Eastern Sierra Avalanche Advisory - 1/2/18

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THIS AVALANCHE ADVISORY EXPIRED ON January 4, 2018 @ 6:30 am
Avalanche Advisory published on January 2, 2018 @ 6:30 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Natural and human triggered avalanches will be unlikely through late Wednesday night, but that does not mean there is no danger. Watch for unstable snow on isolated terrain features where wind has previously built denser, cohesive slabs atop weak, sugary facet crystals. You can often trigger these slabs where the snowpack is shallower or where the overlying slab is thinner. By early Thursday morning, increased winds and snow showers may create new wind slabs at higher elevations.

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Near Treeline

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Below Treeline
Avalanche Problem 1: Persistent Slab
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Old wind slabs sitting atop weak sugary snow on the leeward sides of ridges or the sidewalls of chutes near and above treeline remain the primary concern through Wednesday afternoon. Fresh wind slabs usually remain sensitive to triggering for up to a few days. However, weak layers of sugary faceted snow lie underneath old wind deposits in isolated locations. Some recent observations from the northern half of the forecast area have shown that this sugary snow can still be reactive under stress. 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.

Avalanche Problem 2: Wind Slab
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Beginning Wednesday afternoon winds will increase from the southwest at as a weak systems is pulled north of us. By late Wednesday night/early Thursday morning, areas above ~8500’ may see up to 1” of new snow. New, shallow wind slabs may build on leeward slopes. If you find yourself in the mountains during the wee hours, avoid round, smooth, pillow shaped drifts. Shooting cracks and hollow sounding snow are that wind slabs under your feet could be triggered by your weight.

advisory discussion

Persistent high pressure over California has kept storm systems from entering the Sierra for some time now. Temperatures have remained well above seasonal averages and the snowline has crept ever higher. Northerly slopes continue to have far better coverage than south-facing areas. These calm conditions have created a thin and weak snowpack compared to what we usually expect. Layers of soft, sugary, faceted snow have been observed on north-facing slopes from Mammoth to Virginia Lakes. The most concerning layers lie between 10 and 40cm under the surface, and again just above ground level. In sheltered areas, facets remain exposed on top of the snowpack. Testing these layers has been a mixed bag with some places failing easily under stress while other places have not shown much sensitivity. What does that means for us, as the ridge over the west coast begins to flatten and there becomes potential for new snowfall? It means that the avalanche danger could rise quickly with significant new loading and could be difficult to predict. Stay tuned as the chances of more winter weather increase.

Until temperatures and storm patterns return to something resembling normal, areas south of Mammoth and elevations below ~9500’ will have poor snow coverage. Southerly slopes are mostly bare, or have melt-freeze snow resembling spring conditions – even above 10,000’. Early season obstacles remain exposed or hidden just under the surface on most slopes you want to ride. Firm wind board and breakable crust can make descending northerly slopes challenging. Have fun out there, but stay safe.

Weather and CURRENT CONDITIONS
weather summary

High pressure remains overhead into Wednesday as cloud cover gradually increases. There will be a weak and relatively warm system Wednesday night and Thursday morning as the high weakens, with light rain showers and high elevation snow showers.

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: Partly cloudy. Mostly cloudy. Partly cloudy then becoming mostly cloudy. Slight chance of showers in the afternoon.
Temperatures: 46 to 52 deg. F. 29 to 35 deg. F. 46 to 52 deg. F.
Wind Direction: Light winds. Light winds. Light winds.
Wind Speed: Light winds. Light winds. Light winds.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Partly cloudy then becoming mostly cloudy. Slight chance of snow showers in the afternoon.
Temperatures: 39 to 44 deg. F. 28 to 34 deg. F. 38 to 44 deg. F.
Wind Direction: Light winds. SW SW
Wind Speed: Light winds. Light winds becoming 10 to 15 mph after midnight. 10 to 15 mph. Gusts up to 25 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer
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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