Eastern Sierra Avalanche Advisory - 3/26/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 27, 2017 @ 6:58 am
Avalanche Advisory published on March 26, 2017 @ 6:58 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Mammoth - up to a foot+ of new snow fell in some areas Saturday with moderate SW winds, small to large wind slabs is the greatest avalanche concern for today primarily on NW-N-NE-E-SE aspects in the mid to upper elevations , followed by loose wet snow as temperatures rise during the day primarily on W-SW-S-SE-E in the lower to mid elevations. Clouds forecasted ahead of the next approaching system will help to limit the daytime heating and reducing the concern in the upper elevations but may be possible lower to mid elevations. Natural avalanches are unlikely, and human triggered avalanches are possible.  Careful snow pack evaluation, cautious route-finding and conservative decision-making will be essential. 

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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A fast moving spring storm pushed through the area Friday dropping between 4 and 12+ inches of snow on top of a relatively smooth melt/freeze crust creating a good sliding surface. Snow levels stayed above 8000’ for most of Friday. The storm was accompanied by moderate to strong southwest winds that transported much of that new snow onto NW-N-NE-E-SE aspects but be aware that strong gusty winds and localized channeling can form Wind Slabs on unusual aspects, in sheltered terrain, and further downslope than expected in more complex terrain. Light to moderate winds today (Sunday) will continue to form Wind Slabs primarily above 9500’where the snow remains colder and can be easily transported.

Wind Slabs in the mid to upper elevations will likely be encountered along ridgelines, crossloaded gullies and shallow depressions, in and around terrain features that promote drifting and loading. Be wary of hollow, drum-like sounding snow or cracks shooting. Natural avalanches unlikely, human triggered avalanches possible, careful snowpack evaluation, cautious route-finding and conservative decision-making essential. 

Cornice Warning – Freshly formed Cornices are very sensitive and can fail unexpectedly, give them wide berth while traveling along ridgelines, avoid corniced slopes. 

 

Avalanche Problem 2: Loose Wet
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Low to mid elevations, small Loose Wet natural and human triggered avalanches may be possible as the snow surface thaws and becomes wet during the day. An approaching storm system with accompanying increasing cloud cover and moderate to strong SW winds will reduce and slow surface snow thawing in the lower and mid elevations. However, as the surface snow warms through the day (Easterly > Southerly > Westerly) will become possible, especially near cliffs and rock outcrops. Natural avalanches unlikely but small isolated human triggered Loose Wet avalanches possible in steeper terrain as the snow surface softens in the lower and mid elevations. Large pinwheels or snow penetration of boot top or greater are signs the snow is loosing strength and becoming unstable, steep terrain on these aspects should be avoided.  

A number of factors affect the rate of snow thawing: Aspect (Easterly facing slopes begin to thaw first, followed by south and then west in the afternoon); Elevation: lower elevations warm more quickly than higher elevation, Cloud cover: cloud cover can shade the slopes slowing thawing. However, clouds can help to slow or limit overnight cooling limiting how solid the snow sets-up. Thin cloud cover during the day can trap the heating from the sun filtering through producing strong warming, Air temperature: Warmer air temperatures leads to more rapid snowpack warming. 

Caution - small wet slides are dense/heavy and are capable of carrying a rider into hazardous terrain or possibly entrain enough snow to bury a person, especially if a terrain trap is involved.  Early starts are recommended before the snow become saturated and unsupportable. 

 

Avalanche Problem 3: Loose Dry
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Mammoth - June: Sheltered areas with significant unconsolidated loose snow may see Loose Dry releases where disturbed by riders in steep terrain (see Obervations, 3/25 Long Running Slough in Bardini Chutes, 3/25 Loose Wet Slides Carson Peak).  Likely small in size but they can carry a person into hazardous terrain, and can result in burial if combined with a terrain trap. 

advisory discussion

A series of spring storms have moved through the region bring cooler temperatures and moderate snowfalls. The latest storm to move through the region Friday deposited 6-12 inches in Mammoth and June with 3-6 inches elsewhere. Moderate SW winds helped transport the new snow and form Wind Slabs on NW-N-NE-E-SE aspects where a recently formed melt freeze crust formed a slick bed surface. This resulted in widespread avalanche activity at the Mammoth ski area on the upper mountain and a number of reported avalanches in the surrounding terrain. Elsewhere, the snowfall amounts were significantly less with more limited avalanche activity but remains a concern on NW-N-NE-E-SE aspects in the mid to upper elevations

During the recent warm-up, solar aspects saw a significant loss of snowcover with some slopes becoming completely bare below ~8000’.  Creeks below ~8500’ have begun to reemerge from the melting snowpack. The deep snow cover has created some rather big drops to the creek bed below creating a new backcountry hazard to be aware while approach streams and creeks. 

recent observations
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3/25 Long Running Slough in Bardini Chutes

3/25 Loose Wet Slides Carson Peak

3/25 Many Sluffs, Intentional Skier Triggers In Red Cone Bowl

3/25 Widespread avalanches - Mammoth Mountain Ski Patrol Control Efforts

3/24 Earthquake Dome - Stability Tests

3/24 Small loose wet slides - Mt Wood N. Facing

3/24 Mixed Bag On Bradley

3/23 North Fork Bishop Creek

 

Saturday (Hi), Sunday AM (Low)      High   Low    New Snow, SWE (Sat)

Virginia Lakes (9300’):                          40        26               3, .3”   

Ellery Lake (9545’):                               39        14                 N/A,  .4”

Agnew Pass (9450’):                             48         18                10”, N/A

June Mtn Wx Plot (9220’):                    41         25               5”, .3”

Sesame Snow Study (9014’):                 36        24                 13”,  1.4”

Rock Creek (9700’):                              37        18                 4”, N/A

 

South Lake (9600’):                               47        20                 N/A

 

Weather and CURRENT CONDITIONS
weather summary

Sunday thru Monday – The latest in a series of spring storm is still on track to move into the Sierra later today (Sunday). The bulk of the precipitation moves into the Sierra after 6pm, which will result in a period of moderate to heavy snow for elevations mainly above 7000 feet. The main cold front pushes through Monday morning where snow levels may drop down to around 5500 feet. For areas above 7000 feet, between 5 to 7 inches new snow is forecasted. Monday will be brisk as westerly flow transitions to northerly flow behind the cold front. A few lingering showers Monday afternoon with the cold core of the low over north central Nevada, but overall precipitation chances will be on the decrease. As the area of low pressure pulls away and allows ridging to build in across the region, the Sierra and western Nevada should experience a bit of a warming and drying trend.

Wednesday thru Thursday - The leading edge of this system begins to affect areas near the Oregon border Wednesday evening. The surface front slides south by early Thursday and crosses the forecast area by Thursday afternoon.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy. Slight chance of rain and snow in the afternoon. Mostly cloudy. Chance of snow. Mostly cloudy. Chance of snow showers.
Temperatures: 40 to 46 deg. F. 21 to 27 deg. F. 31 to 37 deg. F.
Mid Slope Winds: Light winds becoming southwest Southwest West
Expected snowfall: 0 in. up to 4 in. up to 1 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy. Chance of snow in the afternoon. Mostly cloudy. Chance of snow in the evening, then slight chance of snow after midnight. Mostly cloudy. Chance of snow showers.
Temperatures: 34 to 40 deg. F. 15 to 21 deg. F. 22 to 30 deg. F.
Ridge Top Winds: Southwest Southwest West
Expected snowfall: 0 in. 1 to 4 in. up to 1 in.
Disclaimer
This Avalanche Advisory 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.

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