Eastern Sierra Avalanche Advisory - 4/19/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 21, 2017 @ 6:32 am
Avalanche Advisory published on April 19, 2017 @ 6:32 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

For Wednesday: Between 3 and 10 inches of new snow fell across the forecast area Tuesday above 8500’ with moderate to strong south and west winds. Winds will continue at snow transporting speeds throughout Wednesday keeping wind slabs a concern. Highs will be in the 40s by afternoon with sunny skies significantly heating the new snow and creating a loose wet avalanche problem. Natural avalanches will be possible, human triggered avalanches likely. Careful snowpack evaluation, cautious route-finding, and conservative decision making will be essential.

For Thursday: After a good freeze Wednesday night, Thursday will be slower to warm but winds will be stronger as a weak storm sweeps north of us. Though less likely than Wednesday, human triggered wind slab avalanches will still be possible in specific leeward areas where snow is still available for wind transport. Human triggered loose wet avalanches will also remain possible as temperatures climb throughout the day, especially on steep rocky slopes. Evaluate the snow and terrain carefully to identify features of concern.

How to read the advisory

Bottom Line

For Wednesday: Between 3 and 10 inches of new snow fell across the forecast area Tuesday above 8500’ with moderate to strong south and west winds. Winds will continue at snow transporting speeds throughout Wednesday keeping wind slabs a concern. Highs will be in the 40s by afternoon with sunny skies significantly heating the new snow and creating a loose wet avalanche problem. Natural avalanches will be possible, human triggered avalanches likely. Careful snowpack evaluation, cautious route-finding, and conservative decision making will be essential.

For Thursday: After a good freeze Wednesday night, Thursday will be slower to warm but winds will be stronger as a weak storm sweeps north of us. Though less likely than Wednesday, human triggered wind slab avalanches will still be possible in specific leeward areas where snow is still available for wind transport. Human triggered loose wet avalanches will also remain possible as temperatures climb throughout the day, especially on steep rocky slopes. Evaluate the snow and terrain carefully to identify features of concern.

Avalanche Problem 1: Wind Slab
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For Wednesday: Up to 10 inches of new snow fell Tuesday and southwest winds were packing it into dense slabs on leeward slopes above about 9,000’. Winds will increase through Wednesday afternoon as a storm passes to the north of our area. Actual wind speeds wont be impressive, but 15-25 mph ideal for building slabs with soft cold snow. Favored areas could have large sensitive wind slabs. Expect to see wind loading just below ridges, on cross loaded slopes, and on the down wind sides of convexities. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet. Watch for blowing snow and recent cornice formation and use these observations to help you make safe terrain choices.

For Thursday: West winds will increase on Thursday and in places with an ample supply of transportable snow, wind slabs could continue to grow on leeward terrain features at upper and middle elevations. A few snow showers near the crest are not out of the question for Mono county, which would contribute additional building material to the wind slab problem. Expect these slabs to be less touchy in most areas, but below ridgelines and across slopes where there is adequate snow to blow around triggering wind slabs will still be possible. Evaluate the snow and your terrain choices carefully.

Avalanche Problem 2: Loose Wet
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For Wednesday: Highs are forecast to be in the mid 40s below 10,000’ and near 40 above 10,000’ on Wednesday, which will begin to melt the new storm snow that fell on Tuesday. As this new snow on the surface warms quickly to its melting point in the middle of the day, especially around rock outcrops, below cliff bands, and in open bowls, loose wet avalanches will be increasingly likely. Small point releases could be widespread, and larger avalanches may be encountered where Tuesday’s storm snow was deepest and on steeper, sun baked slopes where rocks or vegetation provide extra warming. These avalanches will follow the sun around the compass dial. East aspects will receive the first solar radiation in the morning, then south, then west and north by late afternoon. Lower elevations will warm more quickly than higher elevations. Watch out for pinwheels rolling down the slopes around and above you. Pay attention as the snow becomes warmer and wetter. Sinking in to your boot tops in wet, unconsolidated snow is a good sign of instability.

For Thursday: Slopes will heat up more slowly and winds will be stronger, which can help to cool the snow surface. But temperatures will overall be warmer and the sun will still be warming the snowpack as it makes its journey across the sky. You may still find heightened loose wet avalanche conditions on slopes where extra heat from cliffs, rock faces, and trees results in additional localized thawing. Sheltered areas may thaw more quickly.

advisory discussion

An atmospheric river storm dumped up to 3’ of snow on top of an old melt-freeze crust between 4/6 and 4/9. Since then, a moderate storm between 4/12 and 4/14 added a thinner layer of newer snow in many areas, but the diurnal spring freeze-thaw cycle has largely strengthened the snowpack. Warm and sunny weather over the weekend created another melt-freeze crust on the surface, even above 12,000’ in some places. As a spring storm moved through the area on Monday afternoon, new wet snow began sliding on this more recent layer. Snow levels fluctuated during the storm and many areas received rain Monday before snow in the early morning hours on Tuesday. The initial rain soaked and weakened the old snow down to at least the crust that was buried on 4/6. Rain fell again up to almost 9,000’ Tuesday morning before a final shot of snow by Tuesday afternoon. Loose wet avalanches were prevalent during the storm at middle elevations, and while the snowpack was at its wettest slab fractures were observed in the Mammoth Lakes Basin. Temperatures were colder at higher elevations and the strong southwest winds accompanying the storm redistributed new snow onto leeward slopes above about 9,000’.

A hard freeze overnight on Tuesday should have helped to the underlying snowpack re-bond and its strength to rebound. As for the 3-10 inches of wind packed storm snow, it will take time to settle and strengthen. Remember that wind can deposit new snow onto leeward slopes 3 to 5 times faster than it can fall from the sky and even smaller amounts of snow can create large wind slabs in just a short amount of time. So these fast moving spring showers can create wind slabs sensitive enough for you to trigger. In areas where the most snow has accumulated, just under ridge lines, convexities, and the side walls of gullies, hollow-sounding wind slabs will be sensitive to your weight. Blowing snow and cornice formation will continue to point to where the avalanche danger has increased. Use these observations to make conservative terrain choices that keep you out of harms way.

Highs will creep up into the 40s Wednesday and Thursday. As temperatures climb, so will the risk of loose wet avalanches. As the sun heats the new snow to its melting point it will become less stable. Buried melt-freeze crusts like the one that we have now are good sliding surfaces for wet snow avalanches. Extra heat from cliffs, rock faces, and trees can result in additional localized thawing. Large roller balls originating from exposed rocks can be a good indicator the loose wet avalanches are becoming increasingly likely throughout the day.

weather

Breezy winds and increasing clouds will be in place today ahead of a final weak system that will quickly move through far northeast California and into the Tahoe Basin tonight into Thursday morning. This system will create breezy winds this afternoon with gusts in the 40 mph range. Winds across the ridgetops will also become gusty, particularly this evening where peak gusts could reach 60+ mph. Any lingering light showers will quickly move out into eastern Nevada by Thursday morning. Thereafter, the weather turns quieter as an actual ridge of high pressure builds across are area for the weekend.

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. Partly cloudy. Scattered snow showers after midnight. Partly cloudy. Isolated snow showers in the morning.
Temperatures: 33 to 43 deg. F. 18 to 28 deg. F. 36 to 46 deg. F.
Wind direction: Light becoming SW SW W
Wind speed: 10 to 15 mph with gusts to 40 mph in the afternoon. 10 to 15 mph with gusts to 50 mph. 10 to 15 mph with gusts to 45 mph.
Expected snowfall: 0 in. 0 in. 90% probability...0 in. 10% probability...1 to 3 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Sunny. Partly cloudy. Isolated snow showers after midnight. Partly cloudy.
Temperatures: 30 to 38 deg. F. 18 to 24 deg. F. 35 to 45 deg. F.
Wind direction: SW SW W
Wind speed: 15 to 25 mph. Gusts up to 35 mph increasing to 50 mph in the afternoon. 20 to 35 mph. Gusts up to 60 mph. 25 to 35 mph with gusts to 55 mph decreasing to 15 to 25 mph with gusts to 40 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 90% probability...0 in. 10% probability...1 to 3 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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