Eastern Sierra Snowpack Summary - 2016-03-17 07:02

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THIS ADVISORY EXPIRED ON March 19, 2016 @ 7:02 am
Avalanche Advisory published on March 17, 2016 @ 7:02 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
Avalanche Character 1: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

The melt-freeze cycle began for solar aspects at mid to upper elevations on Monday afternoon as strong winds diminished and skies cleared.  Melt-freeze crusts can also be found on lower elevation northerly facing slopes below 8500’.  Clear skies and cool temperatures at night have allowed for a good nightly refreeze for all slopes.  These frozen surfaces will soften throughout the day, beginning with east and southeast facing slopes in the morning, southerly facing slopes by mid-day, and more southwesterly and west facing slopes in the afternoon.  Lower elevation slopes will typically soften more quickly and to a greater depth than upper elevation slopes.  Each day through Saturday temperatures are expected to rise a few more degrees, increasing the rate and depth of these daily thaw cycles.  Signs of slopes potentially becoming unstable are fresh rollerball activity (large rollerballs more so than small), and boot penetration past boot-top, and fresh wet-loose avalanche activity (at which point it may be too late!).  Plan to be off these slopes before they soften too much, and be flexible to change your plan/route/turn-around time based on the evidence you encounter.  Natural and human triggered wet-loose avalanches will become possible on E-SE-S-SE-W slopes >35 degrees depending on the timing of when these slopes soften and become less supportable.  These wet-loose avalanches could have the potential to trigger deeper larger Wet Slab avalanches.  Some risk will develop for these types of avalanches on more northerly facing steep slopes below 8500’ later in the day as these slopes soften as well.  While these kinds of avalanches are slower moving than dry slab avalanches, they are very difficult to get out of.  Even small releases can be dangerous when they end in terrain traps where snow can pile up deeply or when cliffs and rocks and trees are involved.     

Avalanche Character 2: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

The storm slabs and wind slabs which developed as a result of new snowfall and high southwest to west winds from last Thursday through mid-day Monday are settling and bonding well.  This being said, there could be small very isolated windslabs that exist that may still be sensitive to human triggering in upper elevation steep terrain that faces mostly N-NE-E.   

Snowpack Discussion

Avalanche concern for the next 2 days will be focused on loose-wet activity on solar aspects for slopes that face E–SE-S-SW-W as they soften throughout the day as a result of clear sunny skies, above average air temperatures, and low winds. 

The melt-freeze cycle began for solar aspects at mid to upper elevations on Monday afternoon as strong winds diminished and skies cleared.  Melt-freeze crusts can also be found on lower elevation northerly facing slopes below 8500’.  Clear skies and cool temperatures at night have allowed for a good nightly refreeze for all slopes.  These frozen surfaces will soften throughout the day, beginning with east and southeast facing slopes in the morning, southerly facing slopes by mid-day, and more southwesterly and west facing slopes in the afternoon.  Lower elevation slopes will typically soften more quickly and to a greater depth than upper elevation slopes.  Each day through Saturday temperatures are expected to rise a few more degrees, increasing the rate and depth of these daily thaw cycles.  Signs of slopes potentially becoming unstable are fresh rollerball activity (large rollerballs more so than small), and boot penetration past boot-top, and fresh wet-loose avalanche activity (at which point it may be too late!).  Plan to be off these slopes before they soften too much, and be flexible to change your plan/route/turn-around time based on the evidence you encounter.  Natural and human triggered wet-loose avalanches will become possible on E-SE-S-SE-W slopes >35 degrees depending on the timing of when these slopes soften and become less supportable.  These wet-loose avalanches could have the potential to trigger deeper larger Wet Slab avalanches.  Some risk will develop for these types of avalanches on more northerly facing steep slopes below 8500’ later in the day as these slopes soften as well.  While these kinds of avalanches are slower moving than dry slab avalanches, they are very difficult to get out of.  Even small releases can be dangerous when they end in terrain traps where snow can pile up deeply or when cliffs and rocks and trees are involved.     

The storm slabs and wind slabs which developed as a result of new snowfall and high southwest to west winds from last Thursday through mid-day Monday are settling and bonding well.  This being said, there could be small very isolated windslabs that exist that may still be sensitive to human triggering in upper elevation steep terrain that faces mostly N-NE-E.   

recent observations

Observations throughout the region over the past several days have shown that the storm slabs and wind slabs which developed as a result of new snow fall and high southwest to west winds from last Thursday through mid-day Monday are settling and bonding well. 

A party yesterday (Wednesday) reported evidence of a recent significant wet loose avalanche on a southeast aspect on Mt. Gibbs that likely ran yesterday.  R3-D2 size.  The same party reported significant softening of SE aspects of the negatives behind June, with top 10cm moist and soft just after noon.  The snow was loose, wet and somewhat sticky and transitional.

Observations around Redcone Bowl in the Mammoth Crest Area yesterday showed significant surface softening down about 10cm on eastern aspects above Crystal Lake (below tree line) by 1pm (transitional snow).  SE aspect lower down just above Lake George at 1:15pm showed a moist snowpack down 1.5ft, but still very supportable (boot penetration 10-15cm).  Some minor rollerball activity occurring naturally around cliffs and as a result of skiing.        

A party in the Lamarck Col area out of Bishop yesterday observed that above treeline to 12,400’ only southern aspects softened slightly.  They observed no evidence of recent wet-loose avalanche activity, and some minor loose-wet snow entrainment below treeline.  Mostly firm supportable stable windboard above treeline. 

weather

High pressure off the coast will slowly more inland over the Sierra by Friday, keeping skies relatively clear, winds light, and temperatures above average and warming a few degrees each day through Saturday.  Isolated cumulus clouds may develop over the mountains each afternoon.

Today, Thursday, expect sunny skies, light southwest winds near 10mph, and high temperatures in the mid 40s to mid 50s.  For Friday, expect the same except for temperatures to increase by a few degrees. 

Some high cirrus clouds will likely begin to develop by Saturday afternoon, as low pressure begins to move back into the region.  Winds will be on the increase starting Sunday, and there is a slight chance of light precipitation for Monday and Tuesday. Temperatures will drop progressively each day beginning Sunday and be well below average by Tuesday.     

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: SUNNY CLEAR SUNNY
Temperatures: HIGHS IN THE LOW TO UPPER 50S deg. F. LOWS IN THE UPPER 20S TO LOW 30S deg. F. HIGHS IN THE MID 50S TO LOW 60S deg. F.
Wind direction: SOUTHWEST WEST SOUTHWEST
Wind speed: 5-10 5-10 5
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: SUNNY CLEAR SUNNY
Temperatures: HIGHS IN THE MID 40S TO MID 50S deg. F. LOWS INTHE LOW TO UPPER 20S deg. F. HIGHS IN THE MID 40S TO MID 50S deg. F.
Wind direction: WEST NORTHWEST SOUTHWEST
Wind speed: 10-15 10-15 5-10
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. 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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