Bulletin d’avalanche

West Slopes South

Northwest Avalanche Center
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Danger d’avalanche

Fri
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Heavy snowfall, strong winds from an atypical direction, existing weak snow, and multiple crusts all point toward dangerous avalanche conditions on Thursday. Look for red-flag signs of instability, such as new avalanches, shooting cracks, or whumpfing collapses. Avoid slopes over 35 degrees where triggering a slide is more likely.

Avalanche danger will likely be HIGH on Mount St. Helens, where snowfall totals are expected to far exceed other areas of the zone.

Problèmes avalancheux

  • Storm Slabs

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

    Wherever you find more than 6 inches of new snow, start thinking about slab avalanches. They should be thicker on the volcanoes or in wind-loaded terrain. If storm totals are less than about 1.5 feet, using small test slopes and uptrack tests are an excellent way of determining how the new snow is reacting to your weight as a traveler. Otherwise, you will have to dig down to investigate for potential points of failure. Look for heavy snow over weak snow, a poor bond to the old surface, or sensitive interstorm layers. Avalanches will be easier to trigger on slopes over 35 degrees.

    Slabs could overlay weak surfaces in isolated locations, resulting in surprising avalanches breaking widely or above you. 

    (1/6/2024)

    This small slope failed with minimal effort when undercut by at traveler earlier this season. It failed on buried weak grains. You will likely see similar activity on Thursday.

    Photo: Andy Harrington

  • Wind Slabs

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

    We don’t often pair Storm Slab and Wind Slab in the same forecast. When it’s stormy in the Cascades, it’s almost always windy. They kind of go hand in hand. The difference with this storm is that we are receiving significant snowfall while the winds blow from an atypical direction. Easterly winds often result in more cross-loading and mid-slope loading than you might be used to. They also load different aspects than our typical storms. Look for actively blowing snow, fresh cornices, or new drifts to identify wind-loaded aspects.

    Many of these may be hard slabs, which are notoriously difficult to manage. They will be stiff and may feel as if they are hollow, like a drum. Whether slabs are soft or hard, your best bet is to identify and avoid wind-loaded aspects.

    Again, some of these slabs could form over weak grains, meaning avalanches could break in surprising ways - more details in the discussion.

    (2/14/2024)

    Signs of wind loading, particularly during an east wind event. Use these visual clues to help identify wind-loaded aspects.

    Photo: Andy Harrington

Avalanche Discussion

This storm is going to wallop the Mount St. Helens area, and HIGH avalanche danger is expected. Close to 3 feet of snow is possible, plus a warm-up mid-day and sustained moderate winds. Mount Adams may also develop very dangerous avalanche conditions. Travel in avalanche terrain is not recommended in these areas on Thursday. 

As for the rest of the zone, dangerous avalanche conditions are expected as a potent storm drops roughly 8”-1.5 feet of snow Wednesday night and Thursday morning. Snowfall should coincide with peak wind speeds. As noted above, the unique set-up of significant snowfall combined with the atypical east winds makes this an unusual storm. 

To complicate matters, widespread weak surfaces were reported from White Pass, Crystal, and Paradise this week. In some areas, the wind may help to minimize their impact. However, they were observed well after the east winds kicked up on Wednesday, meaning they’ll likely survive and be buried in some locations. Where they persist, expect touchy slabs that could break far distances or even above you. They could be in lower-angled terrain or down in the trees where avalanches are less common.

Even if the weak surface snow doesn’t survive, a series of temperature crusts interspersed with faceted snow has been well documented in most areas of the zone this week. This includes facets on top of the Early February Crust (EFC). These layers will be tested as the snow starts to accumulate. Small avalanches could step down to other crusts or even the EFC, which are all excellent sliding surfaces. Focus your observations on the snow above the EFC and default to lower-angled terrain if you have any suspicions.  We do not have any concerns below this interface at this time.

At lower elevations, this storm will help to rebuild our depleted snowpack. You’ll likely see avalanche activity on specific terrain features such as convexities or the base of cliffs. Snow will fall on firm crusts or possibly even bare ground. Natural hazards such as holes, open creeks, avalanche debris, or rocks may become shallowly buried, making them even more difficult to manage.

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