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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 23rd, 2021 - 7:00AM
Expires
Wed, March 24th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE today, and it is still possible to trigger an avalanche on persistent weak layers buried 1-3′ deep. The most likely place to trigger an avalanche is in steep, previously wind-loaded terrain, with stiff snow near the surface. People have triggered avalanches on these layers in the past two days, clearly showing that these layers are still reactive. Be mindful of this lingering problem while choosing your terrain today, and be aware of increasing likelihood of triggering an avalanche as winds pick up this afternoon.

SNUG HARBOR/LOST LAKE/SEWARD: Winds are once again expected to be stronger as you head towards Seward. Be aware of increased likelihood of triggering an avalanche where winds are transporting snow into sensitive wind slabs.

Tue, March 23rd, 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. 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

Wilson Headwall: A skier triggered an avalanche on a hard wind slab on top of facets on Wilson ridge. The skier was able to self arrest. They were the 4th set of tracks on the slope. More details here.

Looking up at the avalanche path on Wilson Ridge. 03.22.2021. Photo submitted anonymously.

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

A persistent weak layer of buried surface hoar and near surface facets that got buried earlier this month continues to show that it still capable of producing avalanches, with skier triggered avalanches in the Summit Lake area yesterday (details), and near Girdwood on Sunday (details, video below). Both of these avalanches were big enough to bury a person, and similar activity remains possible today. The most likely place to trigger an avalanche is on steep, previously wind-loaded terrain with relatively stiff snow at the surface. As winds pick up during the day today (see problem 2 below), the likelihood of triggering a persistent slab avalanche may increase.

Safe travel today will require maintaining the same mindset we have had for the past few weeks. Be wise with your terrain selection, avoiding steep slopes with terrain traps below, like trees, rocks, cliffs, or gullies. Be careful to identify and avoid slopes that have been previously wind-loaded, with stiff snow at the surface. If you do choose to get into steep terrain, be sure to only expose one person at a time to the avalanche path, and keep an eye on your partners from safe zones.

Sluffs: Steep slopes that have been sheltered from the wind have around a foot or more of loose snow sitting on top of firm surfaces. It will be easy to trigger dry loose avalanches 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.

Avalanche Problem 2
  • 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 are expected to increase this afternoon. Be aware of sensitive wind slabs forming as the winds pick up, especially near ridgelines, below convexities, and in cross-loaded gullies. As mentioned above, these stiffer wind slabs could increase the likelihood of triggering an avalanche on persistent weak layers (buried surface hoar and near-surface facets) below the surface.

Link to video here:

Looking down the path of a skier-triggered avalanche in Ragged Bowl. Note the small debris pile from a pocket that released in the foreground, as well as the larger debris pile at the bottom of the bowl. 03.22.2021.

 

Weather
Tue, March 23rd, 2021

Yesterday: High clouds moved in during the day, with mostly sunny skies in the morning turning into cloudy skies by early afternoon. High temperatures were in the low 20’s to low 30’s F, with overnight lows in the single digits to mid teens F, dipping below zero in the Summit Lake area. Winds were light out of the east, with a few hours of slightly stronger winds in the afternoon around 10-15 mph. No precipitation was recorded.

Today: Skies are expected to clear up during the day, with scattered clouds this afternoon. Temperatures are expected to reach the low to upper 20’s F, with winds increasing to 10-20 mph out of the west. No precipitation is expected.

Tomorrow: Skies are expected to start off with a few clouds, with increasing cloud cover as chances for precipitation increase later in the day and overnight Wednesday. Winds are expected to decrease slightly, dropping back down to 5-15 mph out of the west. Low temperatures tonight are expected to be in the low to mid teens F, with highs tomorrow expected in the low to upper 20’s F. We might get a couple inches of snow Wednesday night into Thursday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 0 0 111
Summit Lake (1400′) 14 0 0 45
Alyeska Mid (1700′) 21 0 0 114

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 E 7 25
Seattle Ridge (2400′) 17 SE 7 17
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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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