Eastern Sierra Avalanche Advisory - 12/21/17

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THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2017 @ 7:00 am
Avalanche Advisory published on December 21, 2017 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

The greatest avalanche concern for Thursday and into Friday is new wind slabs that began forming with yesterday’s 1-4” of new snowfall, and strong NE winds at tree line and above.  These are most likely to be found on the leeward side of ridges, on the sidewalls of gullies, and around other features that promote drifting, mostly on slopes facing E-S-W.  Be wary of smooth hollow sounding snow.  Natural avalanches could still be possible, and human triggered avalanches likely on Thursday, decreasing to possible on Friday. 

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

With 1-4” of new snowfall at most elevations yesterday, and high prefrontal SW winds which quickly switched around out of the North East as snow intensity increased mid-day yesterday and continued through the night and are expected to continue through Thursday, wind slabs likely formed at tree line and above below ridgelines, on the sidewalls of gullies, and around other features that promote drifting.  These can predominately be found on E-S-W facing slopes, and will likely be sensitive to human triggering though-out the day on Thursday and could remain sensitive Friday as well.  Be wary of smooth, hollow sounding drifts, shooting cracks from your skis, and realize that conditions can change dramatically even within a few yards. 

advisory discussion

1-4” of new snow fell mid-day yesterday, with pre-frontal SW winds which quickly shifted out of the NE as snowfall intensity increased.  These winds have continued through the night at more moderate levels, and are expected to continue through much of the day on Thursday.  New wind slabs have surely formed in leeward locations at tree line and above, below ridgelines, in the sidewalls of gullies and around other features that promote drifting.  These are most likely to be found on E-S-W facing slopes.  The light density and small quantity of this snow is not too concerning in areas that did not receive loading from the wind. 

Prior to this storm yesterday the snowpack has undergone a lot of faceting (weakening sugary snow) on E-N-W facing slopes, and areas where these new wind drifts are overlying this weak faceted snow could remain more sensitive to human triggering for much longer. 

Beware that many early season obstacles still exist, and with a few inches of light new snow, my be even harder to see than before this small storm.  

weather summary

High pressure builds back over the region for the end of the week through the weekend.  Expect seasonal temperatures with highs in the low to mid 20s around 10,000’ for Thursday, and into the mid 30s for Friday.  Moderate northerly winds are expected over 10,000’ with gusts into the 70s for Thursday, decreasing quite a bit for Friday. 

Models are showing an increased chance of precipitation as early as Christmas Eve afternoon, but it is still too early to say anything with much confidence.

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: Sunny Clear Sunny
Temperatures: 22 to 27 deg. F. 14 to 19 deg. F. 35-40 deg. F.
Wind Direction: light N NW
Wind Speed: light Gusts up to 40 mph decreasing to 25 mph after midnight 10-15 mph
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 20 to 25 deg. F. 13 to 18 deg. F. 32-37 deg. F.
Wind Direction: N N NW
Wind Speed: 30 to 50 mph with gusts to 75 mph 25 to 35 mph with gusts to 50 mph decreasing to 15 to 20 mph after midnight 15 mph with gusts to 25 mph
Expected snowfall: 0 in. 0 in. 0 in.
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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