Bulletin d’avalanche

Continental

Valdez Avalanche Center
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Danger d’avalanche

Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 2 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

The avalanche hazard is Moderate at all elevations.  Expect to encounter a diverse range of wind-affected surfaces, including wind slabs. In certain areas, these wind slabs could be triggered by a person and may range from 1 to 2 feet in depth. The likelihood of natural avalanches occurring is low.

 

Avalanche forecasts are available from Wednesday to Sunday. The upcoming forecast will be issued on March 6.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    The main concern for avalanches today is wind slabs. Wind speeds are finally decreasing across all local weather stations after five days of strong, continuous north winds. The recent weather has significantly affected the upper snowpack, resulting in a variety of wind-affected snow surfaces, including sastrugi, wind slabs, and stripped snow surfaces. The sensitivity of wind slabs will vary based on their hardness and depth.

    In areas that have experienced strong winds throughout this period, expect to find surfaces that are scoured, mixed with very hard wind slabs ranging from 6 inches to over 3 feet deep. The harder and deeper the wind slab, the more challenging it will be to trigger an avalanche. This is because harder slabs are less likely to bend and crack under a person's weight.

    In more sheltered areas, where the wind was less intense or lasted for a shorter time, wind slabs will be thinner and softer. These are the areas where human-triggered avalanches remain possible. You can distinguish these slabs by whether you can break them by traveling over them or stomping on them. Be aware that the depth and hardness of wind slabs can vary even within a single slope. Triggering a wind slab is more likely at its thinner sections, but it can spread to deeper parts.

    Be vigilant about the depth and hardness of wind slabs and look out for signs of instability, such as shooting cracks or collapsing snow. Wind slab avalanches could reach deeper layers in the snowpack, resulting in larger and more destructive avalanches.

    Current typical snow surfaces consist of a mix of wind affected snow.  3/2  mid elevation East Francis

    Photo: Gareth Brown

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2–3

    Heavy snowfall and southerly storm winds previously overloaded our snowpack, triggering a widespread avalanche cycle on February 22. Several storm slab avalanches were released on a layer of surface hoar that was buried on February 17. Before the storm, surface hoar was observed in isolated areas from Valdez through Thompson Pass and northward along the Richardson Highway. This persistent weak layer may still exist in specific wind-protected basins and is expected to be around 2 feet deep. Currently, there is significant uncertainty regarding the extent and sensitivity of this layer. On February 22, this layer naturally cycled in some high elevation north-facing terrain but may still be present in protected areas at mid and upper elevations.

    At present, strong winds are forming a hard snow surface that is likely to bridge this deeper weak layer, reducing the likelihood of a person directly impacting it. However, in isolated locations, wind slab avalanches may still be able to step down to this layer.

     

    Natural avalanche 2/22 North Three Pigs

    Photo: Gareth Brown

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2.5–3.5

    The natural deep slab avalanche that occurred on Tone's Temple on February 24 has sparked concerns about potential deep-seated instabilities within our snowpack. The specific weak layer involved remains unidentified but seems to be close to the ground. Another smaller deep slab avalanche was noted in the rocky terrain on the south side of Girl's Mountain on February 27. These events are particularly alarming as they happened days after the previous avalanche cycle and reached layers deep within the snowpack. There is considerable uncertainty regarding the sensitivity of the layers involved in these significant avalanches. Currently, it is improbable that a person could directly trigger this problem. However, a cornice fall or another slab avalanche might propagate down to these deeper layers.

    Given the depth of this issue, accurate assessment is nearly impossible. This avalanche problem does not exhibit any typical signs of instability, and conventional snowpack assessment methods are ineffective for layers at this depth.

    The pressing question is whether we will witness further failures on this deep layer. Upcoming heavy snowfall could test the stability of this layer, though such conditions seem unlikely in the near to medium term. Meanwhile, it is recommended to minimize exposure to steep, rocky terrain, particularly on or near extensive continuous slopes.

    Tones Temple deep natural 2/24. Intiated from D1 natural wind slab

    Photo: Gareth Brown

Avalanche Discussion

The narrative of our snowpack continues to be dominated by north winds. We find ourselves nearing the end of a significant outflow wind event, resulting in a diverse array of wind-affected snow surfaces. Across most areas, we observe stripped and hardened old snow surfaces, with wind slabs present on specific terrain features. These wind slabs are now rock hard in most locations. As these slabs harden, their susceptibility to human-triggered avalanches diminishes significantly. However, areas traditionally sheltered from wind may still harbor wind slabs that remain possible to trigger. In these locales, where wind slabs are newer and relatively softer, human triggered avalanches remain possible.

The past week of persistent winds has wrought substantial changes upon our snowpack. The upper layers have suffered notable damage, resulting in a considerable loss of mass. Below 2500 feet, the impact has been somewhat mitigated by the presence of the rain crust from February 20th, which has acted as a barrier, limiting deep gouging and mass removal from the snowpack.

On both the 22nd and 24th, we observed several deep natural avalanches occurring, with failure points within the mid to lower layers of the snowpack. These layers are now unlikely for a person to trigger, as the rock-hard, wind-affected snow at the surface acts to bridge over potential weak layers buried deeper within.  However, smaller wind slab avalanches and cornice fall may still be able to step down to these layers.  

Régions couvertes