Eastern Sierra Avalanche Advisory - 12/5/17

Avalanche Advisory published on December 5, 2017 @ 6:37 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Wind slabs will remain the primary avalanche concern through Wednesday. It will not be out of the question for a backcountry traveler to trigger these recently deposited slabs on specific terrain features at mid and upper elevations. Natural avalanches will be unlikely, human triggered avalanches may be possible. Weak, faceted snow underneath of wind slabs may cause them to break further than expected. Watch for unstable snow on specific features such as the down wind side of ridges and on leeward slopes of about 35 degrees and steeper. Evaluate snow and terrain carefully.

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

A fast moving low pressure system passed to the north of us on Sunday accompanied by moderate to strong westerly winds leaving wind slabs in favored locations on north-easterly mid and upper elevation slopes. Be aware of the potential for these slabs lingering below high elevation ridges and on steep, cross loaded slopes on Tuesday. East winds have dominated the picture since that storm exited the region and have been transporting snow across ridgelines and into chutes and gullies. These winds will continue, especially at upper elevations, through Wednesday. And be aware that strong temperature gradients in the snowpack are quickly faceting and weakening the snow underneath. Be wary of pillow shaped drifts, hollow sounding snow, and cracks shooting out from your feet. Better yet, pay attention to where snow has been blowing to avoid wind slabs before you get too close.

advisory discussion

A complex, early season snow structure has developed in the forecast area. A warm storm in early November left 7 to 20 inches of snow throughout the region. A combination of snow and melt-freeze crusts from that period can be found between 9,500’ to 11,000’+. Then, a couple weeks of dry unseasonably warm weather followed which resulted in the development of small facets above ground capped by an ice crust above. The next system to move into the region rolled though on November 27th, with another wave of warm moisture dropping an additional 1 to 10 inches of snow above 9000’. The storm system came in very warm with moderate South to Southwest winds. Precipitation amounts ranged from 1 to 10” of snow, initially wet and dense. As the system moved through the region temperatures began to fall and the snow density decreased above 9,500’. These two storms have combined to produce a supportive crust/facet combination mid snowpack. A snow/wind event passed quickly through on Sunday leaving little new snow but strong to moderate Westerly winds redistributed surface snow onto North and East aspects creating winds slabs of varying size and sensitivity. Since then, very cold temperatures and upper elevation East winds have become the norm. Weak faceted snow crystals have been developing quickly in the snowpack, especially around the melt-freeze crusts that can be found between 65 and 50cm above the ground. The snowpack is fairly thin overall with early season conditions and hazards (rocks, stumps, logs, open water, etc). With little to no snow below ~ 9000’ to 9500’, the likelihood of human triggered releases is unlikely but as you ascend above 9,500’ triggered avalanches are possible. Thin and isolated wind slabs have been observed on a variety of aspects near and above treeline. These slabs often take hours or days to stabilize. But with continuous Northeasterly winds in the forecast, don’t rule isolated wind slabs out of your hazard and terrain evaluation just yet. Hollow sounding slabs on leeward slopes may still be triggered by a rider’s weight. Faceted, sugary snow grains underneath of these wind slabs may increase their sensitivity to trigger or cause them to propagate further.

The amplified high pressure ridge over the area will bring warmer temperatures as the week goes on. Warmer and wetter snow conditions may develop by mid afternoon on Wednesday. Rock outcrops and treed slopes will warm surface snow the most. Cold overnight temperatures will continue to refreeze the snowpack.

South and north of Mammoth, snowfall amounts have been lower this season, making easily access to ridable terrain more difficult with long approaches.

Other concerns: early season conditions exists. There are plenty of rocks, stumps, down trees just under the snow surface, use caution while playing in the backcountry.

weather summary

High pressure strengthens over the region this week bringing inversions, cold nights, and an extended period of dry conditions. Gusty east winds peaked overnight with Sierra ridge wind gusts generally in the 60-80 mph range. Mono county ridge wind gusts should begin to increase through the early morning.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Clear Sunny
Temperatures: 33 to 38 deg. F. 8 to 13 deg. F. 38 to 43 deg. F.
Mid Slope Winds: E Light winds NE
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Clear Sunny
Temperatures: 31 to 36 deg. F. 7 to 12 deg. F. 35 to 40 deg. F.
Ridge Top Winds: NE E NE
Expected snowfall: 0 in. 0 in. 0 in.
This Avalanche Advisory 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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