Eastern Sierra Avalanche Advisory - 1/10/18

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THIS AVALANCHE ADVISORY EXPIRED ON January 11, 2018 @ 7:00 am
Avalanche Advisory published on January 10, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

ADISORY UPDATE

Avalanche danger is greater today than it has been in weeks due to fresh, newly formed and still forming wind slabs.  West winds increased significantly overnight, and will continue increasing throughout the day.  1-1.5ft of snow fell with light winds over the past 2 days in the Mammoth area above 9,500’, and is ripe for wind transport.  Human triggered avalanches large enough to bury, injure or kill a person will be likely and natural avalanches will be possible at higher elevations above yesterday’s very high wet snow line.  Avoid steep wind loaded slopes >~32 degrees above ~9,500’.  It is possible that one of these avalanches could step down into old weak snow layers and trigger a much larger avalanche. 

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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A warm wet storm dropped 1-1.5ft of snow with light winds over the past 2 days in the Mammoth area above 9,500’ (lesser amounts to the north and south).  Below 9500’, snow was wet and mixed with rain for most of the day.  As temperatures dropped yesterday afternoon and night, upper snow surfaces below ~9500’ froze up, with only potentially a small amount of loose snow fall on top.  Above this elevation however where snow was dry, significant amounts of new soft un-wind-effected snow made for great skiing yesterday afternoon.  Last night west winds increased dramatically and will continue to increase today, transporting this loose snow into sensitive dangerous wind slabs.  Avoid wind-loaded terrain >~32 degrees.  These will likely be non-west facing slopes below ridgelines, cross-loaded slopes, and around other features that promote drifting.  Hard wind slabs can often fail well above the trigger point.  Above ~9500’, and especially above 10,000’, resulting avalanches could easily be large enough to bury, injure or kill a person. 

Avalanche Problem 2: Persistent Slab
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A new snow slab from recent snowfall and wind load could be the missing ingredient to make underlying weak layers in the snowpack truly dangerous.  The underlying snowpack structure in many areas is a thin mix of crusts and loose weak sugary facets.  Snow pit tests have been showing that these weak layers have the potential to propagate fractures, but up until now they have not resulted in avalanche activity.  However, with a significant new slab of snow on top, these weak layers could awaken, and a trigger in the right spot could result in a large destructive avalanche.  A trigger could be a rider or skier hitting a thin sweet spot, the force of a smaller avalanche, or the weight of new snow.  This is a much trickier, and deadlier avalanche problem than others.  While much less likely to be triggered, if it is, the outcome could be very bad.  This snowpack structure is most likely to be found on non-southerly facing slopes above 9,000’, and can’t be investigated without digging.  A lot is still unknown about how these deep layers will react, but with new sensitive wind slabs today that could fail and trigger this deeper layer, it is best to avoid steep slopes above ~9,500’.        

advisory discussion

A complex snowpack and avalanche situation exists at the moment.  The easy and obvious part is fresh sensitive wind slabs today as discussed above.  Beyond that, it is much more complex.  The latest storm did not bring nearly as much snow as anticipated, and snowlines remained quite high for the bulk of it.  While this definitely resulted in less of an increase in overall avalanche danger, it actually added more complexity to the avalanche problem.  Had we gotten 3’+ of new snow, widespread natural avalanches would have been much more likely, hopefully resulting in either the cleaning out of some of the deeper weak layers that have been the focus of recent concern, or burying them deeper where they can begin to start strengthening.  As it turns out, enough snow has fallen likely to have led only to a few isolated natural avalanches, leaving the questionable snowpack even more questionable and perhaps closer to the brink of slope-wide failures.  The wind slab problem is the immediate and obvious problem for today.  Less obvious is the possibility that one of these smaller slides could be the stress necessary to trigger one of these deeper layers and result in a much larger more destructive avalanche.  As the storm slab and wind slab problems settle, then we are left with overall more weight and slab ontop of the weaker underlying pack, where a person could conceivably find a thinner weaker spot that could propagate a failure across an entire slope.  This problem could last for awhile!  It will be important to dig and investigate in coming days, and use conservative decision making to avoid a potentially deadly avalanche until we learn more. 

Weather and CURRENT CONDITIONS
weather summary

As the storm clears out today, West winds are on the increase, with gusts exceeding 60mph over ridgetops.  Temperatures will rebound from yesterday afternoon’s drop into the mid to upper 30s at 10,000’.  A fast moving shortwave will keep skies cloudy today with the chance of scattered snow showers, with no accumulation.  For Thursday, expect partly cloudy skies and breezy conditions as the start of a clear dry high-pressure period which will likely last until the beginning of next week. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Isolated snow showers Partly cloudy then becoming clear Partly cloudy then becoming mostly cloudy
Temperatures: 41 to 47 deg. F. 25 to 30 deg. F. 45 to 53 deg. F.
Wind Direction: W W W
Wind Speed: 15 to 25 mph with gusts to 45 mph 10 to 15 mph with gusts to 40 mph in the evening becoming light Light winds becoming west 10 to 15 mph in the afternoon. Gusts up to 40 mph
Expected snowfall: <0.1 in. 0 in. 0 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Scattered snow showers. Partly cloudy then becoming clear Partly cloudy then becoming mostly cloudy.
Temperatures: 34 to 39 deg. F. 22 to 27 deg. F. 38 to 44 deg. F.
Wind Direction: W NW W
Wind Speed: 25 to 40 mph with gusts to 60 mph 20 to 35 mph. Gusts up to 50 mph decreasing to 40 mph after midnight 20 to 30 mph. Gusts up to 40 mph increasing to 50 mph in the afternoon
Expected snowfall: <0.1 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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