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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, April 20th, 2023 - 7:00AM
Expires
Fri, April 21st, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Large human triggered avalanches releasing in deeply buried weak layers 3-6′ deep are possible. Consequences of being involved in an avalanche this size are severe so we recommend conservative terrain selection if you are accessing upper elevation northerly slopes. Wet loose avalanches are likely to release naturally later in the day as the sun and warm temperatures melt the surface of the snowpack. These are most likely on southern aspects, but could also occur on east or west aspects.

Below 2500′ the avalanche danger is MODERATE. Warm temperatures and sunshine will cause wet loose avalanches to release naturally later in the day. These are most likely on sunny east, south, and west aspects, but with temperatures climbing into the mid 40s F at lower elevations wet avalanches are also possible on north aspects.

PORTAGE VALLEY hikers/bikers/xc skiers: Be aware of avalanches occurring overhead as the day heats up. This area can see large wet slides that can run close to commonly traveled areas.

SEWARD: A layer of buried surface hoar 1-2′ deep has been reported. Large avalanches releasing on low angle slopes may still be possible so extra caution is recommended.

*FRIDAY AVALANCHE OUTLOOK:  There will be no forecast issued tomorrow, Friday, April 21. The next forecast will be Saturday, April 22. With stable sunny weather in the forecast zone we expect very similar conditions to Thursday.

Thu, April 20th, 2023
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

A large avalanche was triggered yesterday by the third skier on slope on a north to northwest aspect on Big League in the Virgin Creek drainage in Girdwood. The depth of the avalanche ranged from 2-4′ deep and it propagated 200′ wide across several ridge and gully features on a complex piece of terrain. The avalanche released about 30′ above the skier who triggered it and the skier was swept 1000′ over multiple cliff bands, finally coming to rest partially buried at approximately 2800′. Due to the trauma of being carried through the steep terrain the skier suffered a fractured femur and required a helicopter rescue. Our best wishes go out to the injured skier and their party. Thanks to the local skier who saw the avalanche crown from Max’s and responded to assist with the rescue efforts. Thank you all for sharing your experience! (see ob here)

Avalanche crown along the N ridge of Big League with the party waiting for a helicopter rescue in the lookers right lobe of the debris. Photo Peter Ostroski 4.19.22

Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

Based on the avalanche triggered in Girdwood yesterday we are elevating the deep persistent problem from an additional concern to problem 1. There is a lot of uncertainty about how widespread this avalanche problem is, but it seems most likely that a buried weak layer of facets from mid-March are the culprit in this very near miss. The depth of the avalanche, wide propagation, and fact that it was the third skier on slope all point towards a deeply buried persistent weak layer. It is possible that the warm air temperatures yesterday afternoon started to warm up the snow on the surface, even on this north facing aspect, and made this avalanche ripe for triggering. This avalanche was definitely unexpected from our perspective and could be an indication that the warming temperatures are starting to reactivate buried weak layers.

Very large human triggered avalanches 3-6′ deep will remain possible during the warm and stable weather over the next two days. Dry snow conditions are holding on at upper elevations on northerly aspects and therefore the snowpack is still behaving as if it is mid-Winter. Assessing snowpack stability using snowpits tends to be unreliable with deeply buried weak layers. Due to the potential consequences of being surprised by another avalanche like the one on Big League, we recommend conservative terrain selection if you are seeking out dry snow on high elevation north aspects.  

Another view of the large skier triggered avalanche on Big League in Virgin Creek in Girdwood. Photo 4.19.23 by Peter Ostroski

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

Temperatures are slowly creeping up and starting to reach into the high 30s or low 40s F even at upper elevations. These warming temperatures combined with strong solar warming could be enough to start causing widespread wet loose avalanches as the day heats up in the afternoon. Below 1500′ the snowpack is capped with a supportable melt freeze crust on the surface that is melting in the afternoon. Most of the wet loose avalanche activity we have observed recently is on steep southerly aspects, but as the temperatures increase these could become more widespread on east and west aspects as well.

The advantage of wet loose avalanches is that they are fairly predictable in terms of timing and aspect. East aspects exposed to the sun tend to heat up first, then southern aspects, and finally west aspects later in the day. You can monitor the conditions by looking for roller balls and wet loose avalanches releasing naturally from steep rocky terrain as an indication that the snowpack is weakening. It is also helpful to pay attention to how deep the wet snow is on the surface where you are travelling because that gives you a sense of how large the wet loose avalanches could become. If you are sinking in up to your ankles in wet snow then that is a sign to move to a shadier aspect because wet loose avalanches that release naturally could grow to become large enough to bury a person if they run down a large slope. So far the mountains have been mostly holding together and there has been less wet snow avalanche activity than normal for this time of year. With the warm temperatures and sunshine over the next two days this could change quickly to the beginning of a spring shed cycle.

Recent wet loose avalanches releasing naturally from steep south facing terrain. Photo 4.19.23

Weather
Thu, April 20th, 2023

Yesterday: Sunny skies and calm winds. Temperatures started out in the high teens to low 20s F and rose to the mid 40s F at lower elevations and high 30s at upper elevations.

Today: Another beautiful day with mostly sunny skies and calm winds expected. Temperatures should rise a little higher today, with highs in the upper 30s F expected at upper elevations.

Tomorrow: High pressure should continue through Friday with clear skies, light winds, and no precipitation. Temperatures will continue to reach into the upper 30s to low 40s F at upper elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35 0 0 88
Summit Lake (1400′) 33 0 0 42
Alyeska Mid (1700′) 35 0 0 82
Bear Valley – Portage (132′) 33 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 30 NE 3 11
Seattle Ridge (2400′) 32 N 1 4
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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.