Bollettino valanghe

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Haines Avalanche Center
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Pericolo valanghe

Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

The avalanche danger is HIGH above 1,500ft. Stress from new snow and warming temperatures make the new/old snow interface likely to fail. The old snow formed during the cold, dry spell last week in areas sheltered from the northwest winds and is a slippery surface for the new storm snow. Use careful terrain assessment to identify and avoid avalanche-prone slopes greater than 30 degrees, including terrain traps and run-out zones. 

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    1–2

    In the last four days, 12-24" of snow has accumulated with rising freezing levels and moderate to strong E-SE winds. This combination will likely tip the scales, causing slab formations and failure where the weak old surface snow exists. A thick layer of near-surface facets from February 27th to March 3rd  will increase the sensitivity of new snow loads. Above treeline, winds could drift snow, causing larger, more dangerous avalanche conditions, especially on W-NW aspects. 

    Use lower angle slopes to manage the problem and look for feedback, such as shooting cracks, on small, steep test slopes. If snow has drifted, be wary of wind loading that could amplify the size of a slide. Even shallow storm slabs are dangerous on larger terrain features or slopes with terrain traps, such as trees, gullies, and cliffs.  In some cases, storm slabs can be remotely triggered and can become persistent slabs (see Problem 2) when their instabilities last longer and their behavior becomes more challenging to manage.

    Storm slabs can be released as wet or dry depending on the freezing level. Test slope failures can be indicators of unstable snow on other similar slopes.

  • Persistent Slabs

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

    Last week's outflow winds formed hard slabs primarily on E-SE aspects with a high degree of spatial variability and varying thickness and sensitivity. These old wind slabs are now considered persistent slabs where weak layers can propagate from shallow to deeper areas. Other buried weak layers are from the February 14th inversion event down 1-2ft and the February 7th sneaky near-surface facets down 2-3ft.  With additional load from the storm slab and creep factor from warming temperatures, older, less reactive layers could begin to wake up. 

    Slope-to-slope assessment is key as persistent slabs are characterized by difficult-to-manage and often surprising behavior. Weak layers may be more or less apparent in certain areas, making mapping the problem tricky. Use your probe to identify strong over weak layering and take measurements of snowpack depth. Utilize cautious route-finding and minimize exposure by choosing safe spots to regroup and traveling one at a time. Cold temperatures from this past week have eased; however, it will take more time and moderate temperatures for layers to heal.

    Wind transporting snow formed sensitive wind slabs that have now become more stubborn persistent slabs. Areas of thin to thick wind slab could vary over a short distance and make it difficult to map instabilities. 

Avalanche Discussion

Keep it heads up as we receive additional load on our snowpack; understanding the problem at the surface is tricky, and the likelihood of other buried weak layers could come back into play. Early-mid March has historically come with avalanche accidents in our valley. As part of your trip plan, establish a mindset for your group before you venture into the mountains. This can help combat human factor bias and heuristic traps. Use weather, avalanche, and terrain checklists. Deploy ALPTruth:  

The ALPTRUTH acronym covers 7 contributory factors to avalanche accidents.  When reviewing over 1000 avalanche accidents, Ian McCammon found that 3 or more of these factors were present in over 90% of accidents.

  • A: Avalanche. Have there been any recent avalanches (within the last 48 hours)?
  • L: Loading. Has snow, wind, or rain loaded the slope in the last 48 hours?
  • P: Path. Will your planned travel path cross an obvious avalanche path? Will your uphill travel cross any potential avalanche paths?
  • T: Terrain trap. Is there a terrain trap? These include gullies, trees, cliffs, or other features. A slide into a terrain trap will increase the severity.
  • R: Rating. What does today’s avalanche report look like for your area? If there’s a rating of “considerable” or higher, you may want to reconsider.
  • U: Unstable snow. Have you heard any cracking, collapsing, whoomping? These are clear signs of instability.
  • TH: Thaw. Has there been recent warming of the snow’s surface due to sun, rain, or air temperature? Warming can increase instability and avalanche risk.

    Thinking ahead:

  • When the next loading event happens, is it slow and incremental or rapid and significant?
  • What are the temperatures, steady and moderate, spring diurnal, or are freezing levels rising?
  • How does the likelihood of propagation increase or decrease with strength vs. structure?
  • Is there weak snow above or below a crust? How strong or brittle is the crust? Does the crust laminate well to layers above/below or not?
  • Could strong solar radiation or warming temperatures contribute to instabilities, including cornice fall?

List of layers in the snowpack & approximate depths:

  • Storm snow, 0-40cm. Recent snowfall accumulation. 
  • Down 10-40cm. Near-surface facets or "recycled powder" are at the surface in terrain protected from the wind.
  • Down 20-50cm. Valentine's inversion melt-freeze crust and buried surface hoar. Includes hard sun-warmed surfaces.
  • Down 40-80cm. A sneaky February 7th near surface facet layer that got buried by Feb. 10-11th snowfall and warming.
  • Down 70-170cm. The late January Pineapple Express crust. An intense precip event saw pooling and percolation warming to 5,000ft.
  • Down 150-180cm. End-of-Year Crust (EOY). Wet and warm ended at the end of last year freezing levels to 4,000ft.

Click here for a detailed season weather & snowpack history.

 

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. Always practice 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. Carry radios, first-aid, and a two-way emergency communication device. Take an avalanche course, and practice companion rescue often with your riding group. Every scenario is different, so it pays to be prepared.

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 can benefit from anonymous lessons learned. Please report incidents to Haines Avalanche Center staff confidentially.

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