Avalanche Advisory: Sunday - Jan 21, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON January 22, 2018 @ 6:40 am
Avalanche Advisory published on January 21, 2018 @ 6:40 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Light to moderate winds increasing throughout the day and ample soft snow available for transport may create new wind slabs at upper elevations where the avalanche danger will be MODERATE by the late afternoon. Blowing snow and cornice formation on leeward slopes will point to where the you are most likely to trigger new wind slabs.

Though isolated in nature, our persistent weak layer seems to again be showing the possibility for failure between 10,500’ and 9,000’ in the Mammoth area where the danger will be LOW: triggering this problem will be unlikely but not impossible and the resulting avalanche could be large. Whumphing and cracking are signs the this layer can suddenly fail beneath your feet.

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.

1. Low

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Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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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New snow from the 19th and 20th was cold and soft. The Mammoth area picked up between 4” and 7” while June and Virginia Lakes saw between 6” and 8”. Windward slopes are primed with light snow available for transport. Light to moderate northerly winds may have created some isolated wind slabs yesterday and last night. And with the passage of a weak disturbance north of us today, Southwest winds are set to increase this afternoon. New wind slabs may form on specific terrain features at upper elevations. Areas that accumulated the most new snow since Friday are more likely to have larger slabs. Wind can deposit snow at 3 to 5 times the rate that it can fall from the sky, so some of these new slabs will be shallow and small, but some could be deeper and large. You are most likely to encounter this problem today on steep upper elevation slopes and on the leeward sides of ridges, under cornices, and the sidewalls cross-loaded gullies.

Avalanche Problem 2: Persistent Slab
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Here is what we know now about the persistent weak layer that has plagued our snowpack this season: Since Monday the 15th the weak sugary snow buried in the northern half of the forecast area seemed to be gaining strength and so did the slab on top of it. Temperatures were warm, humidity was high, and in many places those angular, faceted grains had gotten moist and started to round off at the corners...Until yesterday when observations in the Mammoth Lakes Basin seem to show that the cold dry weather has again set this layer on edge. Whumphing was experienced at 9150’ above Lake George and snowpack tests again showed that the facets can initiate a fracture, and can propagate it across a slope. These are direct signs of instability in this persistent weak layer. However, observations are very limited. We know that these facets still exist lower down in the Virginia Lakes area as well, but we don’t yet know what they can do. Isolated, low probability, but potentially high consequence are the phrases that come to mind when describing the current state of this avalanche problem. When you dig down to look for this layer, expect to see it between 50 and 70cm below the surface. Do your own stability assessment near and below treeline, especially in the Mammoth area, to see if this layer is showing signs of weakness on the slopes where you want to ride. A larger wind slab avalanche that flows down to treeline today and suddenly overloads this weak layer would be of special concern.

advisory discussion

With cold temperatures, light winds, and a few showers through yesterday morning, windward areas are still primed with light winter snow ready for transport. Winds on Mammoth Mountain overnight were in the 20 mph range from a variety of directions. And winds across the region will again increase to between 20 and 30 mph from the SW this afternoon as a weak disturbance passes us by. A few showers and up to 1” of new snow may just be possible too. It’s moderate wind speeds like this that are the best at building wind slabs. Don’t ignore blowing snow, cornice formation, and rounded drifts just because it hasn’t snowed in a few days. These are the best clues for new, sensitive, wind slab formation. Be careful in steep alpine terrain.

Meanwhile, below alpine terrain, failures in our persistent weak layer seem to be showing up again in the Mammoth area. It’s like your couch-surfing buddy who promised to sleep in his van and not your living room, but now it’s cold out and he might be back for a while…

Limited observations yesterday hinted that a concerning layer of sugary facets may again present a problem. Whumphing was felt in the Mammoth Lakes Basin which is a direct sign that we haven’t seen the last of the potential for failure in this layer. Your best course of action, if you want to play near or below treeline, is to dig down and do your own stability tests on these facets. See if you can get a fracture to propagate. Pay attention to whumphs! They are the result of sudden collapse and not something to be ignored.

Below ~9,000’ the new snow will not be enough to cover the rocks and brush poking up from the surface.

Weather and CURRENT CONDITIONS
weather summary

Dry and cool conditions will prevail today with increasing clouds. A weak disturbance may bring very light snow for parts of eastern California tonight and Monday. Winds will also increase this afternoon and evening but this will not be a significant wind event. A stronger and colder storm is expected late Wednesday into Thursday, bringing more snow to the Sierra and possibly to lower elevations.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Slight chance of snow showers through the night. Mostly cloudy. Slight chance of snow showers in the morning.
Temperatures: 29 to 35 deg. F. 18 to 23 deg. F. 32 to 40 deg. F.
Wind Direction: Light winds. SW SW
Wind Speed: Light winds. 10 to 15 mph. Gusts up to 30 mph increasing to 40 mph after midnight. 10 to 15 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon.
Expected snowfall: 0 in. 80% probability...up to 1 inch. 20% probability...1 to 2 in. 80% probability...up to 1 inch. 20% probability...1 to 2 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Slight chance of snow showers. Mostly cloudy. Slight chance of snow showers in the morning.
Temperatures: 22 to 28 deg. F. 13 to 18 deg. F. 26 to 31 deg. F.
Wind Direction: W SW SW
Wind Speed: 10 to 15 mph. Gusts up to 25 mph in the afternoon. 20 to 30 mph. Gusts up to 40 mph increasing to 50 mph after midnight. 20 to 30 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 0 in. 80% probability...up to 1 inch. 20% probability...1 to 2 in. 80% probability...up to 1 inch. 20% probability...1 to 2 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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