Eastern Sierra Snowpack Summary - 2016-01-09 06:57

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THIS ADVISORY EXPIRED ON January 11, 2016 @ 6:57 am
Avalanche Advisory published on January 9, 2016 @ 6:57 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
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.

Moderate SW winds today will likely create small isolated wind slabs on N-NE-E facing slopes, especially just below ridgelines and sides of gullies at mid to upper elevations.  The light unconsolidated snow that has fallen this past week will be easily transported and formed into slabs, as will the additional few inches we are likely to receive today.  While these will likely be sensitive to human triggering, the avalanches will likely not be big enough to bury a person, but could be enough to take someone for a ride into undesirable terrain.  Older wind slabs that formed a few days ago are likely to exist in similar locations, and may still be sensitive to human triggering.  Wind slabs are more obvious to detect and avaluate than other deeper avalanche problems, keep your senses alert!   

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.

The low-density snow that fell in un-wind affected areas has had time to settle.  While this is generally a good thing, some of this unconsolidated snow has now had time to consolidate into low-density slabs, which may not have bonded well to an underlying weak layer (such as a faceted old snow surface or buried surface hoar that did not collapse).  In this case, the possibility of a failure at this level has a better chance to propagate resulting in a larger avalanche as opposed to more harmless sloughing that would occur before this upper layer had time to consolidate.  While rare, it is possible that this combination exists where human triggering is possible that could result in a larger avalanche that could have the potential to bury a person, especially if a terrain trap (gully bottom, depression) were involved.  Quick shallow test pits can be done to help evaluate localized areas for this problem, while realizing that variablity exists across slopes.  

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.

A weak basal snowpack still exists throughout much of the forecast area do to the shallow early season snowpack and cold temperatures.  While it is unlikely for a human to trigger an avalanche at this level, the resulting slide would likely be large with bad consequences.  This layer is becoming less and less of a concern as the snowpack deepens.  However, it is still something to be aware of, especially where shallower snowpack exists outside of the Mammoth area.  Dig your own pits and perform your own tests to evaluate this layer to help you make safe decisions. 

Snowpack Discussion

Avalanche danger has decreased over the last two days, as clear weather has moved in and the snowpack has had time to settle after the series of storms from Jan 4-7 dropped between 1.5 to 3ft of relatively low density new snow at higher elevations.  However, as today progresses, 1-3” of new snow along with moderate SW winds will likely create isolated areas of fresh wind slabs sensitive to skier/boarder triggering near ridge tops and gully side walls at mid to upper elevations on N-NE-E facing slopes.  While resulting avalanches will unlikely be large enough to bury a person, they could take someone for a ride into undesirable terrain.   

Mammoth Lakes Basin seemed to be the heaviest hit during this past week’s series of storms, with less snow to the north, and much less to the south.  Moderate to strong SW winds during this period created wind slabs on N-NE-E facing slopes, mostly just below ridgelines.  While these have had 2 days to settle and bond, there are likely still isolated areas where pockets exist that are still sensitive to human triggering.  The low-density snow that fell in un-wind affected areas has had time to settle as well.  While this is generally a good thing, some of this unconsolidated snow has now had time to consolidate into low-density slabs, which may not have bonded well to an underlying weak layer (such as a faceted old snow surface or buried surface hoar that did not collapse).  In this case, the possibility of a failure at this level has a better chance to propagate resulting in a larger avalanche as opposed to more harmless sloughing that would occur before this upper layer had time to consolidate.  While rare, it is possible that this combination exists where human triggering is possible that could result in a larger avalanche that could have the potential to bury a person, especially if a terrain trap (gully bottom, depression) were involved.

The basal facets are still lurking over much of the forecast area, and are something to still be aware of.  While unlikely that a skier or boarder will be able to trigger an avalanche at this depth, the resulting avalanche would likely be large with bad consequences.  Regions with shallow snowpack such as Virginia Lakes and June area are of greater concern.  Possible trigger points across a slope include shallower areas such as around rock-outcrops or areas that have had some wind striping.   

*Come practice your beacon search skills!!  The Beacon Basin on Mammoth Mountain is up and running.  It is located between St. Moritz (Candy Bowl) and lower Jill’s, a short walk up from Main Lodge.  The site will be maintained jointly by ski patrol and members of Eastern Sierra Avalanche Center.  It is open to the public during operational hours of the mountain.  On Saturdays and Wednesdays free training sessions will be offered by Patrol form 9:30am to 11:00am.

recent observations

Small isolated pockets of wind slab have been triggered just below ridgelines over the past 2 days (Thur & Fri) in the Sherwins and Chicken Wing.  While these slabs have been small (6-8” deep, 20’ wide and running a short distance) they are an indication that larger more dangerous ones could exist at higher elevations and in more committing terrain. 

New well-developed surface hoar was observed yesterday (Friday) widespread across the top of the Sherwin Ridge, and in the starting zones of the upper slopes.  This was also observed on Punta Bardini.  While most of the time these delicate feathery structures get destroyed quickly in the Sierra, if they were to stay intact and get covered and preserved under new snow, they can act as a very weak layer for an overlying slab to fail on.

Surface hoar that developed before new years was found intact and buried under ~20cm of new snow at 9,800’.  CT tests failed easily at this layer, however ECT tests did not propagate.  Perhaps if the overlying snow was/became a more consolidated slab it would be of greater concern?

Compression tests done in a pit near the starting zone in the Sherwins produced consistent failures with Q1-Q2 shears ~60cm and 90cm down (total snowpack depth here was 285cm).  These occurred in density changes in the snow (4Finger hardness layers between pencil hardness layers).  However, these failures did not propagate in Extended Column Test.

Mammoth Mountain Ski Patrol reported limited results in their avalanche control work after these storms, some areas of wind slab, but mostly unconsolidated new snow.

June Mountain Ski Patrol reported some results in their avalanche control work, and evidence that the Negatives went thru a natural avalanche cycle during the storms. 

weather

A weak pacific storm moves through the Sierra today and tonight (Saturday). This will bring light snowfall and modest SW winds between 20-30mph, gusting to 50 over the ridge tops (stronger in the morning, weakening in the afternoon).  1-3” of new snow possible, with 6” possible in isolated spots along the Sierra crest.  High temperatures are expected in the high teens to mid 20s.  Lows Saturday night in the low teens to single digits. 

The sun will come back out Sunday, with high temps in the mid to upper 20s, and west winds in the 5-10mph range, gusting into the 20s.  There is another weak system possible for Monday, with little accumulation expected.

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: CLOUDY. SNOW LIKELY IN THE MORNING...THEN SCATTERED SNOW SHOWERS IN THE AFTERNOON. MOSTLY CLOUDY. SCATTERED SNOW SHOWERS IN THE EVENING...THEN ISOLATED SNOW SHOWERS AFTER MIDNIGHT. PARTLY TO MOSTLY SUNNY.
Temperatures: HIGHS IN THE MID 20S TO LOW 30S deg. F. LOWS IN THE MID TEENS TO SINGLE DIGITS deg. F. HIGHS IN THE MID 20S TO LOW 30S deg. F.
Wind direction: SW W W
Wind speed: 15 TO 25 MPH WITH GUSTS TO 40 MPH 25 TO 30 MPH WITH GUSTS TO 40 MPH DECREASING TO 10 TO 15 MPH WITH GUSTS TO 25 MPH AFTER MIDNIGHT. 10-15MPH
Expected snowfall: 1-3 in. NONE in. NONE in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. SNOW IN THE MORNING...THEN SNOW SHOWERS IN THE AFTERNOON. MOSTLY CLOUDY. SCATTERED SNOW SHOWERS IN THE EVENING...THEN ISOLATED SNOW SHOWERS AFTER MIDNIGHT. PARTLY TO MOSTLY SUNNY
Temperatures: HIGHS IN THE MID 20S TO LOW 30S deg. F. LOWS IN THE SINGLE DIGITS deg. F. HIGHS IN THE MID 20S TO LOW 30S deg. F.
Wind direction: SW W NW
Wind speed: 25 TO 35 MPH WITH GUSTS TO 50 MPH 25 TO 30 MPH WITH GUSTS TO 50 MPH DECREASING TO 15 TO 20 MPH WITH GUSTS TO 30 MPH AFTER MIDNIGHT NORTHWEST 10 TO 15 MPH. GUSTS UP TO 25 MPH IN THE MORNING
Expected snowfall: 3-6 in. NONE in. NONE 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 ESAC who is solely responsible for its content.

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