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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, April 27th, 2024 - 7:00AM
Expires
Sun, April 28th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger will start off LOW and rise to CONSIDERABLE at all elevations today. Wet avalanches releasing as a slab several feet deep are the biggest concern. As the sun starts to heat up the snow surface wet avalanches will become more likely. To avoid being involved with a wet avalanche it is important to monitor the snow surface conditions and move to a shadier aspect when your boot sinks into the wet snow more than ankle depth. Glide avalanches and cornice fall are also a concern, which we can avoid by scanning slopes overhead and trying to avoid travelling underneath them.

Special Announcements

Avalanche Center End of Season Operations: The final forecast for the season is TOMORROW Sunday, April 28.

Closure of Twentymile and Placer River to Sled Access: Per Forest Service order these two riding areas are now closed due to inadequate snow cover at low elevation to prevent resource damage.

Sat, April 27th, 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
Sun, April 28th, 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
Sun, April 28th, 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 have been multiple observations of wet slab avalanches releasing during the heat of the day over the past few days, with the most notable being the W to NW facing avalanche path on Orca in Girdwood. There were multiple start zones that released at elevations ranging from 2000-2400′ on SW through W aspects. The debris ran out to treeline on a NW facing avalanche path all the way down to roughly 300′ in elevation.

Overview of massive wet slab avalanche that released in multiple start zones near 2400′ and ran full path down to roughly 300′. Photo 4.26.24

Avalanche Problem 1
  • Spring Conditions
    Spring Conditions
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Spring Conditions
Warmth has a tricky effect on snow. On the one hand it speeds up the stabilization of the snowpack (reduces the chance of slab avalanches). But a SUDDEN rise of temperature increases the chance of slab avalanches considerably. When this warm period is followed by cooling down, then the chance of slab avalanches reduces. Even more so: the more often the temperature changes, the more stable the snowpack becomes when looking at slab avalanches. Once the temperature becomes too warm we have to deal with wet snow avalanches.

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 is a classic ‘springtime conditions’ type of day, where avalanche danger will start off low while the snow surface is frozen and gradually increase on sunny aspects as the crust starts to melt and wet avalanches become likely in the afternoon. The clear skies and temperatures dropping to freezing level or below overnight should create a very strong crust. How quickly the crust will soften, and avalanche conditions will become more dangerous, will depend on aspect and elevation. Eastern aspects will start to soften first, followed by southern aspects, and then western aspects in the afternoon. At upper elevations temperatures are cooler and light east winds might help keep the surface crust intact later into the day.

Wet slab avalanches are the biggest concern today because they can be very large, destructive, and unpredictable. The large wet slab avalanche that released on Wednesday on Orca in Girdwood is a good reminder of the potential size of these avalanches and how unmanageable they are if a person is caught in one. The best way to avoid these conditions is to monitor the snow surface conditions and look for any wet loose avalanches or roller balls releasing naturally, which can be a precursor to wet slab activity. Another rule of thumb is to move onto a shadier aspect once your boot starts to sink in more than ankle depth.

Wet loose avalanches are also likely as the surface starts to soften and can become quite large if they run down long slopes. Be aware of other groups that might be below you to avoid having a small wet loose avalanche triggered up high becoming a potentially dangerous situation for folks further down slope.

Glide avalanches and cornice fall are also a normal part of the spring shed cycles. These tend to be less predictable hazards in terms of timing, but are most likely to release during the heat of the day. The best way to avoid these hazards is to simply avoid spending time underneath open glide cracks or large corniced ridgelines. Giving cornices a wide berth when you are traveling on a ridge is also a wise move when they are getting direct sunlight.

Close up of the wet slab crowns that released on the SW face of Orca near 2300′. Photo from 4.26.24

Weather
Sat, April 27th, 2024

Yesterday: Skies started out mostly cloudy yesterday morning but trended towards mostly sunny in the afternoon. Winds stayed light with averages of 0-15 mph and gusts up to 25 mph out of the east. Temperatures rose to highs in the upper 40s F at sea level, low 40s F at mid elevations, and low to mid 30s F at upper elevations. No precipitation was recorded.

Today: Mostly sunny skies are expected today with winds decreasing slightly this morning to averages of 0-10 mph out of the east. Temperatures look slightly warmer than yesterday, with highs expected to reach the low 50s F at sea level, mid 40s F at mid elevations, and high 30s F at upper elevations.

Tomorrow: Sunday looks largely similar to Saturday with clear to mostly sunny skies. Winds are expected to shift to the NW at 10-15 mph overnight and for the morning hours before shifting back to the east and dropping off to 0-5 mph on Sunday afternoon. Temperatures will bump up slightly into the mid 50s F at sea level, mid to upper 40s F at mid elevations, and 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′) 36 0 0 82
Summit Lake (1400′) 37 0 0 35
Alyeska Mid (1700′) 38 0 0 93
Bear Valley – Portage (132′) 40 0 0
Grouse Ck (700′) 39 0 0 61

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 28 ENE 8 23
Seattle Ridge (2400′) 32 ESE 6 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.