Snowpack Summary - Tue, Dec. 20, 2016

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THIS ADVISORY EXPIRED ON December 22, 2016 @ 6:55 am
Avalanche Advisory published on December 20, 2016 @ 6:55 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

The greatest avalanche concern for this next 48 hour period begins Tuesday afternoon with the possible development of new small windslabs on a variety of aspects at elevations over 9,000’, as winds are forecasted to increase slightly Tuesday afternoon into Wednesday morning.  Human triggered avalanches will likelybecome possible on steep exposed slopes.  While these are likely to be small, in areas where winds are stronger and more loose snow is available for transport, they could be larger and more dangerous.  

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.

Calm winds over the past 3 days have resulted in diminished concern over wind slab avalanches.  However, as winds are forecasted to rise with gusts reaching 35mph Tuesday afternoon into Wednesday morning (beginning in the SW and shifting to the north) the concern over wind slabs developing that could be sensitive to human triggering will again rise.   This concern could exist on a variety of aspects as the winds shift through this period.  Be on the lookout for snow blowing across slopes, and for smooth hollow sounding snowdrifts, especially in steep exposed terrain, below ridgetops and the sidewalls of gullies.  It could be hard to tell the difference between old stable windslabs and new sensitive windslabs just by their looks.  Be on the lookout for shooting cracks, small failures on safe test slopes, and do your own stability assessments such as shallow hand pits before entering committing terrain.  In areas where the combination exists of these winds materializing, available loose surface snow for transport, and terrain >35deg, human triggered avalanches will become likely. 

Avalanche Character 2: 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.

Deeper in the snowpack there still exists facet/crust combinations in many places mostly between the elevations of 9,500’ and 11,000’ on E-N-W facing slopes.  Limited recent observations suggest that this weakness is gaining strength especially in areas where a deeper snowpack exists.  Recent temperature profiles deep in the snowpack are showing a weak temperature gradient, which will result in these layers strengthening more over time.  In areas with a shallower snowpack such as Rock Creek and south, and on smooth steep slopes without anchors such as talus and trees/logs, there is a low possibility that a human could trigger a failure in one of these deeper weak faceted layers, resulting in a larger avalanche.  If you are exploring in areas with a shallower snowpack, take the time to dig a hole and investigate lower down in the snowpack.

Snowpack Discussion

The concerns over storm slabs and wind slabs that developed with last week’s storm have diminished and mostly stable conditions exist in the backcountry as of Tuesday morning.  However, if forecasted winds increase Tuesday afternoon into Wednesday morning, fresh windslabs will likely form that will be sensitive to human triggering.  Deeper in the snowpack there still exists facet/crust combinations in many places mostly between the elevations of 9,500’ and 11,000’ on E-N-W facing slopes.  Limited recent observations suggest that this weakness is gaining strength especially in areas where a deeper snowpack exists.  Recent temperature profiles deep in the snowpack are showing a weak temperature gradient, which will result in these layers strengthening more over time.  In areas with a shallower snowpack such as Rock Creek and south, and on smooth steep slopes without anchors such as talus and trees/logs, there is a low possibility that a human could trigger a failure in one of these deeper weak faceted layers, resulting in a large avalanche.

Recent temperature profiles have shown a steep temperature gradient in the upper snowpack as a result of cold clear nights.  This has led to a good deal of faceting of the upper snowpack.  While this isn’t a stability concern at the moment, if these facets persist, they could become a weak layer concern with a new snow load. 

Remember, it is still early season, and many obstacles exist just under the surface!  Be cautious!

recent observations

12/19: Bishop - South Lake to Mt. Thompson:  

-about 6-18" of snow at southlake 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 propogate.  Test pit dug at 12,600', N facing slope, 265cm of total snow, see attached.  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 occured near the end of last week's storm.  click here for full observation

12/17: Mammoth - Red Cone:

-Approach: Consistent 24 +/-” (60cm) new snow from the trailhead up to Lake George. Above Lake George new snow depth increases dramatically with 140 cm new at 9900’. New snow had a classic density inversion approx. 6” to 12”(15 cm to 30 cm) thick just below the surface (~ 2”/ 5cm). This density inversion was reactive to Compression Tests (CTV/Q2/30cm from surface, CT3/ Q2/30cm from surface, CT6 Q2/30cm from surface). Extend Column Test was negative (ECTN3, Q2, did not propagate, failed directly under the shovel). Snow pit was dug at 9,900’, NW aspect, 32 degree slope, site was just below treeline. Snowpack depth 83”, 215cm.  

-New snow is settling slowly due to the cold temepratures. Supportive if you have a light touch. Wind picking up from the NW, small snow banners present. Wind loading on E-SE aspects. 

Crowns noted: RedCone Bowl released during the storm cycles. Crown visble from mid bowl to first chute on skier's left, 2-3' X 200', Partially filled. Ships Prow, ~ 2' x 200', Partially filled.  Lake George Headwall, small crown visible in the rocks. Crown 6-12"x50'.

Inverted snowpack still evident. The denser layer near the snow surface noted the previously is gaining stength. Stability test mod failures all Q3. CTM 13 (Q3), CTM 15 (Q3), CTM 15 (Q3). Snow is amazingly well preserved throughout the new snow. Snow flake braches still clearly visble.

Still quite unconsolidated. One hazard not to underestimated is falling and the difficulty of getting back on your skis with snow this deep and unconsolidated. A head long fall could be fatal due to suffication.  click here for full observation

12/17: June - Hemlock Ridge:

-A couple significant crowns near the top of J-7 as a result of patrol control work (2-4ft deep)

-Ample coverage now at 9,200' (yost creek) to ski, skin.  Some bushy areas, still thin, but ~2ft depending on wind deposition.

-North face of Hemlock ridge very wind sheltered, soft snow, except for very top exposed terrain which showed signs of wind deposit.  Several pits dug, showing some CT test failures within new snow with moderate force, but all Q2 results, and ECT tests did not propogate at these levels of failure (subtle density changes) ... not too worrysome.  See attached first video for one of these CT tests at ~9,700ft, north facing slope.  Full profile dug at 10,400' just below summit.  See attached pit profile.  155cm of snow, just over 1m of new snow sitting on a couple ice/facet layers. CT test failed near top within new snow, moderate force, Q2, not too concerning, as ECT test did not fail (and therefore did not propogate) at this level or any level.  CT test did fail with hard force, Q1-2 (sudden collapse, but rough failure) just below upper rain crust in facet layer.  Again, ECT test did not fail or propagate at this level.  See attached 2nd video.   Of note, temperature profile shows VERY STRONG temperature gradient in upper snowpack (which will cause snow to facet and weaken in the upper snowpack, which could create concern with additional snowload, but for now will make skiing and riding even better).  Temp gradient in lower snowpack where these ice layers and facet layers exist is now very low, which will cause these layers to round and strengthen given some time.  

-Above ~9,500' frozen layer of varying thickness exists above the ground.  2-3'+ of new snow above 9,500'.  

-Excellent skiing and riding conditions in this area which remained fairly protected from the wind during this latest storm.  Still early season, hard not to hit/scrape a few rocks/logs ... still be careful of obstacles!.  

weather

A significant valley temperature inversion exists early Tuesday morning with much colder temperatures at lower elevation valleys then mid and upper slope. 

For Tuesday expect temperatures to rise into the low to upper 40s between 8,000 and 10,000’, and into the upper 30s to mid 40s above 10,000’.  Light winds will increase slightly in the afternoon out of the SW with gusts up to 30mph, with mostly sunny skies.

For Wednesday night, temperatures are expected to drop into the low to upper 20s at all elevations.  Winds are expected to shift out of the Northwest with gusts up to 35mph, with party cloudy skies.

For Wednesday, clouds will disperse giving way to sunny skies.  Temperatures will reach the mid 30s to low 40s between 8,000 and 10,000’, and the low to upper 30s above 10,000’.  North winds form the night before will diminish in the morning and become light. 

Long-Term:  There is a chance of snowfall Friday.  At the moment, very modest amounts up to a few inches expected, but really too early to tell.    

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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