Snowpack Summary - Sat, Dec. 24, 2016

THIS ADVISORY EXPIRED ON December 26, 2016 @ 7:04 am
Avalanche Advisory published on December 24, 2016 @ 7:04 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

Heavy snowfall began yesterday morning and continued until midnight last night dropping 1-2ft of new snow accompanied by moderate to strong winds.  Wind slabs on exposed slopes at mid to upper elevations will be the greatest concern, especially as winds are forecasted to continue to be high enough to transport snow throughout the day.  Storm slabs in areas sheltered from the wind will be a concern as well.  Human triggered avalanches will be very likely and natural avalanches will remain a possibility especially on exposed slopes.  Avoid exposed slopes >35 deg until this new snow load has time to stabilize. 

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.

New 1-2’ of new snow accompanied by moderate to strong winds out of the E to S to W in the 30-60mph range, with ridge top gusts exceeding 100mph at times during the past 24 hours, will have created dangerous new windslabs especially in exposed terrain at mid to upper elevations.  Continued moderate winds out of the W to NW today will continue to form these windslabs and keep them sensitive to human triggering throughout the day and into tomorrow, and possibly beyond.  Expect to find these mostly on E to N to W facing aspects to begin with, but as winds shift and continue out of the W to NW, increasing concern will develop even on SE aspects.  Areas of greatest concern will be steeper slopes just below ridgelines and sidewalls of gullies, and also across exposed slopes especially at convexities.  Human triggered avalanches will be very likely, and natural avalanches possible for areas that continue to receive snow deposit.  These avalanches could be large.  This concern will decrease gradually with time and as the winds drop.  Avoid this kind of suspect terrain for the next couple of days, give these windslabs a chance to stabilize, and then as you begin to venture out be sure to do your own quick stability tests on these areas with denser wind deposited snow before entering committing terrain.  Pay attention to the snow surface and be on the lookout for areas with slightly more dense snow, areas with very dense hollow sounding snow, and signs of instability such as shooting cracks.

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.

1 to 2 feet of new snow has fallen over the past 24 hours.  In areas that are sheltered from the wind, soft storm slabs will have formed on all aspects.  On more southerly facing slopes the old snow surface will likely be a slick refrozen crust, and on more northerly facing slopes, the old snow surface is likely to be weak faceted sugary snow.  Additionally, cold clear nights have led to the formation of a substantial amount of surface hoar that has atleast been observed in the lakes basin, some of which has been observed on slopes around 9,000'.  While strong winds likely destroyed much of this, yesteday afternoon it was found to have been preserved and buried atleast in one loaction.  Buried surface hoar can be a very weak and dangerous layer.  All of these situations will lessen how well the new snow can bond to the old and increase the possibility of avalanches on all aspects. In steep terrain over 35deg over 8,000’, natural avalanches could be possible and human triggered avalanches likely until this new snow is given time to stabilize.  With this light new snow, it is easy to dig a quick test pit down just below the old/new snow interface and perform your own stability test to see how well the snow is bonding, and to check for this possible buried surface hoar layer.  Remember, even small slides can be dangerous when combined with terrain traps or objective hazards.

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.

Two facet layers deep in the snowpack are still a concern. These layers can be found in the lower 1/3 of the snowpack, primarily between 9,000’ and 11,200’ on NW-N-NE-E facing slopes. Even though these weaknesses are deeply buried, a triggered release higher in the snowpack could step down into these deeper layers, possibly producing large, very destructive avalanches.  While last week's storm resulted in many avalanches above 10,000' that stepped down to this upper faceted layer, it is still possible that some areas that didn't slide could now after this additional heavy load.     

recent observations

Remote sensor Snow fall amounts:

-Big Pine Creek (10,000’): 15” new snow

-Sawmill near Big Pine (10,200’): 10” new snow

-Rock Creek (9,600’): 14” new snow (total depth=30”)

-Mammoth Pass (9,500’): ~2” water, 20” new snow

-Sesame Street (9014’): ~1.7”water, 15” new snow

-June Mtn Summit (9,148’): 0.7” water, 8” new snow

-Agnew Pass (9,355’): 11” new snow

-Gem Pass (10,750’): 0.4” water, 13” new snow

-Virginia Lakes Ridge (9,445’): 0.4” water, 8” new snow

12/23/2016 - Mammoth Lakes Basin - Buried Surface Hoar found on steep slopes around 9,000'  

12/22/2016 - Tioga Pass Rd - Surface Hoar, Conditions

12/22/2016 - Mammoth Lakes Basin - Surface Hoar, Conditions

12/22/2016 - Mammoth Lakes Basin - Old avalanches, Surface faceting

12/21-2016 - VA Lakes - Snowpack, old avalanches


Yesterday’s and last night’s storm system has done the bulk of what it is going to do, leaving for today, Saturday Dec 24th (Christmas Eve), mostly cloudy skies with light scattered snow showers resulting in up to 1-4” of new snow accumulation.  Winds will be out of the W to NW in the 20-40mph, gusting up to 60mph at higher elevations in the afternoon.  Temperatures will remain chilly with highs expected in the teens to low 20s.  Tonight, temperatures will drop to near 0 degrees F, moderate winds will continue out of the NW with gusts up to 85mph at higher elevations, and continued cloudy skies and scattered snow showers could bring an additional 1-2” of snow.

For tomorrow, Sunday Dec 25th (Christmas Day), expect mostly sunny skies with some clouds increasing in the afternoon, chilly high temperatures in the teens to mid 20s, and decreasing NW winds with gusts up to 35mph at higher elevations. 

 For the beginning of next week conditions will be dry and cold, with light winds.  Thru the week until new years conditions will remain dry as temperatures will gradually warm. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Temperatures: 18 to 23 deg. F. 0 to 6 deg. F. 21 to 26 deg. F.
Wind direction: West Northwest light
Wind speed: 10 to 15, gusting to 35mph 15 to 25, gusting to 50mph Gusts to 25mph
Expected snowfall: 2 in. 2 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Temperatures: 11 to 19 deg. F. -4 to 1 deg. F. 17 to 23 deg. F.
Wind direction: West Northwest Northwest
Wind speed: 20 to 30, gusts up to 60mph 25 to 40, gusts to 85mph 15 to 20, gusts to 35mph
Expected snowfall: 1-4 in. up to 2 in. 0 in.

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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