Avalanche Advisory: Friday - Jan 26, 2018

THIS AVALANCHE ADVISORY EXPIRED ON January 27, 2018 @ 7:00 am
Avalanche Advisory published on January 26, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Strong to very strong SW to W winds over the previous two days combined with 3-10” of new snowfall early yesterday morning will keep avalanche danger at MODERATE today at mid to upper elevations due to wind slabs.  While natural avalanches will be unlikely, human triggered avalanches could remain very possible on wind deposited slopes greater than about 35 degrees.  Evaluate snow and terrain carefully!     

2. Moderate

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

2. Moderate

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

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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While overall less sensitive and touchy, the wind slab problem is more complex and unobvious today than yesterday.  Strong SW to W winds yesterday accompanied by 3-10” of new snowfall early yesterday morning led to sensitive wind slab development in exposed areas at all elevations.  Two days ago before the new snowfall, VERY strong SW-W winds blew enough to transport the old loose snow into some very dense wind slabs.  While still out of the west, winds significantly decreased last night and will stay light today, which means two things:  On the positive side, new wind slab development will have mostly ceased since yesterday, and so overall wind slabs will be less sensitive today.  HOWEVER, on the negative side, there is no longer the obvious clue of visible snow being transported to let you know where wind slabs were deposited previously, and how sensitive they are.  Wind slabs become harder to trigger and strengthen with time, but just how long that takes could be from a few hours to a few days (with cold temperatures extending that range).  It is likely that in some steep areas, human triggering of wind slabs will still be possible today.  Look and feel for smooth firm snow, hollow sounds, and realize that dense wind slabs from two days ago could be covered by less dense snow on the surface.  It is important to do your own localized assessments of how well isolated wind slabs are bonding, and to recognize that great variability exists even across small areas on a slope.  Wind slabs are likely to be found just below ridgelines, the sidewalls of gullies, cross-loaded slopes and around other features that promote drifting.  If at all in doubt, give committing steep terrain more time to settle.     

Avalanche Problem 2: Persistent Slab
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The shallow snowpack throughout much of our area has resulted in the formation of loose sugary layers of faceted snow deeper down in the snow.  Snow-pit tests have continued to show throughout most of the winter that these layers have the potential to fail cleanly, even more so lately with the past week of cold weather.  However an overlying slab of snow significant enough to cause avalanches has been lacking.  It is still unknown if 3-10” of new snow from yesterday morning with plenty of wind to deposit into greater depths at greater densities is enough to be the missing ingredient for a sizeable dangerous avalanche to be triggered.  Areas of greatest concern are non-southerly facing slopes at tree-line and below.  A human hitting a thin weak spot, or the force of a smaller wind slab avalanche, could be enough to act as a trigger resulting in a much larger than expected avalanche, potentially failing far above the trigger.  This is a tricky problem that is hard to detect without digging.  Pay attention to signs such as whoomphing on lower angle terrain, which is an indication that a deeper weak layer in the snowpack has failed (and if the slope had been steeper, that an avalanche could have resulted). 

advisory discussion

Avalanche danger is most obvious during and soon after significant loading events such as heavy snowfall, or wind blowing and depositing snow.  This danger typically becomes less obvious and more tricky over the following few days after a storm, as some slopes can stabilize quickly, but others may take more time.  Today is one of those less obvious tricky days, where it is actually more likely for someone to be involved in an avalanche.  The wind slabs that formed up on that slope yesterday MAY have stabilized by today, and it COULD be fine to ride now, or they may still be sensitive and waiting for a trigger in just the right spot.  Add to this the persistent underlying shallow and weak snowpack that exists in many areas at tree line and below, and the avalanche problem becomes even more complex on days like today.  As the MODERATE danger rating advises, evaluate snow and terrain carefully!  And if there's doubt, give those steep mountain slopes some more time, there's lots of sunshine in the near future.     

recent observations
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Evidence of Natural Avalanches was seen in multiple areas yesterday:

        -VA Lakes

        -Mammoth Crest

        -Negatives behind June

Snowpack and Weather observations:

1/25: June - Hemlock Ridge behind - New snow, patches of wind slab

1/25: Mammoth - TJ and Hammil Bowls - testing wind slabs and persistent deep layers

1/25: Mammoth - Redcone - storm snow

1/25: VA Lakes - New snow and tests

1/24: VA Lakes - New sensitive windslabs

1/24: Mammoth - Red Cone - Wind and persistent layers

Weather and CURRENT CONDITIONS
weather summary

Expect partly cloudy skies today with light westerly winds and seasonable temperatures with highs in the mid to upper 20s around 10,000’.  For Saturday, temperatures will be warmer, with increased northwesterly winds in the morning, and some continued clouds. 

Long term:  Models are in agreement that another long-term high pressure ridge is setting into place, bringing temperatures back up above normal and keeping conditions dry and sunny thru at least the first week of February.

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: Partly cloudy then becoming mostly cloudy. ostly cloudy then becoming partly cloudy. Partly cloudy.
Temperatures: 29 to 35 deg. F. 19 to 24 deg. F. 39 to 47 deg. F.
Mid Slope Winds: light light light
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy then becoming partly cloudy. Partly cloudy.
Temperatures: 23 to 28 deg. F. 16 to 21 deg. F. 33 to 38 deg. F.
Ridge Top Winds: West West Northwest
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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