Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
Voir sur la carte

Danger d’avalanche

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

Points saillants

Heavy snow and rain on top of old, weak snow will create dangerous avalanche conditions at all elevations. People will likely trigger avalanches that break in the old, weak snow or cause avalanches of wet, rain-soaked snow on steep slopes. The shallow rocky snowpack increases the chances of serious injury if someone gets caught. If you see recent avalanches or cracks in the snow, feel the snow collapse, or have doubts about a slope, play it safe and choose not to travel in or below avalanche terrain.

Problèmes avalancheux

  • Persistent Slabs

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

    Yesterday, human-triggered cracking and collapsing occurred as the weak layers buried in the bottom half of the snowpack began to fail. More heavy, wet snow and rain on top of these layers should lead to more widespread instability today. The heavy new snow will form a slab layer on top of the snowpack in sheltered areas, and wind-drifted snow could form larger slabs above the weak snow in more wind-exposed areas. Rain will add weight to the snow above the weak layers and weaken the snowpack. The high snow levels will also push people to recreate where the weakest snow exists on NW-N-NE aspects above 8000-8300 ft. This combination of heavy slabs on top of old, weak snow and increased use is a recipe for serious avalanche incidents. Expect dangerous and unusual avalanche conditions today. The shallow rock-infested snowpack will make serious injuries more likely if someone gets caught in an avalanche.

    Recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds can indicate unstable, persistent slabs. Sometimes, triggering an avalanche represents the first sign of an unstable slope. Persistent slab avalanches can break wider than expected, and people can trigger them from a distance or low on the slope. The consequences, variability, and unusual nature of this problem make conservative decisions prudent. If you see any signs of unstable snow or have doubts about a slope, assume it is unstable. Play it safe, and stick to low-angle, non-avalanche terrain not connected to steeper areas above or beside it.

  • Loose Wet

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

    Wet snow can slide downhill on steep slopes where rain has or continues to fall on snow. Wet loose avalanches will likely occur as the upper layers of the snowpack become wet. These could entrain enough snow to push people into barely covered obstacles or knock people over. Some could involve more snow on long, steep, snow-covered slopes. The rain will also add weight to the snowpack and exacerbate the persistent slab problem mentioned above.

    Wet surface snow, pinwheels, and roller balls indicate unstable wet snow. If you get out in the rain, traveling in lower angle non-avalanche terrain that is not below steeper terrain represents the best way to minimize your exposure to wet loose avalanches.

Avalanche Discussion

A weak faceted (sugary) snowpack exists on most NW-N-NE aspects, especially in areas above 8000 ft, with a few spots holding weak snow down to about 7,400'.  Near and below about 8,500', faceted snow exists near buried rain crusts. Above 8,500', the rain crusts gradually disappear, and almost the entire lower half of the snowpack becomes weak faceted snow. Generally speaking, the higher you go on wind-protected NW-N-NE aspects, the weaker the snowpack. These weak layers are well developed, with depth hoar found at the base of the snowpack in many areas. While no avalanches have been reported on these weak layers, people did trigger collapses that propagated across entire slopes after each of the last two storms. One of these occurred in the Hourglass and one in Broken Glass. The current storms will add more load than either of those previous storms.

There is still the potential to hit many obstacles (rocks, logs, open water, bushes, etc.) that are barely covered (or not) by the shallow snowpack. This is especially true in areas below 7,500' and very rocky areas up to at least 8,000'. Anticipate encountering a different snowpack structure and a different set of travel conditions if you are moving from one place to another around the forecast area.

Régions couvertes