri, February 16th, 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
Fri, February 16th, 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
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

 

A snowmachine triggered a 3′ deep slab in the Placer Valley around 2,500′ in elevation (Squirrel Flats area). They were able to ride away without getting caught or carried. We also noticed multiple natural avalanches in the mid and upper elevations along the Seward  Highway from the Placer Valley through Summit Pass.

This avalanche was triggered by a snowmachine yesterday in the Squirrel Flats area above the Placer Valley. Photo: Jonathan Tymick, 02.14.2024.

 

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

The relentless wind storm we’ve been experiencing this week is expected to continue today, which means we are still expecting to find dangerous avalanche conditions. The current setup is much more complicated than a typical wind slab problem, because of the strength and duration of this wind storm, the dangerous weak layer buried below the surface, and the unusually warm temperatures we are experiencing. We’ll dig into the details below, but the important thing to keep in mind is that we are looking at a really challenging and potentially dangerous snowpack right now so the best way to stay out of trouble is to avoid travelling in steep terrain until the snowpack settles down.

Today will be the fourth consecutive day of sustained 20 to 50 mph easterly winds. At some point we are going to run out of soft snow for the wind to move into fresh slabs, but based on the massive amounts of snow we saw blowing off ridgetops yesterday we are still expecting to find reactive wind slabs forming today. The wind slabs forming with winds this strong will likely be very stiff, which may make it possible to get way out onto a slope before triggering an avalanche. We are also seeing strong winds dipping down into our treeline elevation band so be on the lookout for wind slabs in unusual places today. This loading event is adding stress to a dangerous weak layer, and that combination of a fresh wind slab on top of a weak snowpack was responsible for the fatal avalanche accident in Summit Pass on Tuesday. This avalanche problem is not straightforward. The best way to manage it for now is to keep a conservative mindset and stick to lower angle terrain.

Wet Snow Avalanches will also be possible with another day of unusually warm temperatures. This is the time of year when we notice the sun starting to effect snow surfaces on solar aspects again, and that increased solar input combined with the current heat wave will increase the chances of loose wet avalanches. It may also increase the likelihood of triggering a bigger avalanche on weak snow buried in the upper snowpack (more in problem 2).

Cornice Fall: This wind event has made some very large overhanging cornices. The combination of strong winds, warm temperatures, and a lot of solar warming will increase the likelihood of natural cornice failures. Be sure to limit time spent traveling under them.

These natural avalanches on the east side of Wolverine above the Placer Valley are an example of the complex problem we’re still expecting to see today. They were probably triggered by heavy wind loading, but look to have failed on persistent weak layers deeper in the snowpack. 02.14.2024

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

The weak layer of faceted snow that formed in January is still a major concern for today. This was the culprit in the avalanche accident in Summit Pass on Tuesday, and we have seen it throughout our advisory area. The distribution of the layer is tricky to identify. The snow is probably weakest in sheltered terrain between roughly 1000′ to 2000′, but it is experiencing the most stress at and above 2000′ as the strong easterly winds continue to load the snowpack. The problem layer can be tricky to identify with any of our quick travel tests like hand pits or test slopes, so if you really want to assess the problem you’re going to have to take the time to dig a snowpit. Based on the potential size and consequences of these bigger avalanches, it may be a better idea to just simplify your terrain choices and stick to mellow slopes until this problem heals.

This layer of faceted snow was the layer responsible for the avalanche accident in the Summit Pass area on Tuesday, and it exists throughout the advisory area. 02.14.2024

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

The widespread glide activity is still on our radar, and we’re not sure how this warm spell is going to impact the problem. As always, avoid travelling under slopes with open glide cracks since they release unexpectedly. Some glide cracks are harder to identify since they were buried by last week’s snow, so look carefully and be aware of the potential for falling into a crack that is bridged over by a thin layer of snow.

Weather
Thu, February 15th, 2024

Yesterday:  The wind storm we’ve been experiencing all week continued yesterday, with sustained speeds of 20 to 45 mph and gusts of 55 to 70 mph through the day. Skies were mostly sunny with unusually warm temperatures for this time of year. High temperatures were in the mid 30s to mid 40s F with lows in the mid 20s to mid 30s F. Many stations did not get below freezing last night. No precipitation was recorded.

Today: Today is looking to be similar to yesterday. Temperatures are expected to remain warm, with highs in the low to mid 30s F and lows in the upper 20s to low 30s F tonight. The strong easterly winds are expected to continue today, with average speeds around 25 to 40 mph and gusts of 40-60 mph. Skies should be mostly sunny and no precipitation is expected.

Tomorrow: We are looking at hopefully just one more day of this relentless wind event, with winds slowly starting to back down from 40 to 20 mph through the day. Skies should be clear with high temperatures in the upper 20s to low 30s F, and overnight lows should cool to the mid 20s F. No precipitation is expected.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 38 0 0 86
Summit Lake (1400′) 34 0 0 43
Alyeska Mid (1700′) 37 0 0 95
Bear Valley – Portage (132′) 45 0 0
Grouse Ck (700′) 33 0 0 62

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 31 ENE 32 73
Seattle Ridge (2400′) 30 ESE 12 27
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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.