Bollettino valanghe

Eastside Region

Eastern Sierra Avalanche Center
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Pericolo valanghe

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

A winter storm moves into the area today with strong winds and increasing snowfall. Expect new wind slabs to increase in likelihood and size throughout the day and overnight. Loading over the next 48 hours may reactivate buried persistent weak layers. A good strategy is to progressively step back from avalanche terrain as the storm arrives.

Problemi valanghivi

  • Wind Slabs

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

    Snow transported by southwest winds may form reactive slabs over the course of the day. These are most likely on the leeward sides of ridges and in protected areas behind terrain features that block or redirect winds. Look for blowing snow creating layers of cohesive surface snow. New slabs may have a smooth, rounded appearance and feel hollow underfoot. Shooting cracks, collapses, and small slabs breaking on test slopes are signs that the danger is rising in your area. Step back from steeper terrain and avoid ascending steep slopes below actively loading start zones as new snow accumulates.

    Wind slab avalanches on leeward terrain features near and above treeline.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2.5–3.5

    A persistent weak layer of faceted snow that formed in February has been buried three to four feet deep by recent storms. Patterns in recent avalanches activity show that this layer is capable of producing very large and likely unsurvivable avalanches. Persistent slabs may be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. They can propagate widely and in unpredictable ways. Shooting cracks and collapses are clear warning signs of the problem, but signs of instability may not be widely available because of spatial variability in weak layer and slab thickness. As a result, tracks on a slope might not mean that a slope is safe to ride. Persistent Slabs require a wide margin for error. Patience and disciplined terrain choices are the keys to managing risk with persistent slabs. Play it safe by avoiding travel on or below steep terrain where the problem may exist. 

    A very large persistent slab avalanche on Round Mountain (3/2/2023).

Avalanche Discussion

Uncertainty exists around the timing and amount of new snow during daylight hours. Keep track of accumulations in your area to see if we are above or below forecasted snowfall. For today this will mostly affect the size and likelihood of the wind slab problem. In the longer term, there may be enough loading in the next forty-eight hours to increase the sensitivity of the Persistent Slab problem. The timing of snowfall will determine when we reach a threshold for increasing the likelihood of avalanches on deeper layers. 

Persistent Slab history:

Between February 25th and March 1st, many very large (D3) persistent slab avalanches occurred including a remote skier-trigger in Rock Creek on 2/26. Most of these avalanches released on steep, wind-protected, north and northeast aspects near treeline, but persistent slab avalanches were reported at all elevations and on a range of aspects including avalanches on due west and due east slopes.

The build-up to the present persistent slab problem began with a prolonged period of mostly clear weather from late January to late February created a mixture of faceted snow, wind-affected snow, and melt-freeze crusts in the upper 40cm of the snowpack. By the last week of February, we observed well-developed 2mm faceted grains on most protected, steep, shaded slopes throughout the forecast zone at all elevations.

On February 22nd, a strong winter storm system arrived and dropped 5 feet of snow before skies cleared on the first day of March. During the eight-day storm cycle, there were periods of exceptional wind loading that developed slabs on a variety of aspects on the north side of the compass including on west aspects in the first phase of the storm. Snow totals were greatest in the northern part of the forecast area around Mammoth Lakes and between June Lake and Lee Vining. In these areas, the February PWL is buried up to five feet deep. Further south and in drainages that are further east of the Sierra Crest, the February PWL is between three and four feet below the snow surface and may be easier to trigger.

 

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