Mammoth Basin Snowpack Summary - 2015-12-12 08:02

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THIS ADVISORY EXPIRED ON December 15, 2015 @ 8:02 am
Avalanche Advisory published on December 12, 2015 @ 8:02 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
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 recent winds have formed wind slabs in much of the higher elevations; especially steep terrain features prone to wind loading (e.g., leeward or cross loaded terrain features above treeline where new snow has accumulated). In more protected terrain (mid to lower elevations) soft slabs of 1-1.5 feet are possible on slopes of 35 degrees and steeper where bonding or anchoring is insufficient. The underlying early season snowpack is shallow, variable, and predominantly weak. The additional loading from the next system will further increase the risk of avalanche. 

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.
Snowpack Discussion

Thursday’s storm system came in as advertised with high winds and significant snowfall. The system started out initially warm and wet depositing dense snow on top of a relatively warm snow surface. By Thursday night, temperatures cooled dramatically and snow density decreased (aka Powder!) as a second wave of moisture moved through the region. Mammoth Ski Area recorded 14 – 20 inches of new snow with avalanche control producing failures primarily within the new snow. The snowpack is adjusting to this new load and bonding between layers is suspect. A report of a slide in the trees above Horseshoe Lake from an intentional ski cut further confirms that the snowpack is still adjusting to the add stress of the new snow.  Areas of concern are drifted areas, loaded leeward slopes of 30 degrees and steeper. Overall conditions have greatly improved with good riding on slopes less than 30 degrees. The snow is not too deep, just fun. 

recent observations

The strong winds and heavy snowfall associated with this system has rapidly loaded an already weak and shallow snow pack. This combination generally requires a little time to adjust to the new load and for bonds between layers to strengthen. However, Friday night’s cold temperatures did little to aid settlement and with another strong system due in on Sunday, the stress will only increase. Areas of current concerns are wind slabs and soft slabs that have formed on leeward slopes and side loaded features in the Mid to Upper elevations. Snow pit data (12/11/15) suggests there are at least a couple of week layers to be aware of. The upper most layer of the new snow is relatively light (low density) with weak bonding to the new snow below and is easily transported by wind, forming soft slabs and tender drifts in lee areas. The second layer is a layer of preserved facets just below a melt-freeze crust that formed on the old snow surface. Stability Test results: ECT15, Q2 on 2-3 mm facets, CTM 12, 13, 15, Q2 on 2-3mm preserved facets. The two layers together could result in a failure within the new snow that steps down deeper into the snowpack or fails on the preserved facets with the potential for deep burial. The weather system due Sunday may be the tipping point needed for extensive natural avalanches. Stay tuned and stay alert.

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 11 deg. F.
Max. temperature in the last 24 hours: 16 deg. F.
Average wind direction during the last 24 hours: SSW
Average wind speed during the last 24 hours: 15 mph
Maximum wind gust in the last 24 hours: NA mph
New snowfall in the last 24 hours: 3 inches
Total snow depth: 30 inches
weather

Saturday should provide a brief break for most of the area before the next significant system approaches the region early Saturday night through Sunday. This is a very dynamic system with a strong pressure gradient ahead of it (wind) and a short-lived moisture tap. A Winter Weather Advisory is in effect from 10 am to 10 pm PST Sunday. Timing: snow will begin late Sunday morning with the heaviest snowfall expected late Sunday afternoon into the evening with snow accumulations of 10 to 18 inches possible in the Sierras west of highway 395, 2 to 6 inches east. Snow levels:  near 7000 feet late Sunday morning, falling to 5000 feet late Sunday afternoon. Winds will be southwesterly, 20 to 30 mph with gusts to 55 mph, gusts of 115 mph are possible at ridge top. Anticipate snow and blowing snow with periods of near white out conditions. Snow is expected to taper off by late Sunday night for all areas with the far south seeing the heaviest showers through the night. 

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: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW IN THE EVENING. CLOUDY. SLIGHT CHANCE OF SNOW IN THE MORNING...THEN SNOW DEVELOPING FROM NORTH TO SOUTH IN THE AFTERNOON.
Temperatures: 25 TO 33 deg. F. 20 TO 28 deg. F. 30 TO 38 deg. F.
Wind direction: NORTHWEST WEST SOUTHWEST
Wind speed: 5 TO 25 MPH WITH GUSTS 35 TO 50 MPH 30 TO 45 MPH WITH GUSTS TO 65 MPH 60 TO 70 MPH WITH GUSTS TO 105 MPH
Expected snowfall: 0 in. 0 in. 3 to 8 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW IN THE EVENING CLOUDY. SLIGHT CHANCE OF SNOW IN THE MORNING...THEN SNOW DEVELOPING FROM NORTH TO SOUTH IN THE AFTERNOON
Temperatures: 23 TO 28 deg. F. 17 TO 24 deg. F. 25 TO 30 deg. F.
Wind direction: NORTHWEST NORTHWEST SOUTHWEST
Wind speed: 25 TO 35 MPH WITH GUSTS TO 50 MPH 40 TO 50 MPH WITH GUSTS TO 75 MPH 70 TO 80 MPH. GUSTS UP TO 110 MPH INCREASING TO 120 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 4 to 9 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 USDA Forest Service who is solely responsible for its content.

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