Eastern Sierra Snowpack Summary - 2015-12-31 06:30

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THIS ADVISORY EXPIRED ON January 2, 2016 @ 6:30 am
Avalanche Advisory published on December 31, 2015 @ 6:30 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.

In favored areas where wind transport continues to build shallow windslabs in the mid to upper elevations on slopes of 35 degrees and steeper. 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.

Well-developed facets deep in the snowpack 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. Caution is advised in and around thin areas that can trigger releases onto adjacent slopes or potential steeping down to the facet layer.

Lower elevations, especially south of Mammoth, the snowpack remains relatively thin and semi-supportive and it is possible to trigger releases on steep terrain with multiple objective hazards (trees, rocks, terrain traps, etc.), which could potentially contribute to severe trauma should an incident occur. Avoid steep gullies, unsupported slopes, and exposed areas with shallow windslabs.   

Snowpack Discussion

The snowpack in and around the Mammoth Basin continues to build depth and overall strength after a series of weak storms moved through the region Christmas Eve through Monday with moderate to light snowfalls mixed in with breaks over the course of six days.

The Christmas Eve storm brought 13” of new snow with a total of .6” of water and briefly raising the snow depth from a 60” base to 74” at the Mammoth Pass Snotel site. Light Westerly winds dominated the Christmas Eve storm but shifted to the Northeast (15 to 25 mph) as it exited the region (12/26), which created fresh windslabs in the mid to upper elevations on East to Southeast to South aspects producing variable ski conditions, except sheltered areas. Over the course of the weekend the snow settled a total of 7” and strengthened considerably while windslab sensitivity slowly declined. Another weak system moved in late Monday adding a quick 3” of snow to the snowpack, which helped to cover the surface windslabs and greatly improved the ski conditions while adding minimal load. Overall the snowpack in the Mammoth Basin is gaining strength, which is helping to compensate for the deep basal facets, which continue to be relatively weak but currently not reactive. The concern continues to be thin areas and isolated windslabs on terrain 35 degrees and steeper where a rider could possibly trigger a release which could propagate deeper into the pack or to adjacent slopes on Northwest - North - East - Southeast aspects. As you move south of the Mammoth Basin the overall snowpack depth decreases appreciably with greater facet development in the mid to lower snowpack while the upper snowpack is: thinner, more variable in depth and supportiveness, and is marginally able to compensate for the weaker layers below. Ski conditions are good in sheltered areas in the mid to upper elevations while low elevations are good where coverage is sufficient but is varies greatly in its supportiveness with plenty of objective hazards (rocks, stumps, logs, etc.) 

recent observations

Snow depth is slowly building in the Mammoth Basin. However, field observations Wednesday in the lower elevations along the southern end of the Sherwins shows a shallower snowpack that is weaker and more variable in nature. This combination has produced a well faceted mid to lower snowpack with relatively new and unconsolidated snow on top that has yet gained enough strength to fully compensate for the weak basal layers. Exercise extreme caution on terrain 35 degrees and steeper at all elevations, especially where objective hazards can contribute to the potential for injury or worse, if a slide should occur.

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 8 deg. F.
Max. temperature in the last 24 hours: 26 deg. F.
Average wind direction during the last 24 hours: SW-NE
Average wind speed during the last 24 hours: 5-25 mph
Maximum wind gust in the last 24 hours: 45 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 57 inches
weather

A reinforcing shot of cold air will filter in Thursday as a low pressure closes off over northern CA/NV. A northeasterly gradient will tighten up as the surface high builds over the intermountain west. Cloud cover will increase along the Sierra due to the formation of a stratus layer as moisture banks up against the Sierra with scattered show showers possible over the higher elevations, little accumulation, if any, is expected. Temperatures will cool even more behind this push of colder air. By Thursday evening, temperatures will drop into the teens and single digits across the region. The cold temperatures combined with a steady East-Northeast wind will bring wind chills down below zero for the Sierras with daytime highs struggling to reach the freezing mark through Saturday.

Sunday through Wednesday...

Mainly dry conditions Sunday with widespread cirrus clouds, a small possibility of showers mainly near the Sierra Monday, with a better potential for precip Tuesday and Wednesday. A series of split troughs are forecast to move into the region with tracks varying, which will greatly affect the amount of snow that falls in Sierra. 

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 PARTLY CLOUDY
Temperatures: 18 TO 25 deg. F. 3 TO 9 deg. F. 14 TO 21 deg. F.
Wind direction: NORTHEAST NORTHEAST EAST
Wind speed: 10 TO 15 MPH IN THE MORNING BECOMING LIGHT. 10 TO 15 MPH WITH GUSTS TO 25 MPH. 10 TO 15 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: PARTLY CLOUDY PARTLY CLOUDY PARTLY CLOUDY
Temperatures: 21 TO 27 deg. F. -1 TO 6 deg. F. 12 TO 19 deg. F.
Wind direction: NORTH NORTH NORTHEAST SHIFTING TO THE SOUTH IN THE AFTERNOON.
Wind speed: 10 TO 20 MPH. GUSTS UP TO 30 MPH DECREASING TO 25 MPH IN THE AFTERNOON. 10 TO 20 MPH WITH GUSTS TO 25 MPH. 10 TO 15 MPH WITH GUSTS TO 25 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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