Bulletin d’avalanche

Maritime

Valdez Avalanche Center
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Danger d’avalanche

Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

The avalanche hazard is Considerable above 3000 feet and Moderate below.  Numerous human triggered avalanches were reported yesterday at upper elevations 8-18 inches in depth in all forecast zones.  Strong south winds will occur today that will keep the hazard elevated in specific locations.  Wet loose activity will be likely today at low elevations.

Problèmes avalancheux

  • Wind Slabs

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

    Somewhere around a dozen human triggered D1-D2 avalanches were reported yesterday.  These reports came from every forecast zone and were initiated from at or above 5000' 8-16" in depth involving the most recent storm snow.  The sensitivity of these storm slabs is expected to decrease in some areas and increase in others.     

    Strong southerly winds will be on the increase.  We expect for wind slabs to form at upper elevations that will increase sensitivity of most recent storm snow by forming a deeper more cohesive slab.  Human triggered avalanches will be likely up to 2 feet in depth at upper elevations on steep freshly wind loaded terrain.

    In areas that are unaffected by wind, the likelihood of triggering an avalanche at the most recent storm snow is expected to be decreasing as there has not been a persistent (faceted) weak layer observed at the new/old interface. 

    We have switched the avalanche problem from storm slab to wind slab due to our thinking that the most likely place for a person to encounter unstable snow today will be in areas where active snow transport is occurring and wind slabs are forming.  Light snowfall with accumulations of 2-4 inches possible will increase the depth and sensitivity of wind slabs that form, and is not expected to elevate the hazard in protected areas withhout wind.  At mid elevations stable conditions exist with stability tests producing no propagation and storm totals being lower.  The caveat to this will be on solar aspects were wet loose may be possible in steep rocky terrain and in wind channeled terrain where cross or top loaded slopes may be encountered.

    Watch for signs of wind slab development such as flagging off high elevation ridges, and active wind transport.  Avoid steep terrain where new wind slabs have formed, these will be slightly stiffer than the snow underneath and may have a rippled texture or pillowed look.

  • Loose Wet

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

    Wet loose avalanche activity will be likely today.  Fresh snow that has fallen over the last few days will be shedding off steep solar aspects during the heat of the day.  Temperatures in Valdez did not freeze overnight, this will allow an increase of wet loose avalanche activity in the Maritime climate zone at low elevations.  Expect all aspects to be affected at low elevations today.  At mid to upper elevations wet loose activity may be limited due to cloud cover.

    Wet loose conditions are easy to identify.  Once more than a few inches of snow becomes moist-wet (you can easily form a snowball) then wet loose avalanches will become possible both human triggered and natural on steep terrain.   Roller balls and snow snails are a good visual indicator that wet loose is currently an issue.  Plan your route to avoid exposure to steep low elevation slope when the surface is not frozen.

    Avoid travelling on or below steep terrain where more than a few inches of moist snow exists.

    Natural D2 wet loose on solar aspect of Hippie Ridge

    Photo: Gareth Brown

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2.5–3

     A persistent weak layer consisting of buried surface hoar 2-3 feet beneath the surface exists in our snowpack above 3000'.  This layer has not been observed or reported to be reactive along the road corridor near Thompson Pass in the most commonly used areas.  Large human triggered avalanches have occurred in more protected areas beyond the road corridor where higher snowfall totals have occurred, and the layer in question likely formed more substantially and was buried more intact.  The last avalanche activity reported at this layer occurred on 3/2 with one instance involving a D2-2.5 avalanche that caught and carried a skier causing him to pull his airbag and lose some gear.  It is very likely that other avalanches have been triggered at this layer since and have not been reported.

    This layer has been observed in multiple locations above 3000' and has been observed to be rounding (gaining strength) and also mixed with other grains.   This is positive progress, however, we need to keep in mind that incremental loading of this layer can increase its sensitivity.  Storm slab avalanches may have enough force to step down to this weak layer.  Assesment of this layer will be challenging as it can be difficult to locate and ECT tests will be less effective as this layer becomes deeper with continued snowfall accumulation.  Propagation Saw Tests will be a more effective tool to test this layer.

     

    Propagation Saw Test (PST) in the Tsaina valley 4200' that was reactive on 3/29 BSH. PST END 30/100 down 95 cm.  

    Photo: Gareth Brown

Avalanche Discussion

RECENT AVALANCHE ACTIVITY:  

Many human triggered avalanches have been reported in the last 48 hours involving the most recent storm snow above 5000'.  These have been in the D1-2 range and have been 8-18" in depth.  Wider propagations have been reported in instances where the storm slab was stiffened by wind.

Last human triggered activity at 3/29 layer was reported on 3/2

DISCUSSION:

A low pressure system is currently rolling toward the southern Alaska coast.  As this low approaches, it will interact with a high pressure to the north bringing an increase to southerly winds.  Strong winds are expected in some locations that will form wind slabs that will be likely for a person or machine to trigger above 5000' up to 2 feet in depth.  In areas that are protected from this increase in south wind storm slabs are expected to increase in stability, however may still be possible for a person to trigger 8-16" in depth, especially on steep convex terrain.  Cloud cover will likely limit wet loose activity to low elevations, but may be active in the Port of Valdez as temperatures remained well above freezing.

We continue to be monitoring the 3/29 BSH layer.  Before the most recent snowfall, metamorphism has been observed in multiple locations with rounded buried surface hoar grains being observed.  Before the most recent snow, the chances of triggering an avalanche at this layer were unlikely.  The likelihood of affecting this layer has once again moved to possible due to the uncertainty regarding how the additional stress of new snowfall will affect this layer's sensitivity.  We are moving towards removing this layer from the problem list, however are waiting to see if continued snowfall allows this layer to reactivate.

 

Additional Concerns:

This is the time of year when cornices begin to fail.  You can significantly reduce your overall exposure in the mountains this time of year by avoiding traveling on top of or under cornices.

A very significant Glide avalanche/ landslide in Upper Stuart Creek last week is a great reminder of the destructive forces involved with glide crack failure.  Glide cracks exist in isolated areas and will be a hazard throughout the season until they fail.  Avoid exposure to glide cracks.

Valdez season snowfall: 286  inches

Valdez April snowfall: 11 inches

Valdez height of snowpack: 69 inches

Thompson Pass season snowfall: 698 inches as of 4/5

46 mile height of snowpack: 66 inches

 

Régions couvertes