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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, April 7th, 2021 - 7:00AM
Expires
Thu, April 8th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. Triggering slab avalanche is possible in the top 3′ of the snowpack. Recently wind loaded slopes are the most suspect. Watch for stiff, drifted snow and cracking. Remember triggering a shallow avalanche could step down to older layers of buried weak snow.  Assess the snowpack as you travel, identify areas of concern and evaluate terrain consequences. Give cornices a wide berth and watch your sluff if you find soft snow on steep slope.

Below 1000′ the avalanche danger is LOW. There is a hard melt-freeze crust at the surface and triggering an avalanche is unlikely. As always, watch for changing conditions.

SUMMIT LAKE/SNUG HARBOR/LOST LAKE/SEWARD: These areas have been hit hardest by the recent NW winds, making avalanche conditions especially sensitive. Use extra caution if you are getting out in any of these zones.

Special Announcements

Hatcher Pass:  The road to Hatcher Pass remains closed at MP10. For updates go to AK 511 and follow AKDOT&PF on social media. Click HERE for a link to drone footage of the extent of avalanche debris from the Easter avalanche cycle.

Wed, April 7th, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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.
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 winds are forecast to be mostly light today. However, the winds over the past couple days moved a lot of snow around, impacting surface conditions and creating stiff slabs at upper and mid elevations. There were a number of natural wind slab avalanches observed on Monday as well as a few human triggered ones, including a snowmachine triggered slide on the Seattle Creek Headwall. Today these slabs will likely be less tender and prone to triggering but a caution on steep wind loaded slopes is still advised, especially immediately below ridgelines, in cross-loaded gullies, and below convexities. On some aspects these slabs formed over weak snow (more in Problem 2) and/or on a crust. Due to this structure it possible that a person could make it out into the middle of a slope before triggering an avalanche, or for multiple skiers or riders to put tracks on a slope before somebody triggers an avalanche. Avoid areas that look pillowed and watch for cracks shooting out from your snowmachine or skis, and ‘punchy’ feeling stiff snow over softer snow. Cornices may be sensitive today and triggering a cornice fall could trigger an avalanche on the slope below.

Loose snow avalanches: Steep north facing slopes that are sheltered from the wind have loose snow sitting on top of firm surfaces. Triggering dry loose avalanches (sluffs) in that terrain is possible today.

Natural wind slab avalanche on Tri-Tip peak in Summit Lake that likely occurred Monday afternoon or evening. Photo taken yesterday, 4.6.21.

 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • 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.
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

As mentioned in a number of recent forecasts, we seem to be stuck in a pattern of clear and sunny weather where weak surface snow forms and then it snows a little, and then the wind blows. Each time this happens we end up with another buried weak layer covered by a wind slab. Due to the weak snow (facets and/or surface hoar) it’s hard for the slab to bond to the snow below and we develop a persistent slab issue. With this happening over and over we have a variety of weak layers in the upper 3′ of the snowpack.  Today is an ‘on the fence’ definition day. If you trigger an avalanche it could be a wind slab or persistent slab. Or triggering a shallow wind slab could step down to an older weak layer and trigger a larger avalanche. Some of the recent avalanches likely failed on buried weak layers. The bottom line is that steep slopes with wind affected snow should be approached with caution and the consequences of a slope sliding, and where the snow would go should always be considered.

An Extended Column Test on an East aspect at 3700′ on Fresno propagated at 18 taps on buried facets.

Weather
Wed, April 7th, 2021

Yesterday:  Skies were clear with high temperatures in the low teens at upper elevations and high 20°Fs in the valleys. Winds were northwesterly 5-10 mph with gusts into the teens. Overnight skies remained clear with low temperatures in the single digits to mid teens. Winds were light and westerly.

Today: Increasing clouds with light west winds and temperatures in the teens to high 20°Fs. Snow showers starting this evening and continuing into tomorrow. Overnight temperatures will be in the 20°Fs and winds will be from the southeast 15-25 mph with gusts into the 30s.

Tomorrow: Cloudy with snow showers tapering off in the afternoon. Temperatures will be in the 20°Fs and winds shift to the northwest in the early morning starting out 5-10 mph and increasing to 20-30 mph in the afternoon. Gradual clearing expected overnight with dropping temperatures and strong NW winds forecast to continue into Friday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 0 0 109
Summit Lake (1400′) 12 0 0 47
Alyeska Mid (1700′) 13 0 0 119

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 W 7 18
Seattle Ridge (2400′) 16 W 2 9
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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.
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