Avalanche Advisory - Tue, Jan. 17, 2017

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THIS ADVISORY EXPIRED ON January 19, 2017 @ 6:59 am
Avalanche Advisory published on January 17, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

Tuesday:  Human triggered avalanches will be unlikely in most areas, but possible especially at high elevations over 11,000’ on NW-W-S-SE slopes just below ridgelines and in extreme terrain due to high elevation NE winds that have been transporting snow. 

Wednesday:  Human triggered wind slab avalanches will become increasingly likely on exposed slopes and in and around terrain features that promote drifting and cross-loading due to moderate to high SW winds.  These will most likely be found on slopes facing NW-N-E-SE over 9,000’ with likelihood increasing with elevation.    

Be on the lookout for blowing snow and signs of recent dense wind-deposited snow. Avoid steep slopes that are being wind-loaded, and do your own stability assessments on areas where dense wind-deposited surface snow is found. 

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.

For the past few days, winds have been mostly calm to light out of the NE at elevations below 10,000’.  Yesterday (Monday), some early morning snow transport was witnessed over ridgetops at elevations as low as 10,000’ (Sherwins), and throughout the day over high peaks over 11,500’.  NE winds lead to wind-deposited snow on slopes facing NW-W-S-SE.  For Tuesday, be on the lookout for recent dense wind-deposited snow just below ridgetops and peaks on slopes with these aspects, and in extreme terrain features at high elevations, where human triggered avalanches will be possible. While these are likely to be relatively small, they could sweep a person down a slope.  Winds are forecasted to increase substantially on Wednesday and be out of the SW, with gusts possibly reaching 100mph in the afternoon over ridgetops.  These winds will create windslabs that will be more widespread on exposed slopes and in and around terrain features that promote drifting and cross-loading down to lower elevations.  These will most likely be found on NW-N-E-SE facing slopes.  With the abundance of loose powder snow, transport will be easy, and sensitive slab formation likely. Be on the lookout for blowing snow and signs of recent dense wind-deposited snow.  Avoid steep slopes that are being wind-loaded, and do your own stability assessments on areas where dense wind-deposited surface snow is found.  Avalanches could be large enough to bury a person, or at least sweep someone thru undesirable terrain. 

Snowpack Discussion

Last week’s storm cleared and left the Sierra looking like it hasn’t in many years. Lower elevations down to 6,500’ are now covered with supportable snow, opening up an almost endless possibility of skiing and riding.  Last week’s storm started off wet, with rain up to 10,200’, gradually cooled with less and less dense snow falling, capped off with 1-2ft of fluff around the mammoth area on Thursday for a day of face-shot bliss on mellow angle terrain.  This storm snow has had time to settle well, and an overall stable snowpack exists throughout the forecast area for now. 

The item of most concern at the moment is winds.  For the past few days they have been light out of the NE, with some transport occurring at higher elevations, likely creating some sensitive windslabs at upper elevations.  As winds increase substantially on Wednesday and become out of the SW, windslabs will become an increasing issue, especially with the abundance of loose powder snow.  Most avalanche accidents in the Sierra happen due to windslabs.  Even with no new snowfall for days, winds can create dangerous avalanche conditions in a matter of hours.  Recognize wind-transport, wind-drifted snow, and potentially dangerous terrain and don’t become another wind slab accident statistic. 

Another small potential concern is wet-loose point releases.  As today’s high temperatures reach slightly higher than the past two days (upper 30s to mid 40s at 10,000’) with sunny skies, it is possible for small wet point releases to occur on solar aspects (SE-S-SW) on slopes steeper than 35 degrees as they become wet from the sun.  These will originate near rock-bands.  This means that it is also possible for a skier to trigger a small wet-loose slide on such a slope that could catch them off balance and be an issue in steep extreme terrain. 

*While increasingly unlikely, there is still a possibility that a deep weak layer exists at elevations around 10,500-11,200’ due to rain crusts that formed during warm storms in October and facets that formed around these crusts.  Many avalanches have ripped out on this weakness earlier this month during heavy loading events.  With the ever-deepening snowpack, this layer is becoming harder and harder to trigger, and should be gradually gaining strength on slopes that haven’t avalanched at this level where it still exists.  Deeper layers like this are most likely to be triggered toward the edge of a slope where winds have been stripping an area and the snowpack is the most shallow.  While very unlikely, a resulting avalanche would be huge.

 

recent observations

Widespread recent observations have shown a mostly stable snowpack structure throughout with a few items of concern:  Visible snow transport at high elevations (NE winds);  Some small rollerball activity on steep solar aspects originating near rockbands; Widespread mature surface hoar formation in the Mammoth area; Isolated areas where some stability tests have shown a weak layer of less-dense snow which was deposited last Sunday/Monday toward end of main rain-event when temps cooled, snow fell, and then warmed and rained again.

-Solitude Canyon/Ridge (1/16):  Visible wind transport at high elevations, signs of previous recent wind-deposit mid-slope, north facing at mid elevations (9,500-10,500). 

-Punta Bardini (1/16):  Surface Hoar, low density layer of concern found sandwiched between melt-freeze rain crust layers that formed during early part of last week’s big storm during period of cold between rain episodes (on slopes that were exposed to wind) 

-Wheeler Crest (1/15):  Rain crusts presence.  Stable snow.

-Convict Lake (1/15): Surface hoar and wet point releases.

-Convict Lake (1/14):  Evidence of avalanches within storm, wet point releases 

-Valentine Cirque (1/14):  Big crown seen form town that appears to have occurred naturally post-storm.

weather

Tuesday:  Clear skies, mostly light winds, and high temperatures reaching the upper 30s to mid 40s at 10,000’. 

Wednesday: Cloudy, with moderate to strong SW winds all day, increasing in the afternoon with gusts potentially over 100mph above high elevation ridgetops.  Chance of light snow in the afternoon with little accumulation.  High temperatures expected in the mid to upper 30s at 10,000’.

Long-Term:  A weaker Atmospheric River event is forecasted to impact our area with main snowfall occurring Wednesday night, lighter snowfall thru Thursday, then another more intense pulse Friday.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: SUNNY PARTLY COUDY MOSTLY CLOUDY THEN BECOMING CLOUDY. SLIGHT CHANCE OF SNOW THROUGH THE DAY
Temperatures: 40 TO 45 deg. F. 20 TO 25 deg. F. 37 TO 42 deg. F.
Wind direction: LIGHT LIGHT SOUTHWEST
Wind speed: LIGHT LIGHT, GUSTS UP TO 25MPH AFTER MIDNIGHT 15 TO 25, GUSTS UP TO 45MPH INCREASING TO 65MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. 1 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: SUNNY PARTLY CLOUDY CLOUDY. SLIGHT CHANCE OF SNOW IN THE MORNING. CHANCE OF SNOW IN THE AFTERNOON.
Temperatures: 38 TO 44 deg. F. 17 TO 23 deg. F. 33 TO 39 deg. F.
Wind direction: NORTH WEST SOUTHWEST
Wind speed: 10 TO 15 MPH. GUSTS UP TO 25 MPH IN THE MORNING 10 TO 15 MPH. GUSTS UP TO 35 MPH AFTER MIDNIGHT 20 TO 35 MPH. GUSTS UP TO 60 MPH INCREASING TO 80 MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. 1 in.
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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