Snowpack Summary - Thu, Dec. 22, 2016

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THIS ADVISORY EXPIRED ON December 24, 2016 @ 7:19 am
Avalanche Advisory published on December 22, 2016 @ 7:19 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

* ...WINTER STORM WARNING REMAINS IN EFFECT FROM 10 AM FRIDAY TO 4 PM PST SATURDAY...

Primary concern for the next 48 hours - As the next short wave moves into the region midday Friday, the potential for: Storm Slabs (above ~8,000’) and Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations will rise quickly, possible including the lower elevations depending on new snow amounts below 8,000’. As the storm progresses, human triggered avalanches are very likely especially on wind loaded slopes. Additional concern, thou unlikely, the possibility for surface avalanches stepping down into the snowpack and triggering a much larger Persistent Slab resulting avalanches could lead to serious injury or death. Use extra caution in terrain 35 degrees and steeper, because of the early season variability assess the snowpack of the slope before you commit.  

 

Avalanche Character 1: 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 new snow will likely be slow to bond to the old snow surface due to recent facet formation in the upper snowpack.  As snowfall rates increase Friday afternoon into Friday night, storm slabs will likely form in wind sheltered locations on all aspects above ~8,000’. Density changes within the storm can also create weaknesses where failures can occur within the new snow. Natural storm slab avalanches will become possible as the storm progresses; human triggered avalanches will be very likely through Saturday on slopes greater than 35 degrees.  Small slides can be dangerous when combined with terrain traps or objective hazards.

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 approaching cold winter storm is expected to produce relatively low density snow that will be easily transported by the forecasted moderate to strong SW winds, forming dangerous Wind Slabs on leeward slopes (NW-N-NE-E-SE aspects) and near crossloaded terrain features on slopes greater than 35 degrees.  As the storm progresses, natural Wind Slab avalanches will become possible and human triggered avalanches very likely to certain as snowfall rates increase Friday afternoon/night into early Saturday morning. Large avalanches, resulting in serious injury or death will be possible from Friday evening through Saturday.  Signs of Wind Slabs include smooth dense snow and hollow sounding slabs.  Avoid slopes 35 degrees or greater where the potential for wind slabs exist. Use extra caution traveling below actively wind loading slopes.  

Note: Friday morning into early afternoon, moderately strong Southerly winds at upper elevations could produce small isolated wind slabs on exposed leeward terrain features that could be sensitive to human triggering. 

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 a concern as the stress and strain increases over the course of the storm. These layers can be found in the lower 1/3 of the snowpack, primarily above ~ 9,000’ 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. 

Snowpack Discussion

A strong winter storm is poised to move into the region midday Friday and is forecasted to bring significant snowfall along with moderate to strong SW winds, forming potentially dangerous Wind Slab and Storm Slab avalanche conditions beginning midday Friday and into the weekend. The cold temperatures and moderate to strong SW winds will favor snow transport with tender wind slabs forming on NW-N-NE-E-SE aspects, especially in the mid to upper elevations.  Additionally, the recent cold snap has generated a strong temperature gradient in the upper snowpack, which has produced extensive faceting of the surface snow. This will slow bonding of the new snow, increasing the potential for Storm Slabs Friday thru the weekend. The deep snowpack structure is quite complex, especially above 9,000’ on W-N-E facing slopes, where multiple crusts and weak facet layers exist. However, these weak layers are beginning to recede into the depths of the snowpack and would require a large trigger to fail, such as a surface avalanche triggering deep persistent weakness resulting in large destructive avalanche. Large natural avalanches will become increasing likely and the possibility of large human triggered avalanches are very likely. Use extreme caution on or below slopes greater than 35 degrees.  

Because of the widely varying conditions, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.    

Other concerns: Outside of the Mammoth Lakes basin, early season conditions exist. There are plenty of rocks, stumps, down trees just under the snow surface, use caution while riding and playing in the backcountry. 

recent observations

Red Lake Bowl, Virginia Lakes (12/21/2016): From the THD good snow coverage. Virginia Lake and Red Lake ~2' (65 cm) snow over ice. Light wind effect or skin over 1 Fingerish over 4 Finger with Pencil to knife below until ~ 20 cm above ground where it's possible a layer of facets formed early season (Oct. - Nov.) Strong Temperature gradient in the upper snowpack but quickly subsides below 20 cm from surface. Stability tests produced only moderate to hard failures 21 cm from surface with an irregular failure plane. Strong temperature gradient exists in the top 20 cm with faceting. Below 20 cm the gradient eases with decreasing facet formation. Red Lake Bowl slid during last storm cycle. ~ 1300' wide x 800' vertical, visible crown 1-2', max 6'. Slide broke through 3' of ice! No recent slides noted. Ski conditions above treeline, quasi-breakable wind skin, improving as you descend into the treeline and lower gullies. 

Red Lake Bowl (Dec 21, 2016, 2:37 PM)

Lat/Lon:38.0422, -119.2699

Elevation: 10250 ft.

Aspect: ENE Angle: 30°

Sky Cover: FEW

Precipitation: No Precipitation

Wind Speed: Calm, subjective

Current Temp:-1.5°C Trend:➘

Snow Depth: 256cm

Snow Temperature 20cm: -8.5° C

Boot Penetration: 5cm

Comments: Below 185cm snow is mostly Pencil to knife. ~ 235 cm from surface possible facets to ground.

Stability Tests: ECTN16 Q3 21cm Failed directly under shovel, no propagation. CT13 Q3 21cm CT4 Q3 21cm CT21 Q3

 

South Lake - Mt Thompson (12/19/2016) -Toured from Parcher’s Resort (drivable with 4x4 thru snow to ~1/2mi beyond Parcher’s), to end of Rd at South Lake, to Mt Thompson and back. 6-18" of snow at South Lake level, increasing to ~18-24" at 11,500', to 8ft in places above 12,000'. Lots of evidence of wind in upper elevations (above 11,000') during and right after storm, but no new wind effects for the past few days.  No sensitive windslabs found.  Some handpits/hand shears in areas of wind deposit failed 3-5" down, but all with moderate to hard force.  ECT tests did not propagate.  Test pit dug at 12,600', N facing slope, 265 cm of total snow.  Stability tests showed inconsistent failures, with moderate to hard force. Lots of faceted snow was found in shallow areas around rocks, but the talus nature of the area made these facet pockets very patchy and inconsistent across even short distances of a slope. No signs of instability were noted today. All the possible windslabs have had a few days to stabilize, and seem to be of limited concern now. High temperature gradient in upper snowpack.  The clear cold nights have led to a good bit of faceting of the upper snowpack.  This isn't a stability concern now, but these facets could be a weak layer for new snow to fail on in the future. Also of note, lots of old 1.5-2.5' crowns were visible all around upper elevations on NE-N-NW facing slopes around the 12,000' level, mostly below rock bands. These looked like they occurred near the end of last week's storm. Thanks to Zak Mills and Jason Templeton for breaking the trail most of the way the day before!

 

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: PARTLY CLOUDY PARTLY CLOUDY CLOUDY. CHANCE OF SNOW IN THE MORNING, THEN SNOW IN THE AFTERNOON.
Temperatures: 38 TO 43 deg. F. 22 TO 27 deg. F. 30 TO 36 deg. F.
Wind direction: LIGHT WINDS WEST SOUTHWEST SHIFTING TO THE SOUTH
Wind speed: 10 TO 15 MPH. GUSTS UP TO 25 MPH INCREASING TO 35 MPH AFTER MIDNIGHT. 15 TO 25 MPH WITH GUSTS TO 60 MPH, 20 TO 35 MPH WITH GUSTS TO 70 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 4 TO 8 INCHES in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY CLOUDY THEN BECOMING SUNNY PARTLY CLOUDY CLOUDY. SNOW LIKELY IN THE MORNING, THEN SNOW IN THE AFTERNOON.
Temperatures: 32 TO 38 deg. F. 18 TO 24 deg. F. 24 TO 30 deg. F.
Wind direction: LIGHT WINDS BECOMING NORTHWEST WEST SOUTHWEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON. 15 TO 25 MPH. GUSTS UP TO 35 MPH INCREASING TO 50 MPH AFTER MIDNIGHT. 30 TO 40 MPH WITH GUSTS TO 80 MPH INCREASING TO 35 TO 55 MPH WITH GUSTS TO 100 MPH IN
Expected snowfall: 0 in. 0 in. 5 TO 10 INCHES. 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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