Avalanche Advisory: Thursday - Mar 15, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 16, 2018 @ 7:00 am
Avalanche Advisory published on March 15, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

For areas that received 6-20” of new snow Tuesday night thru Wednesday, CONSIDERABLE avalanche danger will exist above tree line due to lingering and fresh wind slabs from moderate SW winds that are expected to continue thru today. 

MODERATE danger exists due to Storm Slabs in sheltered steep areas, and the remote possibility that a deeper persistent weak layer could be triggered.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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4-20” of new snow fell across our region Tuesday night thru Wednesday, accompanied by moderate upper elevation SW winds.  While snowfall ended early last evening, these SW winds will continue throughout today to redistribute snow onto leeward facing slopes.  This will be of most concern above tree line.  Watch for denser snow deposits on NW thru NE thru SE facing slopes just below ridgelines, in the sidewalls of gullies and around other features that promote drifting.  Cornice formation can give clues to wind direction.  Watch for shooting cracks, and do your own hand pit tests to see how well wind slabs are bonding, and remember that lots of variation can occur across a slope.  While natural avalanches will be unlikely, human triggered avalanches will be likely.    

Avalanche Problem 2: Storm Slab
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For areas that received upwards of 8” of new snow Tuesday night thru Wednesday, storm slabs will remain a concern today for sheltered steep slopes.  While this new snow fell generally right-side-up, with heavy denser snow at the bottom, and light fluffy snow on-top (which is great for stability), there have been reports of atleast one area that received significant amounts of graupel mid-storm.  Snowpits behind June Mountain yesterday afternoon found some areas where a loose graupel layer existed 5cm thick that poured out of the pit wall, and tests showed that the storm slab above this layer failed easily.  This was enough to send the forecaster away from steep slopes above.  Do your our localized assessments to determine how well storm slabs are bonding. 

Avalanche Problem 3: Persistent Slab
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If you are tired of reading about this deep buried weak sugary facet layer, imagine how we feel to have to keep writing about it.  But it’s significant enough that we feel that we must.  While tests are showing that it is taking more and more force to trigger this deep weak layer, they continue to show clean and energetic propagation of fractures.  New snow load adds stress to the snowpack, and it is during these times before the snowpack adjusts that these deep layers become more concerning.  This layer has been found anywhere from less than half a meter deep in areas such as convict, and likely rock creek, to over a meter and a half deep in other areas such as around mammoth.  It will take a large trigger in a shallower area of the snowpack to get this deep layer to fail.  The force of a smaller wind or storm slab avalanche is one example, or a snowmobile landing a jump, or a large cornice drop.  E-N-W facing slopes at mid-elevations are most concerning, but upper elevations are not out of the question (they are just typically much more bridged by dense windslabs and deeper snow).  As more and more snow settles and buries these layers deeper, they will become even harder to trigger.  But if an avalanche results, it will likely be quite large and destructive.  Do your own investigations and weigh this into your decision-making.    

advisory discussion

The latest winter storm dropped from 4-20” of new snow in the mountains from Tuesday night thru Wednesday evening.  The greatest amounts of new snow fell around the Mammoth area, with lesser amounts north, and the least amounts south in the mountains outside of Bishop.  Upper elevation SW winds were moderate throughout however, and 4” of new snow is still enough to be transported into a dangerous wind slab.  Today will be the calm between storms, allowing storm slabs to settle, and wind slabs to bond where they aren’t being continuously built by on-going SW winds today.  The next big storm moving in this evening, with peak snowfall amounts Friday night, will bring another spike in avalanche danger. 

recent observations

Storm Totals (Tuesday night thru Wednesday)

  • VA Lakes (9400’): 6” Snow, 0.8” Water
  • Tioga Pass (9970’): 8” Snow
  • Agnew Pass (9350’): 9” Snow
  • Gem Pass (10750’): 1.65” Water
  • June (9150’): 9” Snow
  • Sesame Mammoth Mountain (9000’): 14” Snow, 2.3” Water
  • Mammoth Pass (9500’): 20” Snow, 2.7” Water
  • Sawmill Pass (10200’): 5” Snow
  • Big Pine Creek (10000’): 4.5” Snow

-3/14 - Mammoth area: Morning ski cuts resulting in 4-12" storm slab crowns.   

-3/14 - June - Dream Peak area: Concerning within storm loose graupel layer 

-3/14 - Mammoth - Punta Bardini: New storm snow, deep weak layer

Weather and CURRENT CONDITIONS
weather summary

*Winter Storm Warning in effect from 8pm Thursday to 11am Saturday.

For today expect a mix of sun and clouds, moderate upper elevation SW winds and highs in the mid 20s around 10,000’.  Last night and today will be a break in precipitation before a more significant, but still moderate, cold Sierra storm moves into our area tonight thru Saturday morning.  1-2ft of new snow is expected for Mono County, with potential for double that at upper elevations along the crest.  Heaviest snowfall should occur Friday afternoon into the night.  Expect less than half of this amount of snowfall for the mountains south toward Bishop.  This will be accompanied by moderate to strong SW winds.  Snow showers could linger Saturday, then expect drier weather for Sunday and Monday, before potential for another significant warmer storm exists for Tuesday night thru Wednesday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Partly cloudy then becoming mostly cloudy. Chance of snow in the afternoon. Mostly cloudy. Chance of snow in the evening, then snow after midnight. Cloudy. Snow through the day. Slight chance of thunderstorms in the afternoon.
Temperatures: 25 to 31 deg. F. 14 to 19 deg. F. 23 to 31 deg. F.
Wind Direction: SW SW SW
Wind Speed: 15 to 25 mph. Gusts up to 35 mph increasing to 45 mph in the afternoon. 20 to 30 mph with gusts to 60 mph. 15 to 25 mph with gusts to 50 mph.
Expected snowfall: up to 2 in. 4-10 in. 5-11 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Partly cloudy then becoming mostly cloudy. Chance of snow in the afternoon. Cloudy. Snow. Cloudy. Snow.
Temperatures: 18 to 24 deg. F. 8 to 14 deg. F. 17 to 23 deg. F.
Wind Direction: SW SW SW
Wind Speed: 20 to 35 mph. Gusts up to 45 mph increasing to 55 mph in the afternoon. 30 to 45 mph increasing to 35 to 55 mph after midnight. Gusts up to 80 mph. 30 to 50 mph with gusts to 75 mph.
Expected snowfall: up to 2 in. 5-11 in. 6-18 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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