Avalanche Advisory: Thursday - Dec 26, 2019

THIS AVALANCHE ADVISORY EXPIRED ON December 27, 2019 @ 6:40 am
Avalanche Advisory published on December 26, 2019 @ 6:40 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

*Update - Up to 5" of new snow overnight, plenty of loose snow from previous days, and shifting moderate winds mean avalanche danger will be CONSIDERABLE today at tree line and above due to the threat of sensitive wind slabs on all aspects, and MODERATE at lower elevations. LOW avalanche danger exists in lower elevation sheltered terrain.   

 

3. Considerable

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

3. Considerable

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

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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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

8-15”+ of low density new snow fell on the 22nd and 23rd with surprisingly light winds, and another fresh 2-5” fell since midnight last night.  SW winds have been blowing at ideal speeds (30-50mph over ridges) for wind slab development over the last 2 days at mid to upper elevations.  These winds shifted out of the E last night, and are expected to continue out of the E today at similar velocities.  Small natural and human triggered wind slab avalanches have been reported over the last 2 days, and human triggering of small avalanches will be likely today in certain types of areas.  Be on the lookout for denser wind deposited snow just below ridge lines, side walls of gullies and other catchment zones on all aspects in exposed terrain.  Slabs that developed 2 days ago may now be more stubborn to trigger while fresh deposits will be quite sensitive and more obvious.  An avalanche today could be large enough to injure or bury someone.  Wind sheltered terrain will offer safer conditions.  Do your own localized assessments, and keep in mind if one of these wind slabs is triggered in an area with an underlying weak faceted snow layer, the resulting avalanche could be much larger than expected.          

 
advisory discussion

Shifting wind directions with light new snow reminds me of an incident a few years ago that occurred on Red Slate Mtn.  A group went to ski steep confined north facing terrain, and rationalized that since the winds were now blowing out of the north and should be stripping snow from north-facing slopes rather than depositing, that their objective would be free from wind slab hazard.  Their assumption seemed confirmed as they descended the north facing slope and found etched away snow surfaces, only to be quite surprised when the 5th skier down triggered a stubborn wind slab and went for a harrowing 1000’+ ride and luckily only blew out his ACL.  The group saw what they wanted to see, and forgot about the previous day’s SW winds which were loading that same terrain.  Incidents like this are good reminders of the complexities of the snowpack, and our own human influences that affect our decision making.      

 

Recent field observations have found many areas of weak faceted snow under either melt-freeze crusts or under thicker old stout wind-board.  Up until recently these weak layers have not led to unstable slopes, for either there hasn’t been a cohesive enough slab above (melt-freeze crust scenario), or the overlying wind-board is very strong and supports itself.  However, if these weak layers exist under areas of recent new wind load, they may now be a real threat.  We haven't found this scenario yet, but our observations have been quite limited.  

  • Melt-Freeze crust / facet combo:  With the new wind-loading, areas may now exist where a cohesive slab does lie on top of the fragile melt freeze crust/facet combination.  A failure could propagate much further than expected across a slope.  
  • Wind-board / facet combo:  A smaller wind slab avalanche could be enough force now to trigger a deeper failure under the stout wind-board, also resulting in a much larger than expected avalanche.

 

Do your own localized assessments in safe areas, and recognize the potential that may exist.    

 

This light snow has made for great riding conditions, but remember it's still a very thin snowpack in many areas! Beware of those lurking rocks and logs!  

 

Our range is huge, so please share your findings with us thru our observation page!

 
Weather and CURRENT CONDITIONS
weather summary

A weak low pressure system is moving down the coast bringing chances of light snow showers with less than an inch of new snow expected today before clearing tonight.  Moderate winds have shifted out of the east for today, and will shift out of the northeast tonight.  Expect high temperatures near 20°F around 10,000’ today, and warming to back near normal for Friday thru the weekend with highs expected in the upper 20s around 10,000’.  Another weak low pressure system moves in for Sunday and Monday, bringing chances of more light snow with minimal amounts of accumulation. 

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: Mostly cloudy. Chance of snow through the day. Snow levels below 7000 feet. Chance of precipitation is 40%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 21 to 27. deg. F. 8 to 13. deg. F. 26 to 32. deg. F.
Mid Slope Winds: East around 15 mph with gusts to 40 mph. Northeast around 15 mph in the evening becoming light. Gusts up to 30 mph. Northwest around 15 mph with gusts to 35 mph.
Expected snowfall: 30% probability up to 1 inch. 70% probability no accumulation. | SWE = less than 0.10 inch. in. No accumulation. | SWE = none. in. No accumulation. | SWE = none. in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Chance of snow through the day. Snow levels below 7000 feet. Chance of precipitation is 45%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 13 to 19. deg. F. 4 to 9. deg. F. 21 to 27. deg. F.
Ridge Top Winds: East 15 to 30 mph with gusts to 45 mph. Northeast 15 to 30 mph. North 15 to 30 mph.
Expected snowfall: 30% probability up to 1 inch. 70% probability no accumulation. | SWE = less than 0.10 inch. in. No accumulation. | SWE = none. in. No accumulation. | SWE = none. 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 Avalanche Advisory is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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