Avalanche forecast

Maritime

Valdez Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

The Avalanche hazard is CONSIDERABLE above 1500 feet.  Heavy snowfall at upper elevations and heavy rain at lower elevations has elevated the avalanche hazard.  Human triggered avalanches are likely up to 4 feet in depth above 2500 feet and wet loose avalanches are possible where rain on snow is occurring.  Natural avalanches will be possible today.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Storm slabs continue to build and will vary in depth and composition depending on elevation.  Thompson Pass received another 6 inches in the last 24 hours, 4 of which fell overnight.  Winds increased to moderate overnight and are expected to taper through the day along with precipitation.  An increase in the wind will be creating deeper pockets of storm snow that will be more sensitive to human triggers.  

    Below 1500' storm slabs are not a concern as mostly rain has occurred at low elevations.

    Between 1500-3000'- There is a stout rain crust that is in the middle of nearly 2 feet of settled storm snow.  This rain crust will limit the snow available for transport to what is above the crust, which was observed to be 6-12 inches depending on previous wind exposure.  The snow beneath the rain crust is being insulated by the snow above and remains moist.  Propagation was observed at the interface of new/old snow and below the mid storm rain crust on 1/8.  

    Above 3000'- We dont have any observations since the latest round of storms began.  Storm snow depths are expected to be greater, and without the mid storm rain crust the snow is available for transport will be significantly greater allowing deeper windloaded pockets to form.

    Light snowfall will occur today through at least the morning hours which will continue to put a stress on an overloaded snowpack.  The mid-storm rain crust at mid elevations will disperse a person or machines weight to some extent, but building storm slabs and propagation in stability tests point to continued elevated hazard.  Human triggered avalanches remain likely today up to 4 feet in depth and may be able to step down or fail at persistent weak layers in the snowpack (see problem 2).  Conservative terrain choices and cautious route finding are recommended today.

    Photo: Gareth Brown

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Multiple potential weak layers exist in the mid snowpack.  These layers are of most concern in the mid elevation band, but some may exist at upper elevations as well in isolated areas.

    First and most concerning is the 12/5-10 rain crust sandwich. This layer consists of 2 rain crusts that are separated by about 4 inches with faceted snow in between.  The upper rain crust has lost most of its strength and propagation in stability tests has been observed multiple times this week failing between the crusts.  This layer exists below 3500' and is on average nearly 3 feet deep in the snowpack.

    Second is a layer of surface hoar that was recently buried on 1/4.  Despite rain on snow, this layer was insulated by dry snow that fell at the beginning of the storm, and has been found intact and reactive in stability tests as recently as 1/8.  The distribution of this layer is less continuous than the rain crust sandwich as an uptick in north wind prior to the storm destroyed this layer in many locations above brush line.  This layer continues to exist below brush line and in isolated wind-protected alpine areas.

    Lastly is the 12/22 BSH which was found to be unreactive prior to this round of storms.  This is still mentioned simply because the stress of above this layer has increased and buried surface hoar can lay dormant, and then activate once a significant load is applied.  Reactivity of this layer is unceratain.

    Digging snowpits is the best way to identify persistent weak layers, and ECT and PST tests are the best way to evaluate their sensitivity.

    Faceting melt form between the 12/5+12/10 rain crust at 2300' Girls/  ECTP11 down 80 cms

    Photo: Gareth Brown

  • Loose Wet

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

    Wet loose activity is becoming less likely as temperatures reached the freezing mark at sea level on 1/8.   Temperatures have returned to 40° at sea level but the transition from dry snow to wet has already occurred.   The peak natural activity has already passed along with peak temperatures and rainfall intensity.   The potential danger of wet loose avalanches is directly correlated with how wet the snow surface is and the vertical fall and steepness of the slope in question.  The larger the slope, the more entrainment that can occur.  Terrain traps will significantly increase the consequences of wet loose avalanches as even small slides can pile up deep debris.

    It is recommended to avoid traveling on or below terrain steeper than 30° where the surface of the snow is unfrozen. 

Avalanche Discussion

Observed Avalanche Activity last 72 hours: 

Natural D2+ D3 avalanche debris has been observed on Snowslide Gulch, South Three Pigs and Mt Tiekel.  There is likely more activity, however cloud cover has prevented natural activity observation.

Forecast Discussion:

  We have received 4 feet or more of snow above 3000' since the 4th.  This amount of snow puts significant stress upon the snowpack.  Untill precipitation stops and snowpack is allowed time to accept this new weight, Human triggered avalanches will be likely.  Precipitation is expected to lighten up today but will be on the increase once again tonight and likely through the weekend.  If you decide to travel in avalanche terrain today use cautious route finding, avoid terrain traps, maintain good group communication and only expose one person to a slope that is steeper than 30° at a time.  Signs of instability such as shooting cracks, collapsing or recent natural avalanche activity are all direct indicators to travel on slopes less than 30°.

It is not just the amount of new snow that is a concern.  But there are also potential persistent weak layers in our snowpack, creating not just a higher likelihood for avalanches to initiate but also leading to the possibility of step-down avalanches that would be much larger and more destructive.

Storm totals are unclear currently, but this is the information we have:

Valdez: ~3" storm total water (mostly rain)

12 mile: 3.57" storm total water (mostly rain)

Thompson Pass: 50" snow  estimated 

Nicks Snotel: 30" snow (strong winds have likely redistributed snow from this site)

Additional Information:

Continue to avoid Glide cracks in the area where they exist from Nick's Buttress through Cracked Ice at mid elevation.  Some of these are being filled in by snow and no recent activity has been noted

If you travel in the Continental zone today, exercise extra caution as there is very limited field data from this location.  It is unclear as to whether deep slab instabilities exist, however propagation at the ground on an ECT was observed in a snow pit as recently as 1/2.

If you get out in the mountains today consider leaving an observation to help increase the understanding of current snowpack stability.  

A weather and snowpack season history is available by clicking on the Weather History tab that is located above the bottom line statement once you have entered the full forecast.

Season Snowfall Data

Location 24 hour Snowfall Inches- 1/8 24 hour Snow Water Equivalent Inches Height of Snow January Snowfall Season Total Snowfall Valdez     0    N/O    19     10     45 Thompson Pass    6    N/O   N/O    58    N/O Nicks    2    N/O    73    33 N/O 46 Mile    3     N/O   20    9 N/O

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