Avalanche Advisory: Tuesday - Mar 6, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 7, 2018 @ 7:00 am
Avalanche Advisory published on March 6, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Avalanche danger will be kept at CONSIDERABLE on NW-N-NE slopes at tree line and below due to deeply buried weak layers and the possible large size of an avalanche that a human could trigger.  While natural avalanches are unlikely, isolated areas exist where the right combination of weak snowpack structure and terrain make a large dangerous human triggered avalanche possible.  Remotely triggered avalanches occurred on Sunday, and evidence of large-scale slope collapses continued thru yesterday.  This is not a normal situation for the Sierra.  Consider terrain and snowpack carefully and be conservative in your decision making to avoid being on or under steep slopes where this dangerous snowpack structure exists. 

2. Moderate

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

3. Considerable

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

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Deep Slab
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Weak loose faceted snow buried deeply in the snowpack is our greatest avalanche concern.  The substantial new snow load from the Thursday thru Saturday storm is now acting as a cohesive slab which can propagate deep failures large distances.  These weak layers can be found from 90cm to over 1.5m deep.  Remotely triggered avalanches with large crowns were reported and observed on Sunday, as well as large scale slope failures with deep cracks spidering across slopes even yesterday.  As time passes and the new snowload continues to settle, these deep layers will be more and more difficult to trigger and avalanche danger will decrease.  It may take a rider hitting just the right spot, the weight of several people in a small area, or other greater forces such as landing a jump to trigger these deep layers.  Areas of greatest concern are NW-N-NE facing slopes below tree-line that had snow coverage prior to this last storm, but suspect slopes could exist above tree-line as well where old thick hard wind deposits are not bridging these deep weak layers.  Sheltered areas, convex rolls, slopes that are unsupported below, and slopes with either fewer or more deeply buried anchors (such as bushes) are more concerning as well.  This problem is not obvious, and tracks on a slope does not mean a slope is safe.  Pay special attention to cues such as whoomphing and shooting cracks, investigate thru digging (over a meter deep), and if in doubt, avoid steep slopes.  A resulting avalanche would likely be large and destructive.     

Avalanche Problem 2: Loose Wet
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Partly cloudy skies, light winds, and warmer temperatures than we have seen in quite awhile could cause slopes to warm enough to show some signs of instability.  Concern over small loose wet avalanches will be greatest at mid to low elevations, especially near rock bands that heat up.  The possibility exists that this warming could be enough to weaken bonds and make a small slab release possible.  If clouds remain few, this concern will be greatest on SE-E-SW facing slopes.  If cloud cover increases, this threat could be greater for more northerly facing slopes.

advisory discussion

The snowpack structure this season is much more complex than we have seen in a long time.  Conditions aren’t what we typically see in the Sierra.  Since the last major storm of Thursday thru Saturday, wind slabs and storm slabs have settled, but deeply buried weak sugary facet layers remain reactive.  There is a narrow band where this concern exists, but it is a big enough concern with the potential for large and destructive human triggered avalanches that we have kept the danger rating CONSIDERABLE until more time has passed to allow the upper snowpack to settle more, and the likelihood of triggering to decrease.  With warmer temperatures, these deep weak layers will gradually strengthen, but they will remain a lurking concern for some time.  You won’t find this a concern in lower elevation slopes which weren’t covered in snow prior to this recent storm.  Upper elevation exposed areas could be less likely as well either due to much deeper snow deposits, or significant bridging from thick hard wind-slabs.  The middle elevations which have been covered with enough snow all season to cover up bushes and other potential anchors are the most concerning in terms of the greatest potential for a large deep slab avalanche occurring.  Mammoth and June areas seem to be the most concerning where this problem is more widespread, as opposed to further south toward Bishop and north toward VA lakes where snow lines were previously much higher and the band of sheltered snow coverage much more narrow.   

Besides avalanche concern, lower elevation slopes which were previously very thin or bare before this last storm have lots of barely covered obstacle.   

Weather and CURRENT CONDITIONS
weather summary

For today expect partly cloudy skies, warmer temperatures with highs in the mid 30s around 10,000’, and light winds.  Clouds will increase tomorrow before a very weak low pressure trough moves in north of us which will bring higher winds for Thursday, but not any precipitation.  Some chance of light flurries exists thru the weekend before a more substantial storm looks likely for early next week.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy. Partly cloudy then becoming mostly cloudy. Mostly cloudy.
Temperatures: 39 to 45 deg. F. 17 to 22 deg. F. 40 to 46 deg. F.
Wind Direction: Light Light Light winds becoming southwest in the afternoon.
Wind Speed: Light Light Light increasing to 10 to 15 mph with gusts to 25 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy Partly cloudy then becoming mostly cloudy. Mostly cloudy.
Temperatures: 33 to 38 deg. F. 13 to 18 deg. F. 32 to 39 deg. F.
Wind Direction: Light Light winds becoming south after midnight. South
Wind Speed: Light Light increasing to 10 to 15 mph after midnight. 15 to 25 mph. Gusts up to 35 mph in the afternoon.
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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