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'.  Recent snowfall is sitting on a weak form of snow, making human triggered avalanches likely 1-2 feet in depth.  An increase in North wind will add to the hazard as well. Watch for signs of instability such as shooting cracks, collapsing and recent avalanche activity that indicates unstable snow.  

Avalanche Forecasts are produced Wednesday-Sunday.  The next avalanche forecast will be produced on 1/1.

Avalanche problems

  • Wind Slabs

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

    North winds up to 40 mph were recorded on Thompson Pass yesterday, with lower speeds elsewhere. North wind is expected to continue through most of the day today before tapering off.  Strong wind is not expected to be widespread again today with the highest amounts expected through wind channeled terrain like the Thompson Pass Gap.  Upper elevation wind is forecasted to be light today.  Wind slabs may have formed on buried surface hoar in specific locations, making these areas very sensitive (see problem above).  Even where BSH does not exist, like areas close to the Thompson Pass Gap or high elevation ridge lines, the likelihood of human triggered avalanches will increase where wind reditribution has occurred.  This type of avalanche problem is most sensitive when it is forming or directly after, we are currrently in that time frame.

    Wind slabs will feel stiffer at the surface and upside down, and can have a pillowed look or a rippled surface texture.  Watch for signs of instability, such as shooting cracks, collapsing, recent avalanche activity, and active wind loading indicating the possibility of unstable wind slabs.

  • Persistent Slabs

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

    A layer of surface hoar that was buried on 12/22 (12/22 BSH) has been identified in the Intermountain and Continental zone up to 3800' on all aspects.  This layer has not been found yet in terrain that is close to the Thompson Pass gap probably due to pre-storm wind that would have disturbed it.  It is still unclear as to whether this layer exists in the Maritime zone.  12/22 BSH is now 10-18 inches beneath the surface after this week's gradual snowfall.

    Surface Hoar is a concern because it is known for being weak in shear,  can persist for long periods, and can fail even after multiple tracks have been laid down.  This layer has not yet shown signs of propagation in Extended Column Tests.  This is likely due to the surface snow being very uncohesive (no slab).  This puts us in a period where snowpack assessment will be more of a challenge due to several factors including: difficulty to identify, and changes in sensitivity spatially due to slab cohesion, weak layer distribution, and slope angle.

    Current instability in the upper snowpack is not expected to be widespread even though a weak layer may be present.  This is because the overlying snow in many locations, has very little slab character currently.  Slab character is a variable that can change with time as snow naturally settles and with location as areas with wind effect may have stiffer snow that is more prone to failure.   

    3-4 inches of additional snowfall accumulated yesterday along with an increase in North winds.  The uptick in winds was most notable through the Thompson Pass gap, less of an effect is expected in other areas, but an increase in the distribution of wind redistributed snow is expected.

      Watch for signs of instability, such as shooting cracks that could indicate the presence of a weak layer.  Pay attention to the amount of new snow that exists and the stiffness of the slab.  The stiffer the surface snow is, the more propagation potential there is.  Using small inconsequential test slopes can be a good tool today especially if you can find areas where the wind has stiffened the surface.  Be suspicious of hand shear results when performed in areas with light unconsolidated snow at the surface.

     

     

    (12/26/2024)

    12/22 BSH at 3800' crud

    Photo: Gareth Brown

Avalanche Discussion

Observed Avalanche Activity last 72 hours: 

12/28- A few dry loose slides reported on the East aspect of Mummy Mountain in the Maritime zone.

Our primary concern of the day will be the layer of surface hoar that was buried on 12/22, is 10-18" below the surface, and has been found in numerous locations away from wind channeled terrain in the low and mid elevation band.  It has not been observed at upper elevations yet, but it may exist there as well.  Currently the snow above the 12/22 BSH is uncohesive (no slab) in many locations.  This has changed in some locations as an increase in North wind will increase cohesion in locations where wind redistribution has occurred.

The most likely area for wind slabs to form will be terrain in close proximity to Thompson Pass Gap and upper elevation ridgelines.  These will be the areas, where it is least likely for 12/22 BSH to exist, however wind slab instability will be a concern.  Areas that are typically more protected and have seen some degree of wind redistribution will be the greatest concern today.  These areas may harbor buried surface hoar, with recent wind creating a slab.

Be aware that a wicked learning environment is likely in place.  This means that you could ski or ride a slope without triggering an avalanche, even though a weak layer may be present.  This would allow for a person to gain confidence.  As more confidence is built and the person thinks they are going through proper terrain progression, they may be getting closer to triggering an avalanche without knowing it.  Eventually, the person may choose steeper terrain where the slab is slightly stiffer and/or the weak layer is more pronounced and a slide is triggered.  

Unfortunately looking for and testing this layer is not always straightforward and can easily be missed even by experienced users.  Test slopes without consequence and some wind loaded snow can be effective in identifying this layer.  Pay attention to shooting cracks even small ones that can indicate the presence of this layer.  Only expose one person to an avalanche prone slope at a time and keep in mind that tracks are not a sign of stability when it comes to persistent weak layers.

Continue to avoid Glide cracks in the area where they exist from Nick's Buttress through Cracked Ice at mid elevation.

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 it is a possibility.

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- 12/29 24 hour Snow Water Equivalent Inches Height of Snow December Snowfall Season Total Snowfall Valdez      N/O    17     27     35 Thompson Pass    3-4 N/O   N/O    N/O    N/O Nicks    2 N/O    53    27 N/O 46 Mile    2   N/O   22    12 N/O

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