Avalanche Advisory: Sunday - Mar 4, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 5, 2018 @ 6:02 am
Avalanche Advisory published on March 4, 2018 @ 6:02 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

As the largest storm of the year has come to an end with well over 5’ of new snow accumulation in some areas accompanied by strong SW winds, CONSIDERABLE avalanche danger persists today due to wind slabs, storm slabs, and the possibility of deep slabs.  Today being the first day of sunshine and low winds after the storm, the allure of fresh powder could very well make today the most dangerous day for a deadly avalanche accident to occur.  Evaluate the snowpack carefully, and keep decisions conservative to allow the snowpack more time to settle after this significant new load. 

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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Significant heavy snowfall began Thursday morning thru Friday night, accompanied by strong SW winds, with continued lighter snow showers and upper elevation SW winds thru yesterday.  This has built widespread wind slabs at all elevations.  As snowfall has stopped and winds have decreased significantly today, these slabs have begun to stabilize, making them not nearly as touchy as in previous days, but their stubborn nature could actually make them more dangerous as they entice riders out onto a slope before they fail.  Below ridgelines on NW-N-NE-E-SE slopes will be areas of most obvious concern, but sidewalls of gullies and cross-loaded slopes of any aspect are suspect due to localized wind channeling.  Watch for denser areas of snow, signs such as shooting cracks, and clues like cornice formation to indicate where these wind slabs may exist.  While natural triggering is not likely today, human triggering is.  Remember that wind slabs can vary greatly across a slope, and that one spot may be fine, but a short distance away may not be. 

Avalanche Problem 2: Storm Slab
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Up to 5’+ of new snow has fallen in some areas, especially around the Mammoth area.  This multi-day storm came in with varying densities, at times upside-down, meaning denser snow over lighter snow which is not a good recipe for avalanches.  Significant new accumulation in areas sheltered from the wind ended Saturday night, giving them time to begin stabilizing.  But just how long that process takes isn’t clear, and it is possible they are still sensitive to human triggering today as they continue to stabilize.  It’s important to do your own localized assessments before committing to steeper terrain.  Watch for shooting cracks and woomphing as signs of continued instability, and be extra cautious of convex rolls where storm slabs are more likely to be triggered.

Avalanche Problem 3: Deep Slab
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Deep in the snowpack in the old snow there are layers of weak sugary snow which have been sitting around for most of the winter season.  Up until now there hasn’t been a significant enough snow load sitting on top to make these layers very threatening.  Now there is.  While these layers are buried deeply, making them hard to trigger, if they are triggered a resulting avalanche would be very large and destructive.  The avalanche on Mammoth Mountain yesterday that made national news is a great example of this.  What is a large enough trigger?  A smaller wind slab or storm slab avalanche, a cornice collapse, landing a big jump, or a rider hitting just the wrong spot.  These are very hard to predict, and require extensive digging to fully investigate.  These layers have been found mostly on northerly aspects where old snow has existed prior to this recent big storm.  As a start, give suspect slopes respect by first allowing wind slab and storm slab problems time to settle. 

advisory discussion

A large amount of new snow has fallen on an underlying thin and weak snowpack.  While some of the avalanche problems we face today are fairly standard, let yesterday’s huge avalanche on Mammoth Mountain be a reminder that our current snowpack structure is not standard.  Use caution, increase your margin of safety, and be a bit more conservative than you may be used to being until confidence in our snowpack can grow. 

As the sun comes out in ernest today, some thought goes to the possibility of warming southerly facing slopes and possible instability associated with that.  The cold temperatures will keep these concerns much less than if forecasted temperatures were to be much higher.  Still, keep an eye out for signs of potential instability due to slope warming on E-SE-SW facing slopes such as roller balls originating near rock bands. 

 

recent observations
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3/4 - Mammoth Mountain: Explosive triggered D4+ avalanche, down to weak old layers in the snowpack near the ground.  

Storm totals starting on Friday March 1st thru last night:

-VA Lakes: 22”, (2.5” water)

-Tioga Pass: 27”

-Agnew Pass: 35”

-June Mtn: 25”, (2.1” water)

-Mammoth Pass: 7.7” water!

-Sawmill Pass: 15”

-BigPine 18”

3/4- Lee Vining - V-Bowl - THIN snow

3/2-  Trailhead Snow Survey: June, Tioga, Lundy, Virginia Lakes, Aspendell

3/2- Natural D1.5 Avalanche Low on the Sherwins

Weather and CURRENT CONDITIONS
weather summary

As the most significant storm of the 2017-2018 winter season has ended, expect skies to clear today with light west winds and chilly temperatures with highs in the upper teens to low twenties around 10,000’. 

High Pressure and warmer average to above average temperature are on tap from Monday thru Wednesday, and then models are wavering on the chances of more systems moving in with possible snow Thursday-Fri and late in the weekend. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming sunny. Clear Sunny
Temperatures: 22 to 27 deg. F. 4 to 9 deg. F. 31 to 36 deg. F.
Mid Slope Winds: West Light Light
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming sunny. Clear Sunny
Temperatures: 16 to 21 deg. F. 2 to 7. deg. F. 26 to 31 deg. F.
Ridge Top Winds: Northwest Light Southwest
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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