Eastern Sierra Avalanche Advisory - 1/18/18

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON January 19, 2018 @ 5:41 am
Avalanche Advisory published on January 18, 2018 @ 5:41 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Avalanche danger is rated LOW today.  This means human triggering of avalanches is unlikely, but not impossible.  Concern still exists in isolated areas surrounding the persistence of weak loose sugary facet snow layers deeper down in the snowpack lying under more firm dense snow, failure in which could produce a large avalanche.  This problem is most likely to be found on E-N-W facing slopes at mid-elevations above 9,500’, especially where snow depths are less than about one meter. Do your own local investigations into these layers to help your decision making, and follow safe travel protocol. 

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.  Recent observations have been showing that this faceted snow is gaining strength.   While the possibility of human triggering remains unlikely 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 to steep slopes.  This is a much trickier problem to identify than wind slabs and storm slabs, and as always variability across a slope can be great. 

As more weight is added to the snowpack with the incoming storm tonight into tomorrow for Mammoth and North, these layers may once again become a greater 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.  However, snow pit tests resulting in failures and propagation of fractures in these loose layers are becoming fewer and farther between.  In isolated areas, these reactive layers have mostly been found between 40 to 70cm down in areas where the snowpack is generally shallow and less than ~1m.  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.  LOW avalanche danger does not mean that a human triggered avalanche failing on these deeper loose layers is impossible in isolated areas!  Although unlikely, a resulting avalanche has the potential to be large and destructive. 

Avalanche danger will most likely increase tonight and tomorrow as a turbulent storm drops new snow with high winds.  Wind slabs will likely become the greatest concern, followed by storm slabs, and concern will once again increase surrounding the persistent deeper weak layers. 

recent observations
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1/17: Mammoth - Deadman Pass - Rounding facets

1/17: Mammoth Lakes Basin - Hollywood Bowl - High elevation surface melt/freeze, unreactive deeper rounding facets

1/16: Mammoth - TJ Bowl - strengthening wind slabs, search for persistent weak layer

1/15: Mammoth - Persistent weak layers - Propagating ECT tests and Propagation saw tests

1/14: Lundy - Variable Conditions On South Slopes Above Saddlebag Lake

Weather and CURRENT CONDITIONS
weather summary

A fast moving winter storm will be moving in today from the Pacific resulting in high winds, cloudy skies and snow showers starting in the evening.  A High Wind Warning will go into effect from 4pm this afternoon until 4am Friday morning, with ridge top gusts expected to exceed 110mph.  A Winter Weather Advisory has been issued from 10pm tonight until 10am Friday morning.  Snowline tonight should begin around 8000’, and will drop significantly through the night down below 5000’ by morning.  1-1.5’ of snow looks likely for the mountains around Mammoth, but  unfortunately much less to the south, with only an inch for the mountains outside of Bishop.          

High pressure returns for Saturday with clear skies and cold temperatures, and then Sunday night into Monday becomes unsettled as another storm moves in off the Pacific.  Unfortunately the brunt of this storm is looking like it will stay north of us with only a few inches of snow likely for Mono County.  Another small storm is on the horizon for Wednesday-Thursday.    

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Slight chance of snow in the afternoon. Mostly cloudy. Chance of rain in the evening. Snow through the night. Mostly cloudy. Snow showers likely.
Temperatures: 42 to 50 deg. F. 22 to 27 deg. F. 26 to 32 deg. F.
Wind Direction: S shifting to SW SW W
Wind Speed: 20 to 30 mph increasing to 25 to 40 mph in the afternoon. Gusts up to 70 mph. 30 to 45 mph with gusts to 75 mph 15 to 25 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon
Expected snowfall: o in. 3-6 (Bishop Mtns: <0.5) in. 2-5 (Bishop Mtns: <0.5) in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Slight chance of snow in the afternoon. Cloudy. Snow. Mostly cloudy. Snow showers likely.
Temperatures: 35 to 40 deg. F. 17 to 22 deg. F. 20 to 25 deg. F.
Wind Direction: SW SW SW
Wind Speed: 45 to 60 mph with gusts to 90 mph increasing to 50 to 70 mph with gusts to 100 mph in the afternoon. 55 to 75 mph decreasing to 50 to 65 mph after midnight. Gusts up to 110 mph. 30 to 45 mph with gusts to 70 mph decreasing to 20 to 30 mph with gusts to 50 mph in the afternoon.
Expected snowfall: 0 in. 3-7 (Bishop Mtns: <0.5) in. 2-5 (Bishop Mtns: <0.5) 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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