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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, March 28th, 2021 - 7:00AM
Expires
Mon, March 29th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′, where strong northwesterly winds have been forming sensitive wind slabs since yesterday afternoon. It is likely a person could trigger an avalanche 1-2′ deep on steep slopes near ridgelines, below convexities, or in cross-loaded gullies. These wind slabs are forming on top of weak surfaces, making it possible to trigger an avalanche remotely from above, below, or adjacent to a slope. The danger is MODERATE below 2500′, where it remains possible to trigger an avalanche on multiple layers of facets and surface hoar buried in the upper 3′ of the snowpack. Cautious route-finding will be key today, especially avoiding steep, wind-loaded slopes.

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

Sun, March 28th, 2021
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. 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
  • 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

Winds picked up yesterday afternoon and continued overnight, with sustained ridgetop speeds in the 15-20 mph range and gusts into the mid 30’s near Girdwood and Turnagain Pass, and 53 mph in the Summit Lake area. We saw these winds moving a massive amount of snow yesterday, and expect this has continued through this morning. This snow has been deposited into sensitive slabs, which are sitting on top of weak facets that rest on crusts on solar aspects and on firm older wind surfaces on shaded slopes. All of this is pointing to an elevated likelihood of human-triggered avalanches around 1-2’ deep today, especially at upper elevations.

Increasing cloud cover during the day may make it difficult to travel into the alpine, but if you are trying to get up high be on alert for these fresh wind slabs. As winds die down this morning, it will become less likely to see natural activity, but human-triggered avalanches will remain likely on steep, wind-loaded slopes. You are most likely to find them at upper elevations below ridgelines, on the downslope side of convexities mid-slope, or in cross-loaded gullies. Pay attention to warning signs of instability like cracks shooting out from your snowmachine or skis or the audible whumpf of the snowpack collapsing under your feet.

Loose snow avalanches: Steep slopes that have been sheltered from the wind have up to a foot of loose snow sitting on top of firm surfaces. We have seen a lot of loose snow avalanches (sluffs) since Thursday, and similar activity will be expected today. 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.

Snow blowing off the ridgeline of Bench Peak at 4250′. 03.27.2021

 

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

Our persistent weak layers (buried surface hoar and facets) that formed earlier in the month are still lurking in the upper 3’ of the snowpack. The strong winds from yesterday and last night have increased the load on these weak layers on some slopes, pushing the needle towards instability. These weak layers are present through most of our advisory area as well as the peripheral zones. In addition to the travel advice mentioned with respect to the wind slab problem mentioned above, it is still important to keep these layers in mind while moving through the mountains. Avoid consequential terrain that raises the stakes of getting caught in an avalanche, and be sure to only expose one person at a time to steep slopes and runout zones if you are trying to access steep terrain.

Multiple weak layers in the upper snowpack from a test pit yesterday. 03.27.2021

Weather
Sun, March 28th, 2021

Yesterday: Skies were partly cloudy to mostly sunny, with high temperatures in the mid teens F at upper elevations and in the upper 20’s to low 30’s F at lower elevations. Northwesterly winds picked up in the afternoon, with sustained ridgetop speeds of 15-20 mph and gusts around 35 mph near Girdwood and Turnagain Pass, and as high as 53 mph near Summit Lake. No precipitation was recorded.

Today: Northwesterly winds will gradually calm down this morning, shifting to the southeast at 5-10 mph later in the afternoon as another round of active weather approaches. Skies are expected to start out partly cloudy, with increasing cloud cover through the day. High temperatures are expected in the upper teens to upper 20’s F, with overnight lows in the single digits to upper teens F. Chances for precipitation start to increase late in the afternoon today, with a system that resembles last week’s storm.

Tomorrow: Snow showers continue tonight and through the day tomorrow, with 6-8” expected near Girdwood and 2-4” on Turnagain Pass. Highs are expected in the low 20’s to low 30’s F, with winds at 5-10 mph out of the south. Fingers crossed for cold temps and plenty of snow!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24 0 0 112
Summit Lake (1400′) 21 0 0 48
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′) 11 WNW 9 36
Seattle Ridge (2400′) 18 NW 11 36
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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.