Eastern Sierra Avalanche Advisory - 1/17/18

THIS AVALANCHE ADVISORY EXPIRED ON January 18, 2018 @ 6:00 am
Avalanche Advisory published on January 17, 2018 @ 6:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

While the overall avalanche danger has dropped to LOW, concern still exists surrounding the persistence of weak loose sugary facet snow layers deeper down in the snowpack lying under more firm dense snow.  While human triggering of these deeper layers is unlikely, it is not impossible, and a resulting avalanche has the potential to be large and harmful.  This problem is most likely to be found on E-N-W facing slopes at mid-elevations above 9,500’, especially for areas with snow depths less than about one meter. 

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The thin snowpack that exists throughout our forecast area has kept layers of loose sugary faceted snow under layers of firm dense snow concerning.  While the possibility of human triggering remains low and isolated, once triggered an avalanche of this type has the potential to be large and destructive.  Consider this potential, and do your own localized investigations thru digging before deciding to commit to steep consequential terrain.  Continue to use caution and good travel technique, such as exposing one person at a time.  This is a much trickier problem to identify than wind slabs and storm slabs, and as always variability across a slope can be great. 

If the storm on track for this Thursday and Friday comes thru with significant new snowfall, these layers may re-awaken and once again become a more widespread concern.    

advisory discussion

Recent observations throughout the accessible snowy terrain continue to find many areas where layers of loose sugary faceted snow exist under more dense snow.  Snow pit tests continue to result in failures in these loose layers, some of which continue to show the potential for fracture propagation.  These reactive layers have mostly been found between 40 to 70cm down in areas where the snowpack is generally shallow and less than ~1m.  Despite these test results, there have been no reports of recent avalanche activity, test slopes have not been found to fail, and there have been no reports of whoomphing since before the weekend.  Just because we have lowered the avalanche danger from MODERATE to LOW does not mean that a human triggered avalanche failing on these deeper loose layers is impossible!  Although unlikely, a resulting avalanche has the potential to be large and destructive.  If the projected storm for this Thursday and Friday materializes, the added snow could once again increase the concern over these deeper weak layers. 

Of note, the small disturbance that passed thru our area yesterday did not result in any new snowfall, so the concern over potential new small wind slabs did not materialize.

Weather and CURRENT CONDITIONS
weather summary

Today is the calm before the storm, with above average temperatures reaching into the mid 40s near 10,000’, cloudy skies, and light winds with moderate gusts at ridgetops.

Thursday, winds will increase significantly with 100mph gusts at higher elevations by the afternoon before snowfall begins Thursday night into Friday.  1-1.5ft of snow looks likely for the mountains.  Snow levels should begin under 8000’, dropping to valley floors by Friday morning.  

High pressure returns for Saturday, and then Sunday night into Monday becomes unsettled as another storm moves in off the pacific, but unfortunately the brunt of this storm is now looking like it will stay north of us. 

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 Mostly cloudy Mostly cloudy. Slight chance of snow in the afternoon.
Temperatures: 47 to 53 deg. F. 30 to 35 deg. F. 42 to 50 deg. F.
Wind Direction: SW SW SW
Wind Speed: Light winds becoming 10 to 15 mph in the afternoon. Gusts up to 30 mph. 10 to 15 mph with gusts to 40 mph increasing to 20 to 30 mph with gusts to 60 mph after midnight. 25 to 35 mph increasing to 30 to 45 mph in the afternoon. Gusts up to 85 mph.
Expected snowfall: 0 in. 0 in. up to 2 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy Mostly cloudy Mostly cloudy. Slight chance of snow in the afternoon.
Temperatures: deg. F. deg. F. deg. F.
Wind Direction: W SW SW
Wind Speed: 15 to 20 mph with gusts to 35 mph. 20 to 30 mph with gusts to 50 mph increasing to 30 to 50 mph with gusts to 75 mph after midnight. 45 to 60 mph with gusts to 90 mph increasing to 50 to 70 mph with gusts to 100 mph in the afternoon.
Expected snowfall: 0 in. 0 in. up to 2 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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