Issued by Clancy Nelson - Eastern Sierra Avalanche Center
Old, stubborn, wind-deposited slabs that lie on top of weak sugary snow will be the primary avalanche concern through Wednesday. These persistent slabs can be found in areas that favored wind loading last week such as the down wind side of ridges and on leeward slopes at mid and upper elevations. Natural avalanches will be unlikely, but human triggered avalanches remain possible. You can often trigger these slabs where the snowpack is shallower or where the overlying slab is thinner. Evaluate snow and terrain carefully.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Remember that you need two things for a slab avalanche: a cohesive layer of relatively stronger snow (slab) on top of a less cohesive relatively weaker layer. This is exactly the scenario playing out in specific areas that saw wind loading last week followed by cold temperatures. Firm wind slabs were deposited at middle and upper elevations on the leeward sides of ridges, the sidewalls of chutes, and in depressions near and above treeline. Ordinarily these wind slabs would have bonded to the underlying snow by now. However, high temperature gradients in the snowpack have created weak layers of faceted snow and in some locations that persistent weak layer lies underneath old wind deposits. Limited observations have shown that this sugary snow is not always reactive to stress, but it is worth doing some extra detective work to see what the snowpack looks like before committing to your terrain choice. Steep and shaded slopes where the snowpack is shallower are often places you could trigger even stubborn persistent slab avalanches. Be wary of hollow sounding snow where wind deposits are thinner. Firm persistent slabs can break uphill of a skier providing little chance of escape.
The snowpack in the Sierra remains thin and coverage is limited to elevations above ~9,000 to 9,500’ around Mammoth, and higher elsewhere. Recent high pressure and mild daytime temperatures have slowly edged snowline higher. Early season conditions exist with many obstacles hiding just under the snow surface.
A fast moving low pressure system passed to the north of us on December 3rd accompanied by moderate to strong Westerly winds leaving wind slabs in favored locations on North-Easterly mid and upper elevation slopes. A period of East winds followed that storm and transported snow again across ridgelines and into West-facing chutes and gullies. Since that time we have been stuck under the Ridiculously Resilient Ridge with cold nights and calm dry weather. Strong temperature gradients in the snowpack quickly weakened the snow underneath those wind slabs. Two main faceted layers (layers of in-cohesive, sugary snow) have been observed. One can be found between 5 and 20cm down in the snowpack under the most recent, hard wind-deposits. The second has been observed between 35 and 45cm down in combination with the melt-freeze crust that was buried on November 27th. This observation from last Wednesday highlights the type of problem that may persist in specific locations. That same day, skiers triggered an avalanche while ski cutting near Deadman Pass. More recent observations around the Mammoth Area have shown slab layers to be less reactive during snowpack stress tests. But with limited data I am unconvinced that things have stabilized everywhere. Be wary of hollow sounding snow, and cracks shooting out from your feet. Do some extra investigation to see what the snowpack looks like before committing to your terrain choice. Below ~9,500’, natural and triggered releases are unlikely due to little or no snow.
High pressure will continue to bring dry conditions along with valley inversions, cold nights, and light winds for the next several days. The strong, amplified ridge that we have been discussing for over a week now will continue to bring dry weather, mild days, and cold nights through the middle of the week. Inversions will still be in place through Thursday due to radiational cooling at night.