Bollettino valanghe

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Haines Avalanche Center
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Pericolo valanghe

Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

The avalanche danger is CONSIDERABLE above 2,500ft and MODERATE above 1,500ft. Strong northerly outflow winds have tapered off after four days. These winds have scoured some slopes and loaded others, making the distribution of buried weak layers challenging. A sharp temperature rise today could increase the sensitivity for human triggering. Identify slopes greater than 30 degrees that could slide and practice safe travel techniques to limit your exposure. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–3

    We should treat buried weak layers as guilty until proven innocent, which involves slope-to-slope assessment. After a blast of northerly outflow winds, and a natural avalanche cycle last weekend, it is uncertain what buried weak have been re-loaded, what layers failed, and whether or not they were wiped out. A high degree of uncertainty requires wide safety margins.

    Known buried persistent weak layers included melt-freeze faceted layers, buried surface hoar, buried near-surface facets, and depth hoar. Last Sunday freezing levels increased to 3,500-4,500ft forming the "January Pineapple Express" (JPE) melt-freeze crust that was promptly buried the next day by 2-3ft of snow at upper elevations and then re-loaded by the W-NW-N winds.

    What is dangerous about persistent slabs and requires extra caution is the potential to propagate an avalanche from a thin area of snow to a deeper one that fails on buried weak layers across the terrain. Wind scouring and wind loading may have brought weak layers closer to the surface, or added stress via wind loading. Avoid high-consequence run-out zones and terrain traps.

    (2/1/2024)

    Faint crown lines on NE-aspect of Old Faithful Mountain at 4,200'. It is difficult to know whether this failure occurred more recently, or this past Sunday when temperatures rose. New snow may have re-loaded over old buried persistent weak layers.

Avalanche Discussion

The snowpack does not like rapid changes. A strong northerly outflow event brought W-NW-N wind through the region from Jan 31st-Feb 3rd. This was accompanied by another sharp drop in temperature with a maximum low of -15F at the Haines Pass. Another flip-flop weather event on Sunday will bring a sharp temperature increase. Look for signs of instability including cracking, collapsing, whumping, and natural avalanche activity.

Last weekend, we got walloped by a two-pulse late "January pineapple express" event with ~6" total SWE (snow water equivalent.) During the first wave, Jan 26-28th, 2-3" of SWE fell as temps climbed, with freezing levels peaking up to 5,000'.  Part two of the event on Monday, Jan 29th, was about the same 2-3"+ of SWE, but in 24 hours, and colder with 2,500-3,000' freezing levels and moderate to gusting strong S-SE winds.

Cold temperatures most of January accelerated the weakening of snow grains throughout the snowpack: at the surface, below wind slabs, and above and below melt-freeze crusts. We should treat these layers as guilty until proven innocent and continue to monitor, track, and assess stability on a slope-by-slope basis. Melt-freeze crusts are a dangerous weakness in a snowpack and have played a role in multiple avalanche accidents, even within our forecast zones.

What we know about the EOY crust is that above-freezing temperatures  ~3,000ft- 4,000ft on December 30th caused the formation of a thick ( 1" +/-), widespread melt-freeze crust down 20-50cm. The strength of this layer and its thick and thin variability depend on the duration and intensity of the warm and inverted EOY event. The cold weather in mid-January faceted and weakened snow around the crust. The late January Pineapple Express has released deeper instabilities in some areas and inconclusive in others.

Other layers to note are from earlier this season, including a few melt-freeze layers present throughout the snowpack formed during other warming or rain-on-snow events.

The people you go out with, how you communicate, and how you practice risk reduction as a team throughout the day are equally important as the conditions. The wide propagation and dangerous nature of persistent slabs are best mitigated with safe travel techniques and terrain management- only exposing one person to a slope at a time, avoid grouping up in run-out zones, and re-consider islands of safety to truly limit your group's exposure. 

Help us gather information about weather, snowpack, and avalanche activity by reading and submitting field observations. Useful observations can be anonymous and include: 

  • Avalanche Activity: Natural or human-triggered
  • Signs of Instability: Cracking, collapsing, or whumping
  • Red Flag Weather: Precipitation, wind loading, warming temperatures and rain-on-snow
  • Persistent Weak Layers: Surface hoar growth, snow depth, persistent weak layers, etc
  • Snow Pits: Stability test results, layers of concern, total snow depth, location, elevation, aspect, etc 

Were you caught, carried, injured, or buried in a slide? We are all human and can benefit from lessons learned. Please report incidents to Haines Avalanche Center staff confidentially.

Regioni coperte