Avalanche forecast

Continental

Valdez Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 2 years ago.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

Highlights

The avalanche hazard is Moderate above 2000 feet and Low below.  Human triggered avalanches are possible up to 1 foot in depth on recently wind loaded slopes.  Natural avalanches are unlikely. 

Avalanche forecasts are issued Wednesday-Sunday at 8 am.

Avalanche problems

  • Wind Slabs

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

    North outflow winds that occurred for about an 8 hour period on the evening of the 15th through the early morning of the 16th have come to an end.  Winds have already switched around to the South with the approach of the next weak system.  Wind slabs up to a foot in depth formed during this period in specific areas and will be our main concern today.  The sensitivity of these will be decreasing from yesterday but will still be possible for a person or machine to trigger.  Light snowfall today is not expected to increase the avalanche hazard.

    Watch for signs of previous wind loading like textured snow surfaces, and fresh windlips that will point towards wind loaded slopes.  Wind slabs are easier than other avalanche to avoid because they can be visually identified unlike persistent slabs and they only exist in specific areas unlike storm slabs.  Wind slabs often have a rounded pillowed look, or may have a rippled texture.  Using small inconsequential wind loaded test slopes is a good way to test the sensitivity of this problem.  Look for cracking on these test slopes that would indicate wind slabs would have the energy to produce an avalanche on a larger slope.

    There is a small chance that wind slab avalanches could step down to deeper layers in the snow in isolated areas and create very large avalanches. (see problem2)

    Flagging off high elevation ridges near Thompson Pass the evening of the 15th

    Photo: Gareth Brown

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2.5–3.5

    Prior to last weekends snowfall event there were multiple persistent weak layers that were being tracked, including a couple of layers of buried surface hoar and a rain crust sandwich with facets in the middle.  These exist primarily in the mid elevation band and a couple of them were beginning to show propagation in stability tests as snow steadily accumulated from the 4th-10th.  The 11th-12th brought 100" of snowfall to Thompson Pass in 48 hours which overloaded these layers in many areas with natural and artillery triggered avalanches the result.  

    These layers are now deeply buried by 1.5-2 meters or more of settled storm snow.  As precipitation has taken a break, the snowpack is allowed to settle and slowly accept the weight of this new snow.  Over time, this decreases the likelihood of human triggered avalanches at these layers due to their depth and an overall decrease in sensitivity.  

    It is now unlikely to affect these deep layers and trigger one of these avalanches.  We are leaving this problem on our list for the time being as a conservative approach until we can test a wider variety of areas to confirm better structure and lower sensitivity than what existed prior to last weeks storm.

    If this problem were to arise it would probably be in an area where the recent storm snow was thin due to wind redistribution during the storm, with steep convex slopes in the mid elevation band.  Thin, rocky areas in steep terrain could be a place where trigger points exist as well.

     

    Natural avalanches from 1/12 that failed at a deep persistent weak layer.  

    It is possible these released sympathetically from the artillery triggered school bus slide.

    Photo: Gareth Brown

Avalanche Discussion

Observed Avalanche Activity last 72 hours: 

NONE.  

More info is available about the last avalanche cycle by clicking the Weather history tab or the Avalanches tab just below the bottom line statement.

Forecast Discussion:

Our snowpack has undergone a major transition after a prolonged period of snowfall that fell mostly above 2000'.  From 1/4-1/13 Thompson Pass DOT has recorded 168" of snowfall, with 100" of that falling in a 2 day period 1/11-12.  This has opened up a significant amount of access as bushes have largely been laid over above 2000'.  

Observations from the field over the last three days have found a snowpack that is accepting the new weight of the snow, and is without any major flaws in the deep storm slab.  There was a lot of wind with the major snowfall of this weekend so areas near wind channeled areas of Thompson Pass have shown variable storm slab depths with deep wind drifts in places.  Further away from the pass storm slabs were found to be more homogenous and right side up.  Deep persistent weak layers are now difficult to detect when probing but are likely 2 meters or more deep at this point.  This makes it difficult for a person to directly trigger them.

Wind slabs that formed on the 16th will be the most likely problem for people to encounter today and are expected to be in the 6-12" range in depth as moderate winds are only occurred for a short period.  These are expected to be more concentrated towards the wind funnel of the Thompson Pass summit.  

We need more data points before we can remove the deep persistent problem.  However, everything we have seen so far after the storm is pointing towards improving stability.

Additional Information:

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.

Glide cracks that existed on the buttresses from Nicks through Cracked Ice on North aspects at mid elevation were stagnant prior to the recent snowfall and are now filled in.  It will be worth watching for them to reappear, but this may be a problem of the past.

Season Snowfall Data

Location 24 hour Snowfall Inches- 1/17 24 hour Snow Water Equivalent Inches Height of Snow January Snowfall Inches Season Total Snowfall Valdez     0    ~    19     10     45 Thompson Pass    0    ~   N/O    170    N/O Nicks    0    -    97     N/O 46 Mile    0     ~   20    10 N/O

Regions covered