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. New snow has begun to accumulate with moderate winds, creating the possibility for storm slab avalanches to be human triggered up to 1 foot in depth.  The amount of new snow that is fallen combined with what we are expecting today is expected to be insufficient to create natural avalanches during the day today.  The hazard will be rising as snowfall continues and becomes heavy overnight.

Avalanche forecasts are published Wednesday through Sunday.  The next forecast will be available on 1/8.  The hazard will likely rise to Considerable on Monday as significant snowfall accumulations are expected with rain at low elevations.

Problèmes avalancheux

  • Storm Slabs

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

    We are at the beginning of what is forecasted to be a significant series of storms that may last through the week.  So far weather stations are reporting 3 inches of new snow with Thompson Pass being on the higher end at 8 inches.  An additional 8-12 is forecasted in the next 24 hours with 2-4 inches of that expected during the daylight hours.  Winds have been moderate so far with gusts to 20 mph on Nicks station during the day before dying off overnight.  Despite a forecast for Southeast winds, the direction remains Northeast as cold air entrenched in the interior wins out for now.

    Expect to find shallow storm slabs with instabilities possible in areas that are loaded deeper by the wind that is blowing from a variety of directions.  This new snow is falling on a variety of snow surfaces including, Surface Hoar at low elevations and potentially in alpine areas protected from recent north winds, old stubborn wind slabs, and old powder.  Areas where new snow is falling on the surface hoar will be the most sensitive.  However, the amount of new snow is expected to be insufficient to create a widespread problem today.  This situation will be changing rapidly as heavy snowfall is expected to arrive tonight.

    Pay attention to the depth of new snow and the amount of wind redistribution in the area you choose to travel.  Hand shear tests and no consequence test slopes are a great way to quickly gather information about how well the new snow is bonding with older surfaces.  Watch for signs of instability such as shooting cracks and collapsing, indicating unstable snow.  Where these exist, it is recommended to move to slopes with a lower angle.

    Expect for the depth, distribution, and sensitivity of this avalanche problem to increase in the days to come as snowfall continues through the week.

     

    Photo: NWS

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1.5–2.5

    There are multiple layers that could be consisdered a persistent slab issue currently. Here they are listed below starting near the surface.  There is a layer of surface hoar that was buried on 1/4 that is significant where it survives, but it's distribution is expected to be much spottier that the 12/22 layer described below.  Inconsistent distribution is due the North wind we experienced from 1/2-4.  Protected areas below brush line will have this layer and isolated alpine areas that were free from North wind.  The snow above this layer is currently insufficient to act as a slab, but that can change in a hurry.  It is unclear at this point how much of an effect this layer will have on future stability.

    There is a layer of surface hoar that was buried on 12/22 that is now about 16 inches beneath the surface.  So far this layer has not been reactive in stability tests and the only human triggered activity was the day after a storm.  The surface hoar grains have been observed to be deforming (rounding) and is mixed with other non-faceted grain types as it has been pushed into the soft snow below.  As new snowfall accumulates a couple things could happen.   This layer has the potential to become more sensitive and become reactive as stress increases, or it could be crushed further into the soft snow beneath and become a non-issue.  It is worth continuing to dig for this layer and test it for propagation potential as heavy snowfall is expected to accumulate this week.  This layer has been found on all aspects and has been identified up to 5000'.

    Below the surface hoar is a rain crust sandwich with factes in between.  These crusts extend up to 3000-3500'.  Below 2000' the upper crust is still supportive and is not a current concern.  Between 2000' and where the crust goes away, the upper crust has lost enough strength to become a weak layer.  Collapsing has been observed with failure on small steep slopes that occurs in the factes between the crusts, with the snow above acting as a slab.  This is a persistent avalanche problem; however, it is only a concern in very specific terrain due to the elevation band where it exists.  Most areas where this layer is reactive there are thick alders that act as anchors and prevent a continous slab from forming.  This layer could be a concern above terrain traps, and steep open slopes without vegetation below 3000'.  New snow will be putting additional stress on this layer

     

    1/2-  Surface Hoar/ Max High 2500' /South aspect.  This layer is now in the process of being buried.

    Photo: Gareth Brown

  • Glide Cracks

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2–2.5

    Numerous Glide Cracks exist on the North facing Buttressess of Nicks through Cracked Ice between 2000'-3000'.  Identify where these are if you decide to travel in this area and avoid traveling on or below them.  Glide cracks act differently than all the other avalanche problems and can fail even when the general hazard is low and without signs of instability.  Stability of Glide cracks has more to do with the relationship with the friction at the ground and less so to do with the weather that is effecting the surface like other avalanche problems.  Little is known about assessing glide cracks, so travel advice is simple: Avoid traveling on or below them.

    Numerous Glide Cracks on the Cracked Ice Buttress

    Photo: Kyle Sobek

Avalanche Discussion

Observed Avalanche Activity last 72 hours: 

NONE

Forecast Discussion:

A pattern change is here!  New snowfall amounts are light at the time of writing, but that is forecasted to change in the next 24 hours as heavy snowfall is expected to arrive tonight.  The NWS forecast discussion states that "Precipitation will remain nearly continuous through much of the first half of the week for the coastal areas."  It is expected this will be the last day of moderate avalanche hazard as the snowpack is being stressed by heavy precipitation.  Snow lines are expected to rise on Sunday night and settle between 1500-2000'.  This will create wet loose avalanche problem at low elevations as rain on snow is expected on Monday.  The snowpack will be going through a test of its strength this week.  Persistent weak layers in the snowpack may be revealed later this week and produce natural avalanches.  Be patient during this time period when the snowpack is adjusting to its new load.

Beneath the surface the 12/22 layer of BSH that has been unreactive should continue to be monitored as an increase in stress from new snowfall could increase it's sensitivity.

A rain crust sandwich from early December, with facets in the middle  exists below 3500'.  This crust facet combo is concerning setup when it is examined on its own without putting into context where it exists.  The elevation band where this layer is the most sensitive is also where there is thick exposed alders currently.  It is expected for sensitivity to increase at this layer with new snowfall in isolated terrain.  This would include steep, vegetaion free slopes below 3000'.  The Buttresses of Nicks through Cracked Ice could be one of these areas, as well as steep slopes above river canyons.

Surface Hoar that formed during the last 6 days of clear weather is significant in places, however the recent uptick in wind has destroyed it in many locations above 3000'.  It is also possible if the freezing line actually does rise to 2500' that rain will eliminate this layer where it was most pronounced.

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- 1/4 24 hour Snow Water Equivalent Inches Height of Snow January Snowfall Season Total Snowfall Valdez     3    N/O    17     3     35 Thompson Pass    8    N/O   N/O    8    N/O Nicks    3    N/O    50    3 N/O 46 Mile    3     .2   24    3 N/O

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