Primary concern for the next 48 hours is strong winds and the formation of wind slabs on NW-N-NE-E-SE aspects in the mid to upper elevations. Secondary concerns, focused on small isolated storm slabs on terrain 35 degrees and steeper on NW-N-NE-E aspects in the mid to upper elevations as well as the potential for isolated Persistent Slab releases where melt-freeze crust bridging is weak or not present.
Forecasted strong westerly to southerly winds over the next 48 hours will form shallow wind slabs on NW-N-NE-E-SE aspects near and above treeline. Isolated wind slabs prone to triggered releases are possible on slopes of 35 degrees and steeper. Typically, Wind Slabs may be encountered below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around freshly formed drifts and hollow sounding slabs on steeper terrain. Though these tender pockets may not be big enough to result in burial, they could entrain a rider and carry them over and into hazardous terrain with potentially high consequences.
Recent new snow rests on top a well-developed melt freeze crust above ~ 9,700’ on most aspects. It shows signs of bonding to the melt-freeze crust below but a slight density change within the new snow (~15cm from surface) shows some slab tendencies but lacks sufficient energy to propagate widely. However, on terrain greater the 35 degrees it may be possible to trigger small shallow slabs, though not be big enough to result in burial, they may be capable of dragging riders into hazardous terrain, rocks or trees as well as possibly triggering deeper instabilities with potentially server consequences.
Above ~ 9,000’ on NW-N-NE-E facing slopes - A significant weak faceted layer (10cm+/-) rests on top of a thick melt freeze crust directly above ground. Were the melt-freeze crust is less well formed or lacking, human triggered avalanche are possible on slopes >35° on NW-N-NE-E aspects, especially above ~10,000’, investigate the snowpack for yourself and perform your own stability tests.
The snow season continues to evolve with the recent new snow in the higher elevations, while lower elevations continue to suffer from low or no snow and regular deluges of rain. The last weather system that moved through the region (12/10/16) was very warm in nature. Despite over 2” of water reported at some sites, most of the precipitation came as rain in the mid to lower elevations, as evidenced by a well-developed rain crust up to ~9,300. Above, the warm air aloft melted much of the new snow as it landed until late Saturday afternoon when cooler air moved in, leaving in its wake 6” to 8” of new snow over the melt freeze crust that formed as the warm saturated surface snow cooled. Below this crust, the mid-snowpack consists of a number of 4 Finger to 1 Finger layers over a 10cm +/- of facets, which rest upon a basal layer of Pencil to Knife hard melt-freeze grains. The big question is how widely distributed is the upper snowpack melt-freeze crust that is bridging over the weak facts deeper in the snowpack. Where the crust is well-developed and supportive it will limit the stress applied to the facets below. However, where the crust is less supportive or not present (uppermost elevations), riders may trigger a release. The likelihood of human triggered releases is currently unlikely below ~9000’ with only 6 to 12” snowpack with minimal layering. Above 9,500’ triggered avalanches are possible. Keep an eye out for wind slabs, shallow unstable storm slabs, as well as the presence of persistent slab weaknesses in the upper elevations on E-N-NW facing slopes. If you are going into the backcountry, take the time to dig a hole and assess the facet layer in the lower snowpack.
South of Mammoth, snowfall amounts have been lower, making access to skiable and rideable terrain even more difficult and long approaches.
Other concerns: early season conditions exists. There are plenty of rocks, stumps, down trees just under the snow surface, use caution while riding and playing in the backcountry.
Red Cone Bowl, Mammoth Basin (12/12/16) – a well-developed rain crust at the snow surface is evident from the Trailhead (~8,500’) and continues up to Lake George (~ 9,000’) where surface snow begins to transition to a mix of melt freeze crust and decomposing new snow. Above ~ 9,300’, it transitions to 4” to 8” (N-NE-E aspects) of new snow over a supportive melt freeze crust that formed over the weekend. The new snow has bonded relatively well to the underlying snowpack but stability tests indicate a shallow slab within the new snow due to a slight density within the new snow. Compression Tests (CTE1, Q2 @15cm from surface) and Extended Column Test (ECT 2, Q2, propagated 20 cm beyond shovel) both highlighted this weakness within the new snow but the slab lacks sufficient energy to propagate across large areas. Further testing continues to highlight a persistent weak layer of facets that formed directly above the dense melt forms layer above ground, which was deposited in October and November. This layer isn’t easily triggered in part due to the well-developed supportive ice crust near the snow surface that is helping to bridge over this weakness. Test results: ECT2, @ 15cm from surface. Failed 20 cm beyond shovel), ECT30, Q1 @ 105 from surface, CT3, Q1 and CT0, Q1 @ 105 cm from surface. Due to limited early season observations, uncertain how widely distributed these conditions exist, additional caution is advised for all backcountry users entering avalanche prone terrain.
While the snow depth is slowly building in the upper elevations of the Mammoth Basin, in the lower elevations, the snowpack remains quite shallow and icy but supportive with plenty of objective hazards (rocks, downed trees, stumps, etc.). Exercise caution.
Tuesday thru Wednesday - Low-mid level moisture will increase this afternoon, with light snow and rain primarily affecting areas along the Crest tonight. Snow amounts around look to be rather minimal. Sites above 10,000 feet near the Crest will receive the bulk of the snow. Tuesday winds are forecast to pick up this evening with SW 25 to 35 mph with gusts to 70 mph and light snow forming along the Crest. Wednesday will see strong SW winds of 30 to 50 mph with gusts to 100 mph in the afternoon with little or no additional precip expected across most of the region. Mountain wave conditions will develop with lee side breaking waves/downslope activity possible.
Thursday thru Friday - the storm continues its previous trend of slowing the development of precipitation into the central Sierra with increasing winds aloft. Thursday strong SW winds of 30 to 50 mph with gusts to 100 mph will continue along the Crest. A High Wind Watch has been issued and remains in effect for now. Snow levels are creeping up a bit more so snow should be limited early on to areas above 8000 feet then fall to around 7000 feet in the early evening. The heaviest precipitation will shift south through the evening into Mono County
Still looking at 2 to 4 inches of liquid precip along the Sierra Crest with up lessar amounts below 80000 feet in the Sierra. Snow levels fall by Friday morning with the possibility of limited snow accumulation in the lower valleys by Friday morning. Much colder air filters in Friday through Monday with highs dropping up to 25 degrees from Thursday to Friday. Lingering snow showers will continue into Friday evening...but then drying begins.
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This Snowpack Summary is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.
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