Eastern Sierra Avalanche Advisory - 1/11/18

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON January 12, 2018 @ 6:26 am
Avalanche Advisory published on January 11, 2018 @ 6:26 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Heightened avalanche conditions exist near and above treeline today because of recently formed wind slabs on leeward terrain features and the continuing sensitivity of a buried weak layer on colder, shady slopes. Natural avalanches may be unlikely, but human triggered avalanches are possible. Below ~9,000’ coverage is still thin and avalanches are unlikely.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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Westerly winds were strong and consistent yesterday in the northern half of the forecast area. Small and large, natural and human triggered wind slab avalanches were observed around Mammoth and in Virginia Lakes in the late morning and afternoon. New wind slabs may take hours to days to stabilize. Gusty winds were recorded overnight and are forecast to increase this afternoon. Be on the lookout for snow being transported across ridgelines onto leeward slopes. Cornice formation is a good indication that the slope below has been wind loaded. Smooth, hollow-sounding drifts and shooting cracks are indications of unstable wind slabs.

Avalanche Problem 2: Persistent Slab
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Stability tests from around the Mammoth area and north show that a layer of weak, sugary, facet snow is still reactive to stress. Several recent reports of rapid settlement of this layer, heard and felt as a sudden “whumph,” indicate that human triggered persistent slab avalanches may be possible. The layer of most concern has been observed on shady slopes near and above treeline and is buried approximately 25 to 45cm under newer storm snow and fresh wind deposits. A failure at that depth could potentially produce a large, dangerous avalanche. Evaluate the snow and terrain carefully.

advisory discussion

A complex snowpack and avalanche situation exists at the moment. The latest storm did not bring nearly as much snow as anticipated, and snowlines remained quite high for the bulk of it. However, consistently strong winds yesterday easily transported snow at middle and upper elevations onto leeward aspects and several natural avalanches released on steep North-easterly slopes. Winds shifted in the afternoon and new wind slabs likely formed below Southeast facing ridges. The wind slab problem, especially in alpine terrain under corniced ridges and on the sidewalls of gullies will be easy to spot today. Less obvious are the deeper layers or weak, faceted snow buried near and above treeline. Failure of one of these layers could result in a larger more destructive avalanche. As the storm slab and wind slab problems from the past few days settle, we are left with overall more weight and slab on top of the weaker underlying pack, where a person could conceivably find a thinner weaker spot that could propagate a failure across an entire slope. This type of failure can often be heard as a loud “whumph” and several riders have noted such whuphing in recent days. This problem could last for awhile! It will be important to dig and investigate the snow under your feet before committing to your terrain choice. Evaluate the snow and terrain carefully and identify features of concern.

Weather and CURRENT CONDITIONS
weather summary

A fast shortwave moving through the Pacific Northwest will bring breezy conditions and high level clouds today and tonight. High pressure builds across the West for the end of the week through the weekend with dry conditions. These mild conditions wont last long as high pressure begins to buckle by late Sunday night into Monday, and a wet and potentially windy pattern is looking probable for the rest of the month.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Partly cloudy then becoming mostly cloudy. Partly cloudy. Sunny.
Temperatures: 43 to 51 deg. F. 25 to 31 deg. F. 45 to 50 deg. F.
Wind Direction: Light becoming SW SW SW
Wind Speed: 15 to 20 mph with gusts to 50 mph in the afternoon. 15 to 25 mph. Gusts up to 55 mph decreasing to 45 mph after midnight. 10 to 15 mph with gusts to 35 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Partly cloudy then becoming mostly cloudy. Partly cloudy. Sunny.
Temperatures: 36 to 41 deg. F. 23 to 28 deg. F. 38 to 43 deg. F.
Wind Direction: NW W W
Wind Speed: 15 to 25 mph with gusts to 40 mph increasing to west 25 to 40 mph with gusts to 60 mph in the afternoon. 30 to 45 mph with gusts to 75 mph decreasing to 25 to 35 mph with gusts to 55 mph after midnight. 20 to 30 mph with gusts to 50 mph.
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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