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Issued
Sun, March 7th, 2021 - 7:00AM
Expires
Mon, March 8th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger is rated MODERATE. It is possible a person could trigger an avalanche on wind slabs that started to form in the past 24 hours and continue to develop today. These fresh wind slabs will be forming on a widespread layer of surface hoar that has been growing for the past few days, which will make them especially sensitive, and may allow avalanches to propagate wider than they normally would. In addition to the wind slab problem, we are still concerned with the potential to trigger a deeper avalanche 1-3’ deep on persistent weak layers that are buried in the upper snowpack.

SEWARD/LOST LAKE: These areas have seen stronger winds since yesterday, and more wind is expected today. This will increase the size and likelihood of wind slab avalanches, and extra caution is warranted if you plan on getting out in these zones.

Sun, March 7th, 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
  • 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

Westerly winds began to pick up yesterday afternoon and are expected to continue until this afternoon, building fresh wind slabs 4-10” thick. These wind slabs are expected to be more sensitive than usual, since they are forming on top of a layer of surface hoar that has been growing during the clear weather for the past few days. That surface hoar will potentially allow wind slabs to propagate wider than they usually do, and it might be possible to trigger them remotely from low-angle terrain connected to steeper slopes. Some parts of the core advisory area will see more wind than others, with the strongest winds expected on Seattle Ridge, the south end of Turnagain Pass towards Johnson Pass, and the Placer Valley. These areas are expected to have more sensitive conditions, while areas around the Girdwood Valley and the east side of Turnagain Pass are expected to see less wind, lowering the likelihood of fresh wind slabs forming.

Fresh wind slabs will feel stiffer than snow that has been sheltered, and may appear as a smooth, rounded pillow. They will usually present clear warning signs like shooting cracks, or the ‘whumpf’ of the weak layer collapsing below them. Be suspect of the terrain features that commonly harbor fresh wind slabs– just below ridgelines, below convexities, or in cross-loaded gullies.

Sluffs: Steep slopes that have been protected from the wind and the sun have 2-6” of soft snow on the surface. This snow is poorly bonded, which makes it easier to trigger dry loose avalanches (sluffs). Be aware of the potential for sluffs to gain volume and speed in steep terrain, since these can be dangerous if they carry you through steep terrain.

A fresh layer of surface hoar at 2600′ on Eddies yesterday. This was present from valley to ridgetop, and will be very sensitive if covered by wind slabs today. 03.06.2021

Sun crust at 2000′ on  a southwest aspect. The warmth of the sun is starting to effect the snow surface. 03.06.2021

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

We are still concerned with two persistent weak layers buried in the upper 3’ of the snowpack. These layers have been on our radar for weeks now, and they are slowly gaining strength. But they continue to show some unstable results in stability tests, and produced human-triggered avalanches last weekend. These weak layers appear to be the most sensitive at elevations below 2000’, where weak snow is sitting on top of a rain crust that formed in late January. It can be difficult to navigate this type of avalanche problem since it will not always present clear warning signs prior to avalanching. The best way to reduce your risk with a widespread persistent weak layer is to avoid terrain where triggering an avalanche will have serious consequences. Specifically, these are features where an avalanche would run over rocks or cliffs, or through trees, or where it would pile up especially deep in an abrupt transition or a gully. We want to stop preaching about these weak layers just as bad as you want to stop hearing about them, but for now they are still a concern.

Weather
Sun, March 7th, 2021

Yesterday: Skies were mostly sunny yesterday, with some periods of cloud cover and light westerly winds at 5-10 mph at ridgetops. High temperatures were in the upper teens to low 30’s F, with overnight lows in the single digits to mid teens F.

Today: Northwesterly winds are expected to continue this morning, with ridgetop speeds expected at 5-20 mph. In the core advisory area, the strongest winds are expected around Seattle Ridge and the Johnson Pass area. Further south, the mountains around Seward are expected to see more wind, with sustained speeds around 30 mph until this afternoon. High temperatures are expected to be in the low to upper 20’s F, with clear skies and no precipitation.

Tomorrow: Low temperatures are expected to be in the single digits to low teens F, with winds shifting to the south overnight and temperatures rising tomorrow into the upper 20’s to low 30’s F. Skies are expected to be mostly sunny during the day, but low pressure is expected to move in starting tomorrow night, increasing cloud cover and winds. Our chances for precipitation increase later in the day Tuesday, but at this point it is looking more like a dripping faucet than a firehose.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20 0 0 111
Summit Lake (1400′) 15 0 0 45
Alyeska Mid (1700′) 22 0 0 113

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 10 W 7 16
Seattle Ridge (2400′) 17 WNW 3 12
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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.