Avalanche forecast

Continental

Valdez Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

The avalanche hazard is Considerable above 4000 feet in the Continental zone, the hazard decreases with elevation.  Human triggered avalanches are likely 1-2 feet in depth above 3000 feet in areas where more than 8 inches of fresh snow has fallen in the past 24 hours and is being drifted by strong northeast winds.  Natural avalanches are possible.  Avoid slopes steeper than 32° where fresh storm snow is being loaded into deeper slabs.  The hazard will be lower in areas protected from the wind.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Snowfall has returned to our area with snow falling across the entire forecast zone as of 6am.  The coast has picked up a couple inches, 46 mile received 3-4 while Thompson Pass is experiencing blizzard conditions.  A blizzard warning is in effect for Thompson Pass and the upper elevations of Valdez with an additional 6-12 inches of snow forecasted.  

    The snowfall that we are expecting today will be low density and will be accompanied by strong winds (40-50 mph) out of the NE.  The low density snow that will arrive today will be easily moved into deeper slabs on the lee side of ridges, terrain features and cross loaded terrain.  Expect for these areas to be sensitive to human triggers  around 1-2 feet in depth in the specific locations mentioned above.  

    New snow will be falling on surface hoar created during the clear and cold weather of last weekend  in specific locations.  Surface hoar has been found below 4000' in places that were unaffected by strong outflow winds of last weekend.  More on this in the forecast discussion below

    Watch for signs of instability such as shooting cracks, collapsing and recent natural avalanche activity that would indicate elevated hazard.  For better stability and riding conditions seek out wind sheltered terrain.

     

    Wednesday 12/5 snow forecast

    Photo: NWS

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2.5

    New snow and wind has added stress to a potential persistent weak layer that may exist in isolated locations at high elevation in the Continental zone.

    A natural avalanche was observed on 12/1 around 5400' on The Shovel "40.5 mile peak" that had a very wide propagation.  Crown not only included the entire start zone, it wrapped around a rocky terrain feature indicating a persistent layer as the failure plane.  Crown width estimated at 1500' mapped on google earth, depth was difficult to tell but probably in the 1-2 foot range.  A few other naturals were visible on upper ramps of 40.5 mile, although they were not nearly as wide.

    It is possible this weak layer is isolated to high elevation areas (above the Thanksgiving rain crust) in the Continental zone.  This layer may have developed during the clear and cold period just before the Thanksgiving warmup.  It is likely this layer was only  preserved in areas that were sheltered from the strong winds that accompanied the 11/20 cold spell.  A lack of natural activity from other areas would support this idea.  Below 4500' this layer was likely rained away and in wind exposed terrain above 4500' likely destroyed.

    This layer has not been identified in snow pits or reported by others.  Therefore, there is a large amount of uncertainty dealing with distribution and sensitivity of this layer.  More information will need to be gathered.

Avalanche Discussion

The main stability concern for today involves new snow that is being drifted by strong northeast winds.  Upper elevations, and areas in close proximity to Thompson Pass will be the most prone to human triggers today.  The hazard will remain low at lower elevations where accumulations and winds are expected to be lighter and a strong base exists consisting of stout rain crusts.

Today, instability should largely be concentrated  with what is going on at the surface.   Surface instabilities are easier to deal with and predict because often times signs of instability will be present to warn us of hazard.  In addition the root cause of the instability will be quite literally in your face, new snow and strong wind.  The mid and lower snowpack have been found to be strong in most locations with persistent weak layers currently only a concern in isolated upper elvation areas of the Continental zone.  

This new round of snow is falling on surface hoar in specific locations.  This includes areas below 4000' where a strong inversion line sat last weekend, and in areas that were protected from the strong NE winds of last weekend.  It is unlikely that the current snowfall will overload this weak layer today.  The thinking being that areas that are receiving the highest snowfall amounts and most significant wind loading, will not have buried surface hoar present.  Temperatures at upper elevations were not conducive to significant surface hoar growth and areas receiving strong wind today would have received strong wind last weekend and detroyed the weak layer.

Areas where buried surface hoar does exist will likely be wind protected today.   With a moderate amount of low density snow forecasted cohesive slabs deep enough to overload this layer are unlikely, but may be a concern in isolated areas in the future.

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