The new snow will likely be slow to bond to the old snow surface due to recent facet formation in the upper snowpack. As snowfall rates increase Friday afternoon into Friday night, storm slabs will likely form in wind sheltered locations on all aspects above ~8,000’. Density changes within the storm can also create weaknesses where failures can occur within the new snow. Natural storm slab avalanches will become possible as the storm progresses; human triggered avalanches will be very likely through Saturday on slopes greater than 35 degrees. Small slides can be dangerous when combined with terrain traps or objective hazards.
The approaching cold winter storm is expected to produce relatively low density snow that will be easily transported by the forecasted moderate to strong SW winds, forming dangerous Wind Slabs on leeward slopes (NW-N-NE-E-SE aspects) and near crossloaded terrain features on slopes greater than 35 degrees. As the storm progresses, natural Wind Slab avalanches will become possible and human triggered avalanches very likely to certain as snowfall rates increase Friday afternoon/night into early Saturday morning. Large avalanches, resulting in serious injury or death will be possible from Friday evening through Saturday. Signs of Wind Slabs include smooth dense snow and hollow sounding slabs. Avoid slopes 35 degrees or greater where the potential for wind slabs exist. Use extra caution traveling below actively wind loading slopes.
Note: Friday morning into early afternoon, moderately strong Southerly winds at upper elevations could produce small isolated wind slabs on exposed leeward terrain features that could be sensitive to human triggering.
Two facet layers deep in the snowpack are a concern as the stress and strain increases over the course of the storm. These layers can be found in the lower 1/3 of the snowpack, primarily above ~ 9,000’ on NW-N-NE-E facing slopes. Even though these weaknesses are deeply buried, a triggered release higher in the snowpack could step down into these deeper layers, possibly producing large, very destructive avalanches.
A strong winter storm is poised to move into the region midday Friday and is forecasted to bring significant snowfall along with moderate to strong SW winds, forming potentially dangerous Wind Slab and Storm Slab avalanche conditions beginning midday Friday and into the weekend. The cold temperatures and moderate to strong SW winds will favor snow transport with tender wind slabs forming on NW-N-NE-E-SE aspects, especially in the mid to upper elevations. Additionally, the recent cold snap has generated a strong temperature gradient in the upper snowpack, which has produced extensive faceting of the surface snow. This will slow bonding of the new snow, increasing the potential for Storm Slabs Friday thru the weekend. The deep snowpack structure is quite complex, especially above 9,000’ on W-N-E facing slopes, where multiple crusts and weak facet layers exist. However, these weak layers are beginning to recede into the depths of the snowpack and would require a large trigger to fail, such as a surface avalanche triggering deep persistent weakness resulting in large destructive avalanche. Large natural avalanches will become increasing likely and the possibility of large human triggered avalanches are very likely. Use extreme caution on or below slopes greater than 35 degrees.
Because of the widely varying conditions, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.
Other concerns: Outside of the Mammoth Lakes basin, early season conditions exist. There are plenty of rocks, stumps, down trees just under the snow surface, use caution while riding and playing in the backcountry.
Red Lake Bowl, Virginia Lakes (12/21/2016): From the THD good snow coverage. Virginia Lake and Red Lake ~2' (65 cm) snow over ice. Light wind effect or skin over 1 Fingerish over 4 Finger with Pencil to knife below until ~ 20 cm above ground where it's possible a layer of facets formed early season (Oct. - Nov.) Strong Temperature gradient in the upper snowpack but quickly subsides below 20 cm from surface. Stability tests produced only moderate to hard failures 21 cm from surface with an irregular failure plane. Strong temperature gradient exists in the top 20 cm with faceting. Below 20 cm the gradient eases with decreasing facet formation. Red Lake Bowl slid during last storm cycle. ~ 1300' wide x 800' vertical, visible crown 1-2', max 6'. Slide broke through 3' of ice! No recent slides noted. Ski conditions above treeline, quasi-breakable wind skin, improving as you descend into the treeline and lower gullies.
Red Lake Bowl (Dec 21, 2016, 2:37 PM)
Elevation: 10250 ft.
Aspect: ENE Angle: 30°
Sky Cover: FEW
Precipitation: No Precipitation
Wind Speed: Calm, subjective
Current Temp:-1.5°C Trend:➘
Snow Depth: 256cm
Snow Temperature 20cm: -8.5° C
Boot Penetration: 5cm
Comments: Below 185cm snow is mostly Pencil to knife. ~ 235 cm from surface possible facets to ground.
Stability Tests: ECTN16 Q3 21cm Failed directly under shovel, no propagation. CT13 Q3 21cm CT4 Q3 21cm CT21 Q3
South Lake - Mt Thompson (12/19/2016) -Toured from Parcher’s Resort (drivable with 4x4 thru snow to ~1/2mi beyond Parcher’s), to end of Rd at South Lake, to Mt Thompson and back. 6-18" of snow at South Lake 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 propagate. Test pit dug at 12,600', N facing slope, 265 cm of total snow. 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 occurred near the end of last week's storm. Thanks to Zak Mills and Jason Templeton for breaking the trail most of the way the day before!
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.