Snowpack Summary - Thu, Dec. 15, 2016

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THIS ADVISORY EXPIRED ON December 17, 2016 @ 6:59 am
Avalanche Advisory published on December 15, 2016 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

A significant winter storm is moving into our region Thursday morning, and will bring substantial amounts of snow thru Friday morning accompanied by high SW winds.  Dangerous avalanche conditions will develop.  Beginning Thursday afternoon, and especially Thursday night and into Friday natural avalanches will become very likely above 8500’ and human triggered avalanches an almost certainty especially on wind loaded slopes facing W-N-E-SE.  This new snow deposit will also increase the possibility of large deeper failures in the weak faceted layers of the old snowpack.  Resulting avalanches could lead to serious injury or death.  

Avalanche Character 1: 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.

As snowfall rates increase throughout Thursday, strong SW winds in the 40-70mph range with ridge top gusts reaching 120mph will form dangerous wind slabs where blown snow is deposited on slopes greater than 35 degrees.  Natural wind slab avalanches will become likely and human triggered avalanches an almost certainty as snowfall rates increase Thursday afternoon/night into early Friday morning.  A resulting avalanche could be large enough to result in serious injury or death.  This elevated avalanche concern will continue through Friday even as snowfall ends and high SW winds continue to transport snow.  Wind slabs will most likely be found on slopes facing W-N-E-SE.  Due to the strength of these winds, even treed slopes that may normally be sheltered from the wind could have sensitive wind slabs.  Signs of wind slabs include smooth dense snow, and can potentially be hollow sounding.  As snowfall rates increase avoid slopes greater than 35degrees where the potential for wind slabs exist.  Avoid being bellow such slopes as well.   

Also of note for Thursday morning before snowfall rates increase: strong southerly winds at upper elevations over the past several days could have formed small isolated wind slabs on exposed leeward terrain features that could be sensitive to human triggering.  

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

As snowfall rates increase Thursday afternoon and intensify Thursday night, storm slabs will form in wind sheltered locations on all aspects.  These will be of greater concern to back country travelers by Friday morning.  How well these new storm slabs bond to the old snow surface will vary depending if the old surface was glazed and icy, firm and smooth, or soft.  Density changes within the storm can also create weaknesses where failures can occur within the new snow.  Fortunately this storm is coming in right-side-up (starting warm then cooling), generally resulting in less dense snow falling upon more dense snow, which will allow for these new storm slabs to stabilize more rapidly than if denser snow fell on less dense snow.  None-the-less, Natural storm slab avalanches will become possible during the heavy snowfall, and human triggered avalanches will become likely and remain likely through Friday on slopes greater than 35 degrees.  Remember, even small slopes that end in a terrain trap such as a small gully bottom can potentially bury a person. 

Avalanche Character 3: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

Our early season snow pack is quite complex, especially at elevations over 9,000’ on slopes facing W-N-E where multiple rain crusts and facet layers have developed.  As snow load and stress increases on the underlying snowpack during this storm, the likelihood increases that a failure could result along one of these buried weak faceted layers and result in a large avalanche.  Also, the stress of a smaller avalanche occurring in the newly deposited windslab/stormslab could be enough to trigger one of these deeper weak layers resulting in a much larger avalanche. 

Snowpack Discussion

The incoming winter storm with significant new snowfall forecasted and high SW winds will result in dangerous avalanche conditions beginning Thursday afternoon thru Friday and into the weekend.  The underlying snowpack structure is quite complex especially above 9,000’ on W-N-E facing slopes where multiple crusts and weak facet layers exist.  In addition to the new snow instabilities that are very likely to develop in the form of wind slabs and storm slabs, deeper failures in the snowpack at these weak layers will become more likely as well with increased snow load and stress.  Large natural avalanches will become likely and the possibility of large human triggered avalanches an almost certainty.  Use extreme caution on or below slopes greater than 35 degrees.        

weather

Winter Storm Warning in effect from 10am Thursday thru 10am Friday.  High wind warning in effect thru 10pm Thursday.  A significant winter storm is moving into our area Thursday morning with strong SW winds in the 40-60 mph range gusting up to 120 mph over ridgetops until late Thursday night, when they will lighten up slightly for Friday, still out of the SW in the 30-50 mph range with ridgetop gusts expected in the 90s.  Snowfall is expected to start late Thursday morning, with 4-8” of snow expected around the 10,000’ level by nightfall, then intensifying Thursday night with up to an addition 2’+ expected.  This storm will begin warm with snowline staying around the 8,500’ level until midnight Thursday, when it will drop rapidly to valley floors.  Click here for snow level timing, then click on "snow level forecast".  Thursday daytime temperatures will be in the mid to upper 30s around the 10,000’ level, and in the 40s around the 8,000-9,000’ level.  Friday expect lingering snow showers in the morning, then clearing in the afternoon with much colder temperatures in the teens to mid twenties.  Forecasted snowfall totals decreases south of Mammoth toward Bishop. 

For the weekend expect clear and cold conditions with calmer winds shifting to northwest.

Disclaimer

This Snowpack Summary 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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