Eastern Sierra Snowpack Summary - 2016-01-07 07:29

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THIS ADVISORY EXPIRED ON January 9, 2016 @ 7:29 am
Avalanche Advisory published on January 7, 2016 @ 7:29 am
Issued by Doug Lewis - 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.

New snow fell throughout most of the day Wednesday combined with strong SW winds at ridgetop elevations, creating potentially dangerous windslabs over low density storm snow, especially on mid to high elevation leeward slopes facing N-NE-E – SE.  Areas just below ridgelines, and cross-loaded slopes are of greatest concern.  A change in aspect or exposure can mean the difference between soft unconsolidated snow to dangerous denser windslabs. Watch for changing conditions throughout ascents and descent and be prepared to alter your objectives. 

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.

More than a foot of snow has fallen in the mid to upper elevations (Mammoth Pass – 1.08” Snow Water Equivalent, 13” new, 8% density).  Avalanche danger on all aspects and elevations on slopes of 35 degrees and steeper has risen. The new snow has fallen on well-developed facets, which formed prior to this storm exists in many low to mid elevation areas, creating a poor bonding surface for the new snow. In addition, widespread Surface Hoar developed last week in protected areas at low to mid elevations. While in many areas the Surface Hoar was likely destroyed prior to this most recent snowfall, Surface Hoar was observed (1-6-16) on a relatively unprotected NE slope above 9800” in the Deadman Pass area. Sheltered areas near treeline and below are the most likely areas where it may still  be preserved and possibly forming a weak layer. These conditions are most likely encountered in the June Lakes and VA lakes regions and may be especially sensitive to human triggering, especially as the snow settles forming a more cohesive slab over this potential weakness. 

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.

Basal facets still persist at the bottom of the snowpack through most of the forecast area.  Stability tests have been highly variable, some showing column failure and propagation upon isolation, and others not failing at all.  This is more of a concern where the snow pack is shallower, such as June Lakes, Virginia Lakes, and points south of Mammoth Lakes Basin. Recent tests in the in shallower areas are producing failures at this level as well.  While unlikely to find the sweet spot, with the added stress of new snowfall, it may be possible to trigger a failure at this level. Relatively small slides could certainly step down into these deep facets and potentially produce large and catastrophic avalanches.  Areas where the snowpack is shallower, such as around rock outcrops, sides of gullies, and parts of slopes where wind has stripped some snow from, are areas where these (not so) sweet spots may exist.

Snowpack Discussion

Another winter storm exits the state today (Thursday) and in its wake has dropped over a foot of new snow at mid to upper elevations.  Avalanche danger has increased on all aspects and elevations with the new load.  Wednesday saw strong Southwesterly winds (20 to 30 mph, gusting 40-50 mph) along ridgetops, which deposited potentially dangerous wind slabs on leeward slopes facing SE-E-NE-N aspects, especially at mid to upper elevations.  Lower elevations, caution in steep protected slopes (>35degrees) at lower elevations where possible storm slabs have been deposited over facets, which can inhibit bonding of the new snow to the old snow surface. 

Additionally, there is a concern over the widespread Surface Hoar that formed before New Years in protected areas at low to mid elevations.  Most of the Surface Hoar was likely destroyed prior to this storm in many areas with the exception of isolated protected areas where it is likely preserved. Ski conditions have improved greatly but caution is advised in the lower elevations where objective hazards (rocks, down trees, stumps, etc.) are only light covered and the recent cold temperatures have weakened the mostly supportive mid-pack to a point where it is much less supportive in the lower elevations. 

recent observations

White Wing: Overall shallow snowpack, 60cm - 1m +(where drifted). Some cornice formation at the summit form strong SW winds, and very small pockets 4-5" deep of windslab failing while traversing top of ridge. Compression tests produced consistent failures near old snow/new snow interface with poor quality sheers (unconsolidated slab).  Compression tests also produced consistent failures near the top of the basal facet layer, with Q2-Q3 quality sheers. Extended Column Tests (ECT) tests did not produce failures in the deep facets. ECT tests did produce non-propagating failures at the old/new snow interface.

In exposed area where the snow is thin, large depth hoar was observed with obvious striations and cup shapes. Also, identified buried Surface Hoar layer that developed prior to New Years 20cm down from the surface. Additional Compression Tests failed consistently on the Surface Hoar layer (between 1 and 12 taps - Q1).  ECT tests were inconclusive with failures confined to the shovel width with an unconsolidated slab above the surface hoar.  As the new snow consolidates and settles with time it will form a more cohesive slab over the buried Surface Hoar, which could then become a potential weak layer.  Compression tests also resulted in failures in Depth Hoar layer at various depths (moderate force 11-14 taps, Q2-3). ECT tests did not produce propagation at this level.  

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 16 deg. F.
Max. temperature in the last 24 hours: 21 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 20 mph
Maximum wind gust in the last 24 hours: 25 mph
New snowfall in the last 24 hours: 13 inches
Total snow depth: 75 inches
weather

The resent storm system that brought a foot or better of new snow along Mammoth Crest and the higher elevations continues to move out of California and into Southern Nevada with the jet well to South, which limited orographic snowfall though some convective precip bands deposited several inches in a relatively short time with the highest accumulations in Mono county. As the upper low moves off into southern Nevada, snow showers will decrease with a cold night possible Thursday night if skies clear with more freezing fog possible depending on the low level moisture. Short wave ridging (High Pressure) is then expected into Friday with continued cold temps and light winds. The next weather system is Friday night into Saturday as a shortwave trough quickly pushes through the region. This is not a strong system with initial snow estimates in the 4-8” range through the Sierra with upwards of a foot possible right along the crest. This system is fairly quick moving with shower activity beginning to dissipate by late afternoon.

Sunday through Monday: the overall weather pattern from this week appears more likely to continue through the middle of next week, with relatively weak weather systems moving quickly through the region every 1-2 days. Temperatures will begin to gradually climb to near normal next week with valley winds generally light, though increasing with shortwave passage.

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: CLOUDY. SCATTERED SNOW SHOWERS. CLOUDY. SCATTERED SNOW SHOWERS. MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS.
Temperatures: 18 TO 25 deg. F. 4 TO 11 deg. F. 21 TO 28 deg. F.
Wind direction: WEST WEST WEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 25 MPH. 10 TO 15 MPH. GUSTS UP TO 25 MPH IN THE EVENING. 10 TO 15 MPH IN THE MORNING BECOMING LIGHT.
Expected snowfall: 0-1 in. 0-1 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SCATTERED SNOW SHOWERS IN THE MORNING THEN SNOW SHOWERS IN THE AFTERNOON. CLOUDY. SCATTERED SNOW SHOWERS. MOSTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING. SLIGHT CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 0-1 deg. F. 0-1 deg. F. 0 deg. F.
Wind direction: WEST NORTHWEST NORTHWEST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 30 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH. 10 TO 15 MPH IN THE MORNING BECOMING LIGHT.
Expected snowfall: 0-1 in. 0-1 in. 0 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 the Eastern Sierra Avalanche Center who is solely responsible for its content.

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