Eastern Sierra Avalanche Advisory - 2/25/17

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THIS AVALANCHE ADVISORY EXPIRED ON February 26, 2017 @ 6:56 am
Avalanche Advisory published on February 25, 2017 @ 6:56 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
bottom line

Although new snowfall ended Wednesday morning, moderate upper-elevation winds out of the SW to W, and yesterday out of the N in areas, have continued to load leeward slopes.  Isolated human triggered avalanches will be likely and natural avalanches still possible above tree line in steep areas that have continued to receive wind-loaded snow.  These sensitive areas are most likely to exist on the leeward side of ridges, sidewalls of gullies, and around other features that promote drifting.  Carefully evaluate the snowpack and terrain before getting on steep upper elevation slopes.      

How to read the advisory

Bottom Line

Although new snowfall ended Wednesday morning, moderate upper-elevation winds out of the SW to W, and yesterday out of the N in areas, have continued to load leeward slopes.  Isolated human triggered avalanches will be likely and natural avalanches still possible above tree line in steep areas that have continued to receive wind-loaded snow.  These sensitive areas are most likely to exist on the leeward side of ridges, sidewalls of gullies, and around other features that promote drifting.  Carefully evaluate the snowpack and terrain before getting on steep upper elevation slopes.      

Avalanche Problem 1: Wind Slab
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  • Size ?
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  • Trend ?
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Although its been over 3 days without new snowfall, moderate SW to W winds, and yesterday in areas out of the N, have continued to load leeward slopes, especially at upper elevations.  Significant snow banners (blowing snow) were observed yesterday over high peaks from Whitney to Bishop, and these appeared to mostly be Northerly winds.  Reports of evidence of recent natural wind-slab avalanche activity continue to come in, some appearing to potentially have occured within the past couple of days.  As moderate SW to W higher elevation winds continue this morning with gusts up to 60mph, decreasing to 45mph in the afternoon, isolated areas will continue to be wind-loaded.  Wind-slabs heal with sensitivity decreasing over time, but just how long this process takes, from a few hours to a couple days, isn’t know.  Be wary of firmer denser snow that could be hollow sounding, snow being visibly transported, and cornices that could break underneath you and indicate a likely wind-loaded slope below.  Do your own localized stability assessments in safe places, and avoid steep slopes, especially convex areas, where suspect wind-slabs may exist.  Even a small avalanche could knock someone off balance and result in injury or death in dangerous terrain, or bury someone in a terrain trap where a steep slope ends in a depression or gully.    

advisory discussion

It's been 3 whole days since we have received any new snowfall, quite a dry spell for this season!  The snowpack continues to settle in sheltered locations, with snow sensors showing an average of about 2" of decrease in total snow depth over the past 24 hours.  Even though there has been no new snow, the winds have continued to transport snow over the past 3 days, especially In exposed higher elevation areas.  Mostly out of the SW and W, these winds continue to form wind slabs that, depending on how old they are, are likely to be sensitive to human triggering in isolated locations.  We have eliminated the concern over loose-wet-point releases today on solar aspects, as no evidence of this type of instability was seen yesterday, and cloudier skies and even cooler temperatures are expected today.  For sheltered locations, the storm slab threat is also low as the snowpack continues to settle and stabilize.  Even though there are some layers that look concerning in some snowpits, stability tests have not been shown to be producing concerning results.  With an eventual new snowload, this could be a different story.             

recent observations

-1/24 - Mammoth - Sherwins: Wind trasnport, recent evidence of wind-slab avalanche

-1/24 - Crowley - Red Moutnain: Winds, windslabs

-1/24 - Bishop to Lone Pine - High Peaks: Lots of snow banners off high peaks from northerly winds

-1/23 - June - Dream Peak Trees: Old storm avalanche activity, deeper melt-freeze layer with facets, creep cracks 

*Report came in yesteday of huge slide debris from avalanche likely occuring toward the end of the last major storm in Lee Vining Canyon that filled the approach narrows to the ice climbing area.  Just a heads up to ice climbers of the avalanche threats that exist there!

weather

A small disturbance moves through our area today and tonight, bringing cloudy skies and the possibility of scattered snow flurries, with no real accumulation expected.  Expect temperatures to stay cold, with highs in the upper teens around 10,000', and SW to W winds gusting up to 60mph in the morning and decreasing into the 40s in the afternoon.

For tomorrow (Sunday), expect mostly cloudy skies with light snow showers possible in the afternoon, again with no real accumulation expected.  SW to W winds will be on the increase, with gusts reaching into the 70mph range by the afternoon above 10,000', and temperatures will be slightly warmer at this elevation with highs in the mid 20s. 

Long-Term:  The previously speculated upcoming Atmospheric River event is looking very weak, with light snowfall expected Sunday night into Monday with not much more than a few inches of new snow expected.  However this system will bring increased winds and colder temperatures.  For Tuesday through the end of the week high pressure returns with warming temperatures and clear skies.   

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Partly cloudy then becoming mostly cloudy. Isolated snow showers. Mostly cloudy. Scattered snow showers in the evening, then isolated snow showers after midnight. Mostly cloudy. Slight chance of snow showers in the afternoon.
Temperatures: 21 to 27 deg. F. 8 to 14 deg. F. 26 to 32 deg. F.
Wind direction: SW light SW
Wind speed: 10 to 15 mph with gusts to 40 mph in the morning becoming light. Light winds 15 to 25 mph with gusts to 45 mph in the afternoon
Expected snowfall: 0 in. up to 1 in. o in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Partly cloudy then becoming mostly cloudy. Isolated snow showers. Mostly cloudy. Scattered snow showers in the evening, then isolated snow showers after midnight. Mostly cloudy. Slight chance of snow showers in the afternoon.
Temperatures: 14 to 20 deg. F. 3 to 8 deg. F. 20 to 26 deg. F.
Wind direction: W W W
Wind speed: 20 to 30 mph. Gusts up to 60 mph decreasing to 45 mph in the afternoon. 10 to 15 mph with gusts to 30 mph in the evening becoming light. 10 to 15 mph with gusts to 40 mph increasing to southwest 25 to 35 mph with gusts to 75 mph in the afternoon.
Expected snowfall: 0 in. up to 1 in. o 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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