Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Human-triggered avalanches remain possible today.  Unstable slabs of wind drifted snow and cornices could exist near ridgelines, in gullies, and on other open areas exposed to the wind on most near and above treeline terrain due to strong and shifting winds. Large avalanches that fail on weak layers deep within the snowpack may also remain possible in some areas, especially where the snowpack was shallow before the recent storms. The avalanche danger is MODERATE.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    The NE winds picked up again overnight and should remain strong enough to transport snow and scour slopes in some areas today. Some slabs of wind drifted snow may form on slopes loaded by these NE winds and some older wind slabs may still linger on slopes loaded by the SW winds from earlier this week. In many cases, the new wind slabs which would be more sensitive to human triggering should remain small, and the older larger wind slabs should be more difficult to trigger. Still, problematic human-triggered wind slab avalanches will remain possible today, especially on heavily wind-loaded slopes or in complex or extreme terrain. Large cornices also exist above many slopes.

    Blowing snow, cornices above a slope, wind pillows/drifts, ripples in the snow surface, and other wind created textures can all help identify where wind slabs may exist. Evaluate these areas carefully. If any doubts exist about the wind slabs, change plans and move into an area without the wind slab. Travel in more sheltered terrain will help avoid the wind slabs and wind-scoured firm slopes, but you will still need to assess the potential for the persistent slabs mentioned below. 

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    While data seems to indicate that triggering a deep persistent slab avalanche is becoming more difficult, human-triggered deep persistent slab avalanches may still remain possible in some places. Uncertainty and variability associated with the deep weak layers combined with the severe consequences of a deep persistent slab avalanche mean that this problem still warrants caution. A deep persistent slab avalanche would be large and destructive like the ones that occurred earlier in the week on Waterhouse Peak (Jan 31), Mt. Watson (Jan 31 or Feb 1), and maybe one on Flattop (likely sometime between Jan 31 and Feb 2). Most of the areas of concern for this deep slab avalanche problem exist on the Eastern side of the forecast region on NW-N-NE aspects where a weak and shallow snowpack existed before the recent storms. This includes portions of the Carson Range on the NE, E, and SE sides of Lake Tahoe and locations east of the Sierra Crest in the Luther Pass, Carson Pass, and Ebbetts Pass areas.

    Deep slabs represent a very unpredictable and difficult to assess avalanche problem. Sometimes digging deep holes or probing can help determine whether or not the buried weak layer exists, but often triggering a large deep slab avalanche is the first and only sign of instability. Sticking to slopes less steep than 30 degrees that are not connected to steeper slopes represents the best way to avoid this problem. Avoid complex terrain with multiple trigger points and unsupported slopes. 

     

    Persistent slab avalanche on Mt. Watson from Jan 31 or Feb 1.

Avalanche Discussion

* Some skier triggered cracking occurred on small wind- loaded test slopes near Spooner Summit yesterday. Snowpit tests near Elephants Back (Carson Pass area) also showed signs of potential wind slab instability. Observations from the Deep Creek ridgeline (between Truckee and Olympic Valley) did not find signs of lingering wind slab instability.

* Observers reported scoured, uneven, and firm conditions in exposed above treeline terrain on Elephants Back. Sunny aspects in this area started developing some wet surface snow in the afternoon. Some soft cold snow still existed on wind-sheltered N facing slopes. On Mt. Watson (near Northstar) a shallow layer of soft cold snow also existed above a breakable crust on shaded northerly aspects.

* An investigation of the Mt. Watson avalanche (more details here) revealed it to be a persistent slab avalanche that likely failed on Sunday or Monday (Jan 31 or Feb 1) and may have been triggered by a person. 

* Snowpit data from Elelpant's Back and Mt. Watson showed continued consolidation in the snowpack and tests did not yield unstable results on the deeper weak layers (Jan 22 rain crust or Dec 11 facets). Snowpit tests targeting these old weak layers are starting to show less signs of instability in some areas and continued lingering signs of instability in others. This inconsistency is common with persistent weak layers. 

* Evidence of widespread avalanche activity continues to be reported from during and after the last storm cycle both above and below treeline in both upper and mid-slope start zones on NW-N-NE-E aspects. Many filled in avalanche crowns with wide propagation consistent with persistent slab avalanches continue to be reported. Additional old avalanches were noted above Forestdale Creek (Carson Pass area) and on Flattop up the Deep Creek drainage (photo1, photo 2) yesterday. 

Regions covered