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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Highlights

The avalanche hazard is Considerable above 4000' in specific locations.   Fresh wind slab created by north wind remains the main concern for today.  Human triggered avalanches are likely 6-18 inches in depth on the lee side of high elevation ridges and cross loaded terrain.  Hard snow over soft is a characteristic of wind slab and shooting cracks is a direct indicator of unstable snow .    The hazard will be lower in areas that are unaffected by wind.

Avalanche problems

  • Wind Slabs

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

    After a bit of a break from the wind on 12/2, north winds have gained strength again with gusts to 60 mph recorded on Thompson Pass at 6 am this morning.  Wind slabs were reported as being sensitive on the lee side of high elevation ridges with one skier triggered D2 avalnche reported on 12/2 that was 8-10 inches deep, no one caught or carried.  Wind slabs are expected to remain sensitive to human triggers 6-18 inches in depth in specific locations as a restrengthening of the north wind will continue to load lee slopes.  

     Pay attention to the direction of previous wind and amount of snow redistribution in the area you choose to travel.  Avoid steep terrain that has been loaded in the last 48 hours, especially areas in close proximity to Thompsom Pass where winds have been the strongest.  Wind slabs can be indentified as having a pillowed texture as well as stiffer snow over soft.  Shooting cracks and natural avalanche activity are a bullseye indicator of unstable snow.

     

    Photo: Gareth Brown

  • Persistent Slabs

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

    A natural avalanche was observed 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

Tha main stability concern for today has to do with wind slabs that have recently formed and are currently forming due to strong north wind over the last couple of days.  Unstable wind slabs are most likely going to be found at upper elevations on the lee side of high elevation ridges (E-NW) and in cross loaded terrain.

Below 4000' a rain crust exists with 6-12 inches of dry snow that fell last week on top.  A stable snowpack has been observed in these locations.  Below 4000' where the snow is not affected by wind triggering an avalanche is unlikely.  The exception could be cross loaded terrain where sensitive wind slabs may be lurking, so we have maintained a moderate rating for the mid elevation band.  

The first evidence of a persistent weak layer was observed yesterday in the Continental zone.  A widely propagating avalanche likely from 12/1 was observed on the upper elevations of The Shovel (40.5 mile peak).  Crown extended across the entire start zone and wrapped around a rocky terrain feature indicating a persistent weak layer as the failure plane.  It is possible the presence of this weak layer is confined to high elevation ( above the Thanksgiving rain crust) continental zone.  Although more data will need to be gathered.

Moving forward Surface Hoar was observed in the continental zone on 12/2 up to 4000'.  This will be the next weak layer of concern once snow returns to our area mid week and becomes buried.  Tracking the distribution of surface hoar will be important before the next storm arrives.  You can Help!!

 

Leave an observation if you observe surface hoar on your tour.  How big?,to what elevation?  And as always if you encounter signs of instability or avalanche activity we'd love to hear about it!

Next forecast will be posted Wednesday December 6th.

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