Eastern Sierra Avalanche Advisory - 4/21/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 22, 2017 @ 6:27 am
Avalanche Advisory published on April 21, 2017 @ 6:27 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

Natural and human triggered Loose Wet avalanches will become increasingly possible as slopes warm throughout the day. Heightened avalanche conditions may exist after substantial warming. Watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases. Pay special attention near rock bands, outcroppings, and exposed vegetation.

-Hard snow surfaces and slide-for-life conditions may be found early in the day.-

How to read the advisory

Bottom Line

Natural and human triggered Loose Wet avalanches will become increasingly possible as slopes warm throughout the day. Heightened avalanche conditions may exist after substantial warming. Watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases. Pay special attention near rock bands, outcroppings, and exposed vegetation.

-Hard snow surfaces and slide-for-life conditions may be found early in the day.-

Avalanche Problem 1: Loose Wet
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Spring conditions will continue to dominate the area today as high pressure settles in for the weekend. Highs will be just a few degrees warmer in some places, but in others they will rise almost 10 degrees over yesterday’s warmest thermometer readings. Things are expected to warm quickly today, with most areas reaching above freezing by around 8am. Light, generally north winds will provide little cooling for the snow surface. Some higher elevation areas had dry winter snow yesterday that may be warming for the first time today, which will heighten the risk of loose wet avalanches in those areas. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into deeper snowpack layers. Even small point releases are heavy and can be hard to escape, and they can be a sign that larger avalanches are possible. As the snow surface warms to its melting point throughout the day loose wet avalanches will be increasingly possible, especially around rock outcrops and below cliff bands. Time of day is critical for wet point releases. East aspects receive the first radiation of the day, then south, then west, and finally north as the spring sun moves high into the sky. Lower elevations will warm more quickly than higher elevations. Watch out for pinwheels rolling down the slopes around and above you.

The cornices that formed during the past few days may become fragile with all this warming. Be wary of standing on or under them. Large cornices that break are a hazard and may trigger larger avalanches on the slope below.

advisory discussion

Spring has returned to the Sierra after another fast moving storm swept though on Tuesday and between 3 and 12 inches of new snow fell across the forecast area above 8500’. Snow levels fluctuated during the storm and many areas received rain Monday before snow in the early morning hours on Tuesday. 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. Temperatures were colder at higher elevations and the strong southwest winds accompanying the storm redistributed new snow onto leeward slopes above about 9,000’. Mammoth Mountain Ski Patrol reported large results from avalanche control work on Wednesday morning. Most of these avalanches were triggered in wind slabs with small hand charges and ski cutting.

Winds continued to blow from the west on Wednesday and Thursday and there was enough soft snow still available for transport that snow banners and localized drifting were seen from Mammoth south to Rock Creek. Snowpack observations in Rock Creek showed that some of these wind slabs were slowly healing, but also that the snow at higher elevations is still soft and cold. Near Bishop things were already warming up yesterday. But in other areas, today may be the first time this dry snow becomes wet which will increase the possibility of loose wet avalanches. Rocky outcrops, gullies, and trees will trap the sun’s heat today and reflect in back down onto the snow, also elevating the hazard of wet sloughing. Large pinwheels or ski penetration of boot top or greater are signs the snow is loosing strength and becoming unstable. Starting early and being out of steep, rocky terrain before things get too warm is the best strategy for avoiding loose wet avalanches.

Early in the day, firm, slide-for-life conditions may exist. Be careful of your exposure to a slip and fall on steep terrain where consequences are high.

weather

A ridge over northeast CA and western NV will bring lighter winds, dry conditions, and much warmer afternoons into Saturday. Light low-level easterly flow will remain in place today with high pressure centered over eastern Oregon and western Idaho. This will slightly limit the warming potential with highs around average today.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Friday Friday Night Saturday
Weather: Sunny. Clear. Sunny.
Temperatures: 48 to 58 deg. F. 32 to 37 deg. F. 51 to 61 deg. F.
Wind direction: SW
Wind speed: Light. Light. Light becoming 10 to 20 mph with gusts to 40 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Friday Friday Night Saturday
Weather: Sunny. Clear. Sunny.
Temperatures: 39 to 49 deg. F. 30 to 36 deg. F. 41 to 51 deg. F.
Wind direction: NW SW SW
Wind speed: 15 to 20 mph with gusts to 30 mph. Light becoming 10 to 15 mph with gusts to 25 mph after midnight. 10 to 15 mph with gusts to 45 mph increasing to 20 to 30 mph with gusts to 60 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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