Eastern Sierra Avalanche Advisory - 3/29/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 31, 2017 @ 6:37 am
Avalanche Advisory published on March 29, 2017 @ 6:37 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

For Wednesday: As temperatures climb into the upper 40s and low 50s Wednesday, so will the risk of loose wet avalanches on sun-heated slopes at middle and lower elevations. Heightened avalanche conditions will exist on specific terrain features. Evaluate the snowpack and terrain carefully to identify features of concern.

For Thursday: A fast moving and turbulent cold front will push its way through the Sierra with strong and sustained west winds and a chance of light snow or hail. There will be a chance of human triggered wind slab avalanches on Thursday at middle and upper elevations where snow accumulates. Generally safe avalanche conditions will exist, but watch for unstable snow on isolated terrain features.

-Hard snow surfaces and slide-for-life conditions may be found on steep exposed slopes early in the day or as strong winds prevent daily melting.-

How to read the advisory

Bottom Line

For Wednesday: As temperatures climb into the upper 40s and low 50s Wednesday, so will the risk of loose wet avalanches on sun-heated slopes at middle and lower elevations. Heightened avalanche conditions will exist on specific terrain features. Evaluate the snowpack and terrain carefully to identify features of concern.

For Thursday: A fast moving and turbulent cold front will push its way through the Sierra with strong and sustained west winds and a chance of light snow or hail. There will be a chance of human triggered wind slab avalanches on Thursday at middle and upper elevations where snow accumulates. Generally safe avalanche conditions will exist, but watch for unstable snow on isolated terrain features.

-Hard snow surfaces and slide-for-life conditions may be found on steep exposed slopes early in the day or as strong winds prevent daily melting.-

Avalanche Problem 1: Loose Wet
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Enjoy the balmy and beautiful spring weather that is expected today (Wednesday). But remember that as the snow surface warms quickly to its melting point, especially around rock outcrops, below cliff bands, and in open bowls throughout the day, loose wet avalanches will be increasingly possible. Time of day is critical for wet point releases. East aspects receive the first radiation of the day, then south, then west as the sun moves across the sky. Lower elevations will warm more quickly than higher elevations. Watch out for pinwheels rolling down the slopes around and above you. Even small point releases are heavy and can be hard to escape, and they can be a sign that larger avalanches are possible.

Avalanche Problem 2: Wind Slab
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Winds shifted to the north Monday night and transported snow onto southerly aspects throughout Tuesday, especially above about 10,000’. Older, harder to trigger, but potentially larger wind slabs may linger on high elevation, southerly aspects Wednesday. Winds will become gusty across the ridge tops Wednesday night as the next spring storm sweeps into the region. Precipitation amounts will be small, but snow levels may drop to highway 395 by 11am on Thursday. By Thursday afternoon, shallow new wind slabs may be found on isolated wind loaded terrain at upper and middle elevations. Expect to see wind loading just below ridges, on cross loaded slopes, and on the leeward sides of convexities in areas where the most snow accumulates. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet.

advisory discussion

Fast moving and windy spring showers, dropping between 1 and 12+ inches of snow, have been affecting our area for the past two weeks. Between these quick hitters, warm temperatures and sunshine have been returning to heat the new snow and cause wet point releases near rocky areas and in steep terrain. The latest storm, on Sunday, left 1 to 3 inches of snow by Monday morning, primarily in the northern half of the forecast zone. Southwest winds associated with this storm created 6” to 18” thick wind slabs on steep, middle and upper elevation terrain. As showers exited the region early Monday, winds shifted to the north and northwest and temperatures rebounded to the 40s and even the 50s.

Temperatures are forecast to climb even higher on Wednesday. Slopes that did not rise to freezing levels yesterday may warm quickly. Rocky outcrops and trees will trap the heat and reflect in back down onto the snow, elevating the hazard of loose wet avalanches. Large pinwheels or ski penetration of boot top or greater are signs the snow is loosing strength and becoming unstable. Pay attention as you travel, notice conditions changing and trends. Perform your own stability tests and identify potentially hazardous terrain.

On Thursday, a cold front will push through our area bringing some snow showers, possibly some thunder and lightening, and most importantly wind. Winds will be strong and sustained starting Wednesday night and a High Wind Warning will be in effect through 8pm on Thursday. Though precipitation amounts are expected to be 1” or less, remember that wind can deposit new snow onto leeward slopes 3 to 5 times faster than it can fall from the sky. It’s this kind of rapid loading that creates wind slabs sensitive enough for you to trigger. In isolated areas where the most snow accumulates, just under ridge lines, convexities, and the side walls of gullies, you may find hollow-sounding wind slabs. Blowing snow and cornice formation will point to where the avalanche danger is increasing. Use these observations to make terrain choices that keep you out of harms way.

Early in the day Wednesday, and as winds cool the snow surface Thursday, firm, slide-for-life conditions will exist. Be careful of your exposure to a slip and fall on steep terrain where consequences are high.

weather

*High Wind Warning in effect from March 30, 08:00 AM PDT until March 30, 08:00 PM PDT*

Dry conditions, light winds, and warm temperatures stick around for one last day before a storm pushes into the region late tonight into Thursday. The storm on Thursday will bring a strong cold front through the Sierra with gusty winds as well as rain, snow, and pellet showers. Cooler temperatures will remain through Friday.

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 mostly cloudy. Mostly cloudy then becoming cloudy. Snow showers likely in the morning. Slight chance of thunderstorms through the day. Chance of snow showers in the afternoon.
Temperatures: 47 to 52 deg. F. 29 to 35 deg. F. 34-43 deg. F.
Wind direction: NW W SW
Wind speed: Light. Gusts up to 25 in the morning. 10 to 15 mph increasing to southwest 20 to 30 mph after midnight. Gusts up to 65 mph. 30 to 45 mph with gusts to 100 mph shifting to the northwest 25 to 35 mph with gusts to 80 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 80% probability...up to 1 in. 20% probability...2 to 4 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Partly cloudy. Partly cloudy. Mostly cloudy then becoming cloudy. Chance of snow showers through the day. Slight chance of thunderstorms in the afternoon.
Temperatures: 42 to 48 deg. F. 24 to 30 deg. F. 29 to 35 deg. F.
Wind direction: N W W
Wind speed: 10 to 15 mph with gusts to 25 mph in the morning becoming light. 10 to 15 mph increasing to 35 to 55 mph after midnight. Gusts up to 80 mph. 40 to 60 mph. Gusts up to 110 mph decreasing to 95 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 80% probability...up to 1 in. 20% probability...2 to 5 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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