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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, April 13th, 2024 - 7:00AM
Expires
Sun, April 14th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations today. A weak layer buried 1-2′ deep has the potential to cause large human triggered avalanches today, and could even be triggered from low angle terrain adjacent to steeper slopes. The sun is due to make an appearance this afternoon, which could quickly create conditions for wet loose avalanches at lower elevations and on solar aspects.

Special Announcements

Avalanche Center End of Season Operations: This is the final week of daily 7am avalanche forecasts. Beginning April 15 we will forecast on Tuesday, Thursday, Saturday, and Sunday mornings. For the non-forecast days, we will give an avalanche outlook on the day prior. The final forecast for the season will be April 28.

Headed to Hatcher Pass today? Check out Hatcher Pass Avalanche Center to get the latest information on avalanche conditions.

Sat, April 13th, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sun, April 14th, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sun, April 14th, 2024
Alpine
Above 2,500'
2 - Moderate
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 remote triggered avalanche from the Girdwood area was reported yesterday, which released about 2′ deep and took out some old ski tracks. As far as we know no one was caught or involved with this avalanche. These are concerning signs that the old snow interface from early this week still has reactive persistent weak layers that are capable of producing human triggered avalanches. In the Girdwood area this layer is buried roughly 2′ deep, while in Turnagain Pass the layer is likely closer to 1′ deep on the north end of the pass and shallower on the southern end of the pass.

Remote triggered avalanche from Girdwood area. Photo 4.12.24 from Anonymous

Avalanche Problem 1
  • 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

Just as we were starting to hope that the layer of buried surface hoar and surface facets that was buried on Tuesday (4/9 interface) was becoming unreactive, we heard about a large human triggered avalanche in the Girdwood valley yesterday. The fact this avalanche was triggered remotely, meaning a person triggered the slide from adjacent lower angle terrain, and that it took out a bunch of old ski tracks are strong signs of a lingering persistent weak layer in the snowpack. The Girdwood area was heavily favored by the snowfall on Tuesday, with 1.5-2′ falling compared to 6-10″ in Turnagain Pass, which means the weak layer is buried more deeply and the potential size of human triggered avalanches is larger in the Girdwood area.

We recommend carefully evaluating how well the snow from this week is bonding to the old snow surface before committing to steeper terrain today. Persistent weak layers can cause avalanches on lower angle slopes than typical and can produce wide avalanches that connect across multiple terrain features. On southern aspects these weak layers are likely sitting on top of a crust, which could make them more likely to produce larger and wider avalanches.

As the sun starts to come out this afternoon it could melt any surface crusts in the upper snowpack, which may increase the potential for triggering a persistent slab 1-2′ deep. Since there is no significant weather stressing the snowpack today we think the chances of natural avalanches are pretty low, however a falling chunk of cornice or wet loose avalanche could act as a trigger for this deeper weak layer. To avoid the problem entirely you can always stick to lower angle terrain.

Another photo of Girdwood area avalanche from 4.12.24

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

It looks like the sun might wait until the afternoon before making an appearance today, but since it is mid April it doesn’t take long for the sun to start melting the surface snow if skies clear. Keep an eye out for wet snow on the surface and roller balls or small avalanches releasing from solar aspects, especially in thin rocky areas. Wet loose avalanches are typically small, but on large slopes they can pick up a lot of snow on the way down and turn into larger avalanches. You can always switch to a shadier aspect to avoid wet avalanche problems.

Strong sun can also cause cornices to start shedding, and there are a lot of ridgelines still holding onto humongous cornices. Try to avoid spending time underneath cornices if they are receiving direct sunlight and give them a wide berth if you are travelling along a corniced ridge. If you are lucky enough to find some sheltered north facing terrain still holding soft snow, dry loose avalanches are also likely in steep terrain today.

Example of roller balls on steep solar terrain from earlier this winter. Photo 3.21.24

Weather
Sat, April 13th, 2024

Yesterday: The clouds never quite cleared out yesterday afternoon, with lingering mostly cloudy sky cover throughout the day. Snow showers brought 1-3″ of new snow and despite the warm temperatures at low elevations, snow line looks like it remained at sea level. Winds backed off in the morning and averaged 0-15 mph out of the east with some gusts up to 25 mph before noon. Temperatures at low elevations reached into the mid to upper 30s F while staying in the high teens to low 20s F at upper elevations.

Today: Snow showers are expected to linger in the area this morning with no real accumulation expected. Clouds cover should start to clear out this afternoon and evening. Winds are expected to be light, averaging 0-10 mph from variable directions. Temperatures should reach the mid 30s F at lower elevations with highs in the mid 20s F at upper elevations.

Tomorrow: Clearing skies are expected to dominate on Sunday, with light NW winds averaging 5-15 mph. Temperatures are expected to stay relatively cool for this time of year, with highs in the mid to upper 30s F at low elevations and mid 20s F at high elevations. No new snowfall is expected on Sunday and the clearing trend looks like it will last through Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 1 0.1 101
Summit Lake (1400′) 30 1 0.1 47
Alyeska Mid (1700′) 30 3 0.2 116
Bear Valley – Portage (132′) 35 0 0.2
Grouse Ck (700′) 36 2 0.2 77

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 ENE 6 28
Seattle Ridge (2400′) 24 SE 4 15
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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.