Eastern Sierra Avalanche Advisory - 3/8/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 10, 2017 @ 6:25 am
Avalanche Advisory published on March 8, 2017 @ 6:25 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

For Today: There will most likely be small point releases in specific areas and potentially large avalanches in isolated areas. Watch for unstable snow like pinwheels or roller balls. The highest danger will likely be found on steep slopes where solar radiation is the most intense. Middle and upper elevation slopes may still have isolated wind slabs on the leeward side of ridges and other features where drifting has occurred that could be sensitive to human trigger. Evaluate snow and terrain carefully.

For Thursday: Temperatures will be high but this will not the the first time this week that the snowpack has been warmed and we will have generally safe avalanche conditions. Watch for unstable snow on steep, isolated terrain features. Although less likely than Wednesday, natural and human triggered loose wet avalanches may still be possible.

How to read the advisory

Bottom Line

For Today: There will most likely be small point releases in specific areas and potentially large avalanches in isolated areas. Watch for unstable snow like pinwheels or roller balls. The highest danger will likely be found on steep slopes where solar radiation is the most intense. Middle and upper elevation slopes may still have isolated wind slabs on the leeward side of ridges and other features where drifting has occurred that could be sensitive to human trigger. Evaluate snow and terrain carefully.

For Thursday: Temperatures will be high but this will not the the first time this week that the snowpack has been warmed and we will have generally safe avalanche conditions. Watch for unstable snow on steep, isolated terrain features. Although less likely than Wednesday, natural and human triggered loose wet avalanches may still be possible.

Avalanche Problem 1: Loose Wet
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    Decreasing Danger

Air temperatures are forecasted to reach almost 50 degrees F at 10,000’ for the next two days with clear sunny skies, and low winds. As snow surfaces warm and moisten throughout the day, likelihood will increase for small loose point releases to occur, some of which could entrain enough snow to bury a person. Even small point releases are good indicators that slopes are becoming unstable and that larger avalanches may be possible. These smaller slides can knock you off balance and can carry you into dangerous terrain if you aren’t careful. Pay attention as the snow surface warms and becomes unsupportable. Watch for pinwheels and point releases originating around rock bands and outcrops. Time of day is critical for wet loose avalanches. East aspects receive the first radiation of the day, then south, then west in the late afternoon. Wet avalanches often occur the first time temperatures rise and the likelihood of these point releases may decrease somewhat for Thursday.

Avalanche Problem 2: Wind Slab
  • Character ?
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  • Trend ?
    Decreasing Danger

Strong to moderate SW winds were steady through Monday, and with 8-13” of new snow throughout the region they had an easy time of building wind slabs on leeward slopes. Wind affected snow has been reported at all elevations. As temperatures warm, winds calm, and time passes these slabs will decrease in sensitivity. Though more stubborn to human trigger, and probably less destructive in size, be aware of the potential for these slabs lingering in less sun-affected areas: below high elevation ridges and on steep, cross loaded slopes on Wednesday. Triggering wind slabs will be less likely on Thursday as time passes and temperatures warm.

advisory discussion

The latest storm that moved through the area on Sunday was accompanied by periods of intense snowfall and moderate southwest winds. Precipitation rates of 1 to 2” per hour were recorded and sensitive wind and storm slabs developed throughout the period.

Precipitation Totals and Temperatures (as of 4:00 AM, 3/6/17)
Location                                  Snow Water
VA Lakes (9445’)                     13”     1”
Tioga Pass (9798’)                  12”     N/A
Ellery Lake (9645’)                   N/A    .3”
June (9148’)                             8”      .3”
Gem Pass (10750’)                  N/A    .6”
Mammoth Sesame (9014’)      10”     .8”
Mammoth Pass (9,500’)           N/A   N/A
Rock Creek (9600’)                  10”    N/A
Saw Mill-Big Pine (10200’)       10”    N/A
Big Pine Creek (10000’)           N/A   N/A

As the system exited the area on Monday and Tuesday, generally southwest winds remained blowing 15-40 mph at the top of Lincoln Mountain in Mammoth; gusts often hit 60 mph or more. Wind effects were reported from north of June down to Aspendell in Bishop. Mammoth Mountain Ski Patrol triggered wind slab avalanches with 3 foot crowns at mid mountain locations. Yesterday these wind slabs were still touchy, cracking around skis, on northerly slopes above 9000’. Though time will decrease your likelihood of triggering these wind slabs, shooting cracks and hollow sounding snow beneath your feet should set off alarm bells in your head.

Most recently, a warming trend has dominated the Sierra and it’s only going to get warmer through Friday. Wet snow instabilities have been seen on steep rocky slopes at lower and middle elevations. Sinking into wet, unsupportable snow and pinwheels rolling down the hill from rocky outcrops and from beneath your skis are some of the first signs that slopes are warming and becoming unstable.

At lower elevations where a melt-freeze crust remains below the snow surface, there is a buried weak layer that just might be triggered by larger point releases running down and overloading it. This layer of large faceted crystals has been seen near Rock Creek and in the June Lake area and is buried below the crust and about a foot of softer snow. Triggering this layer may be unlikely for now, but intense warming may change things over the next few days.

weather

Mostly sunny skies will be the norm this week as the storm track moves north into Oregon and Washington. Temperatures will have a spring time feel the next few days with afternoon highs climbing into the 40s and 50s. Winds will generally be light from the northwest.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Partly cloudy Partly cloudy then becoming clear Sunny
Temperatures: 38-48 deg. F. 22-32 deg. F. 42-52 deg. F.
Wind direction: Northwest Var Var
Wind speed: Light Light Light
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Partly cloudy Partly cloudy then becoming clear Sunny
Temperatures: 28-38 deg. F. 19-29 deg. F. 32-42 deg. F.
Wind direction: Var Var Northwest
Wind speed: Light Light 10-15 in the morning becoming light
Expected snowfall: 0 in. 0 in. 0 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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