Eastern Sierra Avalanche Advisory - 1/6/18

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON January 7, 2018 @ 7:00 am
Avalanche Advisory published on January 6, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

With up to 7” of new snow falling last night and today, and moderate to strong SW winds, natural avalanches will be possible today, and human triggered avalanches will be likely.  The greatest avalanche concern will be fresh wind slabs above 9000’ on the leeward side of ridges, sidewalls of gullies and around other features that promote drifting.  Another potentially more dangerous avalanche concern is the persistent weak layers deeper in the snowpack that exist in many areas that the weight of new snow, or even more likely the stress of a smaller avalanche, could trigger, resulting in a much larger more destructive avalanche.  Cautious routefinding will be essential, especially at higher elevations.   

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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With up to 7” of new snow expected at higher elevations, and moderate to strong SW winds gusting up to 80 mph at ridgetops, new sensitive wind slabs will form on the leeward side of ridges, sidewalls of gullies, and around other features that promote drifting.  It is possible that some of these wind slabs could fail naturally as winds continue to build them, and they will very likely be sensitive to human triggering today.  As the storm clears this afternoon, and the temptation rises to travel into the high country, avoid smooth dense new snow on steep slopes greater than about 32 deg.

Avalanche Problem 2: Persistent Slab
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The underlying snowpack structure below last night and today’s new storm deposits is a variable mixture of melt freeze crusts, wind crusts, and facets.  The bottom line is that In many areas weak snow exists under more dense snow.  Snow pit tests over the past couple of weeks have been resulting in failures between these layers, and propagation of these failures, but slopes have not been failing.  However, now with the addition of new snow, this added weight could be enough to tip the balance where slope failures are now possible.  Especially concerning is the stress of a smaller surface avalanche triggering these deeper layers and resulting in a much larger destructive avalanche. 

Avalanche Problem 3: Storm Slab
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In areas above 9000’ that are sheltered from the wind that receive more than a few inches of new snow, human triggered avalanches will be possible today.  It is unknown how well this new snow will bond to the underlying snow surface, which could be anything from melt-freeze crusts to small facets.  Do your own analysis before committing to steeper sheltered terrain. 

advisory discussion

Our high pressure ridge of doom has finally broken down, and we are seeing our first snowfall of substance since before Christmas.  While it is a modest storm, with only up to 7” or so of snow expected above 9000’, the SW winds will surely form much deeper deposits on leeward terrain at mid to upper elevations that will be today’s greatest avalanche concern.  Below 9000’ the snow coverage has barely been enough to cover the ground, especially in areas south of Mammoth.  Therefore the rain falling today below that elevation, which normally would be a concern for dangerous wet avalanches, is not.  The dry clear weather that has persisted for weeks, combined with a shallow snowpack, has led to the formation of weak faceted snow in many areas throughout the forecast area, both at the surface in protected northerly facing areas, and deeper down in the snowpack around crusts and more dense slabs.  While snow pit tests over the last couple of weeks have been showing failures and propagation in these buried weak layers, slope failures have not been seen.  The addition of new snow weight, and especially the stress of a smaller avalanche, could awaken these layers and a resulting avalanche could be quite large and destructive.  This 7” storm likely will not create widespread failures, but the much larger storm we are expecting in a couple of days very well could.  Thursday morning's tinny precipitation event, which left up to an inch of new snow at higher elevations, was quite warm with drizzle up to 9,500'.  That moisture combined with dense clouds and warm air, left melt freeze and rain crusts up to 11,000' on all aspects.  Small facet formation was observed ontop of that crust in many areas, as well as some small surface hoar.  While the surface hoar did not likely survive, the slick crusts, with some facets ontop, will likley act as a good sliding layer for new snow, whether it be in the form of wind slab or storm slab.

Weather and CURRENT CONDITIONS
weather summary

Snow and rain are expected to continue today thru mid-day, with up to 7”+ of new snow possible at higher elevations.  Winds will be moderate to strong out of the SW at mid to upper elevations.  Snowline is expected to remain above 8,500’.  Clearing will occur tonight, with a slight chance of precipitation, and then a sunny day Sunday is expected with warm temps around 40 deg F at 10,000’, with lighter SW winds. 

Long Term:  Forecasts continue to increase the chances of greater and greater amounts of precipitation for our area as a subtropical low pressure system moves in Monday night into Tuesday morning.  2-3” of water are expected, meaning 2-3+ feet in the mountains.  Snow levels are expected to start high, but then drop below 7000’, but the timing of this drop is where the uncertainty lies.  This system will be centered more over Mono County as opposed to more typically further north.  Dry conditions are expected for the rest of the week into the weekend. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy. Snow and rain in the morning, then chance of snow and rain in the afternoon. Partly cloudy then becoming clear. Slight chance of snow in the evening. Sunny then becoming partly cloudy.
Temperatures: 38 to 46 deg. F. 25 to 30 deg. F. 44 to 52 deg. F.
Mid Slope Winds: Southwest West light
Expected snowfall: 3-7 in. up to 1 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy. Snow and rain in the morning, then chance of snow and rain in the afternoon. artly cloudy then becoming clear. Slight chance of snow in the evening. Sunny then becoming partly cloudy.
Temperatures: 31 to 36 deg. F. 23 to 28 deg. F. 37 to 42 deg. F.
Ridge Top Winds: Southwest West Southwest
Expected snowfall: 4-10 in. up to 1 in. o 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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