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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, April 1st, 2021 - 7:00AM
Expires
Fri, April 2nd, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE at all elevations. Triggering a slab avalanche that fails on a layer of buried weak snow 1-3′ deep is still possible on slopes 35° and steeper. In addition, watch for small fresh wind slabs in upper elevation terrain as the NW winds continue today. Pay attention to any surface snow warming, avoid travel under glide cracks and don’t get surprised by your sluff.

SUMMIT LAKE/SNUG/LOST LAKE/SEWARD:  Stronger NW winds continue today in these zones and natural wind slab avalanches are possible. Additionally, slabs may step down into buried weak layers and create a larger avalanche.

 

Special Announcements
  • Skookum Valley is closed to snowmachines as of today, April 1st. This closure occurs annually as per the Chugach NF Forest Plan.
  • Heading to Hatcher Pass? Be sure to check the Thursday Conditions Summary at hpavalanche.org.
Thu, April 1st, 2021
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. Dangerous avalanche conditions. 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
  • 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

With temperatures in the Alpine forecast to be in the single digits and low teens today, it seems like the arrival of spring is saying a big “April Fools’!” And instead of March coming in like a lion and going out like lamb, it’s more like most of the month has been the movie Groundhog Day. The weather forecast this morning is pretty similar to many of the days in March (and February). That being said, there is no reason to complain about cold soft snow sticking around on shady aspects. The only real sign of spring today is the sun crust on SW through SE slopes.

The main avalanche issue continues to be the possibility of triggering an avalanche on buried weak layers in the top 3′ of the snowpack. We know there are a couple of layers of buried facets and surface hoar across the forecast area. Snowpack tests have produce varied results, with some showing the potential for an avalanche to be triggered and some pointing towards stability. It has been a week since the last reported human triggered avalanche that failed on a buried weak layer. However, natural avalanches occurred in Summit Lake on Monday. Persistent weak layers in the snowpack often present this feeling of uncertainty and we aren’t ready to forget about them.  Steep terrain with wind loaded snow over the weak snow is the most suspect today. Be on the lookout for areas that were previously wind loaded and those with any new wind loading from the NW winds yesterday and today. Lurking slabs will likely be at upper and mid elevations, immediately below ridgelines, in cross-loaded gullies, and below convexities.

As always, use good travel protocol, and think about terrain consequences.   Hard wind affected surface snow might allow you to get well out on the slope before the avalanche releases and it might not be the first person that triggers the avalanche.

Wind Slabs: Observers yesterday reported a bit of snow moving with the northwest winds and if winds bump up today we could see more of that. Watch for pockets of fresh wind slab in upper elevation terrain. Look for shooting cracks and avoid hollow sounding snow in steep terrain. There is a blowing snow advisory for Whittier this morning.

Loose snow avalanches: Steep slopes that are sheltered from the wind have loose snow sitting on top of firm surfaces. It will be easy to trigger dry loose avalanches (sluffs) in that terrain today, and they can pick up enough volume and speed to carry a person. While it is unlikely they will be big enough to bury you, they can be dangerous if they drag you into terrain traps like cliffs, trees, rocks, or gullies.

Sun effect: Colder temperatures and NW winds should keep most surface snow from warming too much today. However, steep protected southerly slopes (especially under rocks) may get some warming from the afternoon sun. If crusts soften and/or you see any roller balls, move to a shadier aspect and colder snow.

Buried surface hoar in a snow pit on Tincan yesterday, 3.31.21. This same weak layer was reactive on Eddies on Tuesday but not on Tincan yesterday. This really illustrates the variability in the snowpack and the lingering possibility of finding the wrong spot and triggering an avalanche.

 

Weather
Thu, April 1st, 2021

Yesterday: Skies were mostly cloudy with temperatures in the teens in the alpine and high 20°Fs to low 30°Fs near sea level. Winds were westerly 5-10 mph with gusts into the teens. Overnight skies were mostly cloudy with temperatures in the teens and low 20°Fs. Winds were northerly 5-10 mph.

Today: Chance of snow this morning with decreasing clouds today and skies becoming partly sunny in the afternoon.  Ridgetop temperatures will be in the single digits to low teens and valley bottoms will be in the high 20°Fs. Winds will be from the NW 5-15 mph with gusts into the 20s. Overnight skies will be partly cloudy with temperatures in the single digits to mid teens.  Winds will continue from the NW 5-10 mph with gusts into the 20s.

Tomorrow: Skies will be partly cloudy with cloud cover increasing through the day. Temperatures will be in the 20°Fs to low 30°Fs and winds will be light and westerly.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 0 0 110
Summit Lake (1400′) 25 0 0 50
Alyeska Mid (1700′) 24 0 0 117

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 W 6 18
Seattle Ridge (2400′) 19 NW 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.