Eastern Sierra Avalanche Advisory - 3/15/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 17, 2017 @ 6:30 am
Avalanche Advisory published on March 15, 2017 @ 6:30 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

Natural and human triggered loose wet avalanches will become more likely as extensive solar radiation saturates the snow. The majority of these avalanches should be small, but larger avalanches may be possible where the snowpack retains more heat. Timing of these avalanches will depend on aspect, elevation, winds, and cloud cover. East facing and low elevation slopes become unstable earlier. South and West facing slopes, and higher elevations, become more dangerous later in the day. Winds cooling the snow surface will create firm, “slide for life” conditions. Be careful of your exposure to a slip and fall on steep terrain. Evaluate snow and terrain carefully.

How to read the advisory

Bottom Line

Natural and human triggered loose wet avalanches will become more likely as extensive solar radiation saturates the snow. The majority of these avalanches should be small, but larger avalanches may be possible where the snowpack retains more heat. Timing of these avalanches will depend on aspect, elevation, winds, and cloud cover. East facing and low elevation slopes become unstable earlier. South and West facing slopes, and higher elevations, become more dangerous later in the day. Winds cooling the snow surface will create firm, “slide for life” conditions. Be careful of your exposure to a slip and fall on steep terrain. Evaluate snow and terrain carefully.

Avalanche Problem 1: Loose Wet
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Same

Air temperatures will remain above normal through Thursday. Southwest winds will cool the snow surface and could keep things firm until later in the day, but cloudy skies this afternoon and tonight may keep daytime heat from leaving the snowpack. This situation could create conditions favorable to larger wet avalanches at middle and lower elevations by Thursday. This balance between extended solar warming and cooling from increasing winds may make conditions trickier to predict for backcountry travelers. Remember that even small point releases show that slopes are becoming unstable. 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, especially around rocky outcrops, in cirques and gullies, and on treed slopes. Sinking into wet snow up to your boot top, pinwheels, and small point releases are good indicators of instability. Time of day is critical for loose wet avalanches. East aspects receive the first radiation of the day, then south, then west in the late afternoon.

advisory discussion

One phrase has come to mind over the past week (besides “Hawaiian shirt” and “flip flops”): “Melt-Freeze.” Temperatures have been trending ever warmer in the Sierra and turning our former powder into creamy corn snow by mid day on most slopes. The melt-freeze process – with the spring sun warming the snow throughout the day and cold nights refreezing it – generally makes the snowpack more stable over time. Prolonged warming, however, can start to weaken the snowpack on a deeper level. On slopes with tree cover and in rocky gullies where solar radiation doesn’t dissipate as readily as on open slopes, melt water has been saturating the snow down to a foot deep or more. As snow temperatures stay warmer, and water content is higher, loose wet and wet slab avalanches become more likely. Tuesday, South and East aspects warmed early. Surfaces became wet by 11am at elevations up to about 11500’. Slopes also warmed up to alpine elevations. Skiers triggered some small wet slabs at 13000’ on Mt. Tom yesterday. As skies become cloudy this afternoon and into this evening, solar radiation will have an even harder time escaping. Loose wet avalanches may become more widespread and larger, especially near rocks, cliffs, and on slopes with forest cover. Small point releases and roller balls indicate that wet snow is becoming unstable.

 

Light winds from the south have been cooling the snow at upper elevations. Firm wind board can be dangerous in it’s own way since a slip and fall on steep terrain could make for a long slide down and have higher consequences. Winds are set to be stronger from the southwest today through Thursday morning. This could result in slopes taking longer to soften.

weather

A fast moving low pressure system moving through the Pacific Northwest will bring increasing winds and clouds to the Sierra today. High pressure builds back into the region Thursday with lighter winds by Friday. Cold air remains well to the north of our area, so high temperatures will still be well above normal for mid-March.

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: Sunny then becoming partly cloudy. Mostly cloudy then becoming partly cloudy. Partly cloudy.
Temperatures: 48 to 56 deg. F. 28 to 34 deg. F. 49 to 57 deg. F.
Wind direction: SW SW SW
Wind speed: Light becoming 10-15 mph with gusts to 40 in the afternoon. 15-25 mph with gusts up to 50 mph decreasing to 40 mph after midnight. 10-15 mph with gusts up to 50 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Sunny then becoming partly cloudy. Partly cloudy then becoming clear. Sunny then becoming partly cloudy.
Temperatures: 41 to 49 deg. F. 24 to 30 deg. F. 43 to 51 deg. F.
Wind direction: SW SW SW
Wind speed: 15 to 25 mph with gusts up to 40 mph increasing to 50 mph in the afternoon. 20 to 30 mph with gusts up to 55 mph decreasing to 45 mph after midnight. 20 to 35 mph with gusts to 60 mph decreasing to 15 to 25 mph with gusts to 45 mph in the afternoon.
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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