Bollettino valanghe

Continental

Valdez Avalanche Center
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Pericolo valanghe

Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

The avalanche hazard is Considerable above 2000'.  Expect to find a wide variety of wind affected surfaces.  Human triggered avalanches are likely in specific areas and natural avalanches remain possible.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    The primary avalanche concern today is wind slab.  At Thompson Pass, north winds have consistently exceeded speeds in the 50s, with gusts surpassing 60 mph over the past 72 hours. This prolonged period of intense wind has led to significant snow redistribution, resulting in scoured surfaces in many areas and the formation of wind slabs elsewhere. Yesterday's light snowfall further contributed to wind slab development in the Thompson Pass and northern regions. Although winds will remain strong this morning, they are expected to gradually diminish in both intensity and distribution over the next 24 hours.

    The characteristics of wind slabs, including their depth, thickness, and sensitivity, will vary depending on location. In areas experiencing the strongest winds, anticipate encountering scoured snow surfaces mixed with deep, compact wind deposits that exhibit stubborn and unreactive behavior to triggers. Conversely, in more sheltered locations, wind slabs are likely to be thinner, softer, and more responsive to human triggers. With time, the sensitivity of these wind slabs will decrease as yesterday's new snow has already been redistributed, slabs settle with time, and as wind strength diminishes,  further snow redistribution will stop.

    Pay attention to the depth and hardness of windslabs and watch for signs of instability such as shooting cracks or collapsing.  Wind slab avalanches may be able to step down to layers deeper in the snowpack creating larger more destructive avalanches.

    Photo: Gareth Brown

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    Heavy snowfall and Southerly storm winds loaded our snowpack causing a widespread avalanche cycle on 2/22.  Several storm slab avalanches released on a layer of surface hoar that was buried on 2-17.  Before the storm, surface hoar was observed in isolated areas from Valdez through Thompson Pass and to the North on the Richarson Highway. This persistent weak layer may still exist in specific wind-protected basins and is expected to be buried around 2 feet deep.  There is currently high uncertainty regarding the extent of this layer and its sensitivity. This layer cycled naturally in some high elevation north facing terrain on 2/22, but may still exist in protected areas at mid and upper elevations.

    Currently, strong winds are creating a hard snow surface that is expected to bridge this deeper layer making it less likely for a person to directly affect.  Wind slab avalanches may be able to step down and affect this layer in isolated locations.

     

    Natural avalanche 2/22 North Three Pigs

    Photo: Gareth Brown

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2.5–3.5

    The Natural Deep slab that released on Tones Temple on 2/24 has raised questions about potential instabilities deep within our snowpack. The weak layer is still unknown but appears to be near the ground. An additional smaller deep slab was observed in rocky terrain off the south side of Girls Mt on 2/27.  These avalanches have been concerning because they occurred several days after the last avalanche cycle and dug into layers much deeper in the snowpack.  There is quite a bit of uncertainty regarding the sensitivity of the layers involved in these large avalanches. It is currently unlikely for this problem to be triggered directly by a human. A cornice fall or slab avalanche could possibly step down to these deeper layers. 

    Due to the depth of this problem, assessment is near impossible. This avalanche problem is not associated with any signs of stability. Additionally, standard snowpack assessment strategies are not possible with layers this deep.

    The big question is whether or not we will see any additional failures on this deep layer.  Future snowfall that provides a heavy load may be a good test of this situation, although this looks unlikely to occur in the short-medium term.  In the meantime minimizing exposure to steep rocky terrain especially on or near wide continuous slopes is advised.  

    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 polished old snow surfaces, with wind slabs present in specific terrain features. These wind slabs have progressed beyond mere firmness; they now exhibit a rock-hard consistency. 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 susceptible to human influence. In these locales, where wind slabs are newer and relatively softer, a Considerable avalanche hazard persists.

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.  Step down avalanches and cornice fall may still be able to affect these layers.  

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