Eastern Sierra Snowpack Summary - 2016-01-02 07:13

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THIS ADVISORY EXPIRED ON January 5, 2016 @ 7:13 am
Avalanche Advisory published on January 2, 2016 @ 7:13 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.

Windslab in favored areas where wind transport continues to build shallow windslabs in the mid to upper elevations on slopes of 35 degrees and steeper. These can be found on NW-N-E-SE aspects near and above treeline and exposed areas in the lower elevations, especially in gullies, open bowls, rolls, and near cross loaded terrain features. Thickness and supportiveness varies with elevation with the thickest slabs in the mid to upper elevations where there’s plenty of snow for transport producing slabs of Pencil hardness or greater. Lower elevations you’ll likely encounter thinner slabs with varying degrees of supportiveness. Avoid freshly drifted and loaded leeward slopes, especially where terrain features can contribute to cross loading, corniced slopes, or where the terrain can create an objective hazard (cliffs, large rock & boulders, or terrain traps). 

Avalanche Character 2: Deep Slab
Deep Slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to Storm or Wind Slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

Deep slab concerns continue due to the well-developed basal facets that formed early season at all elevations. The facets currently are not overly reactive but remain a weakness within the snowpack that are capable of failing with severe consequences. The basal facets have been found at all elevations throughout the region with one notable exception, near Mammoth Crest, where the snowpack is deepest. The thicker snowpack has helped to insulate the basal facets from the cold temperatures, which limits further deep facet formation and the added weight is slowly compressing and strengthening the deeper layers overtime. Elsewhere, the snowpack is thinner and the basal facets are well established and remain a significant weak layer. Currently this layer isn’t overly reactive but on terrain 35 degrees and steeper triggered releases remains a possibility, especially where the overlying snowpack is thin (near terrain features, sides of gullies, break-overs). Caution is advised in and around thin areas that can trigger releases onto adjacent slopes or potential steeping down to the facet layer or where multiple objective hazards (trees, rocks, terrain traps, etc.) could potentially contribute to severe trauma should an incident occur. Avoid steep gullies, unsupported slopes, and exposed areas with shallow windslabs overlaying basal facets.   

Snowpack Discussion

The snowpack continues to slowly settle and gain strength overall, especially along the Mammoth Crest where the snowpack depth is greatest. Elsewhere, more of a mixed bag depending on total snow depth with the well-documented basal facets lurking throughout the range. The issue is how thick and strong are the layers above the basal facets and what is required to trigger a failure. No recent avalanche have been noted which is a good indicator of improving avalanche conditions but triggered releases (skier, boarder, snowmachines, cornice falls, etc.) are possible on slopes of 35 degrees and steeper at the mid to upper elevation, especially in thin areas where a failure can trigger deeper release on adjacent slopes or a localized windslab releasing and possibly triggering slopes below because of the sudden loading. 

recent observations

The recent cold snap has generated a significant temperature gradient in the upper snowpack, which is breaking down the most recent snowfall and driving strong facet formation in the upper pack. This is creating nice facet skiing in sheltered areas with a mostly supportive mid-snowpack in the mid to upper elevations with less depth and consistency in the lower elevations. Lower elevations have less snow but plenty of objective hazards and variability outside of the Mammoth Lakes Basin, which has a bit more snow overall and less variability.

Wednesday Observations: Hemlock Ridge (June Lakes Region), the snow is patchy at the lowest elevations and slowly increases in depth with elevation. The strong temperature gradient in the upper snowpack continues to drive strong facet formation in the upper pack with the recent addition of Surface Hoar (3-6mm, 4-8mm in favored areas) in June Lake. This makes for nice skiing currently but could be become a major weak layer as it is buried by next week’s storm systems.

June Lake: No signs of recent avalanche activity. Extensive facet development in the upper snowpack with Surface Hoar extending from valley bottom to over 9700’! Snow stability tests (9670’, Aspect NNE, Slope 38 degrees): Upper most layer (14cm) well-developed facets with Surface Hoar (3-6mm, 4-8mm in favored areas near the valley floor) with a mostly supportive mid-pack of windslab or denser snow overlaying 30 plus cm of Depth Hoar at Ground. The mid-pack appears to be compensating for the basal weakness, which was clearly indicated in stability tests with all failures at the mid-pack basal facet interface, some columns failing on isolation. ECT tests clearly propagated across the same mid-pack basal facet interface (ECTP 14,17). This reinforces the concern for potentials deep slab failures, possibly triggered from shallower areas on slopes 35 degrees and steeper. Observations further confirmed widespread windslab deposited by strong Southwest – West winds deposited on NW-N-E-SE aspects near and above treeline and exposed areas in the lower elevations. The recent formation of facets and Surface Hoar along the surface has made for good skiing in sheltered trees in the June Lake area, despite the cold conditions. However, this could become a weak layer if it is buried under the snow forecast for next week.  I’ll be watching this closely next week.

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 18 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: 15-30 mph
Maximum wind gust in the last 24 hours: 56 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 51 inches
weather

The recent cold Low pressure over northern CA/NV with well below normal temperatures and steady east flow the over the mountain west will shift northward bringing slightly warmer temperatures for the next couple days. Saturday morning, increasing cloud cover from the next approaching system. The increasing clouds from the next system will inhibit significant warming of temperatures during the day, but should allow temperatures to stay warmer overnight. As the approaching low pressure system moves closer to the CA coast, the energy will split into southern CA/NV, which is not favorable for significant snow for the Sierra with only a chance of snow forecast for the Sierra Sunday night into Monday.

Tuesday through Wednesday: A fairly active pattern is still expected for next week with several weather systems pushing through the Sierra. None of these systems appear to be overly strong. However, the storms will help increase the snowpack in the Sierra with up to a couple of feet possible along the Crest over the course of the week. Temperatures will be near to slightly below normal for the period with snow expected in the mountains and a mix of rain and snow in the valleys for each of these winter systems. The best overall chance for precipitation appears to be from Tuesday through Wednesday, as Pacific moisture is pulled in ahead of this trough. This broader area of moisture will bring a period of at least some light precip to much of the region with snow amounts east of the Sierra crest dropping off more quickly if the main energy splits farther south. 

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: MOSTLY CLOUDY MOSTLY CLOUDY MOSTLY CLOUDY
Temperatures: 24 TO 31 deg. F. 13 TO 20 deg. F. 29 TO 36 deg. F.
Wind direction: SOUTHWEST SOUTH SOUTH
Wind speed: 20 TO 30 MPH WITH GUSTS TO 45 MPH 20 TO 30 MPH WITH GUSTS TO 45 MPH 25 TO 35 MPH WITH GUSTS TO 50 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: MOSTLY CLOUDY MOSTLY CLOUDY MOSTLY CLOUDY
Temperatures: 25 TO 31 deg. F. 15 TO 21 deg. F. 28 TO 34 deg. F.
Wind direction: SOUTHWEST SOUTH SOUTH
Wind speed: 30 TO 40 MPH WITH GUSTS TO 60 MPH 30 TO 40 MPH WITH GUSTS TO 60 MPH 30 TO 40 MPH WITH GUSTS TO 65 MPH
Expected snowfall: 0 in. 0 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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