Eastern Sierra Snowpack Summary - 2016-01-19 05:39

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THIS ADVISORY EXPIRED ON January 21, 2016 @ 5:39 am
Avalanche Advisory published on January 19, 2016 @ 5:39 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Sensitive wind slabs will continue to form throughout the day today as snowfall continues (2-7” of new expected) and high winds out of the southwest increase up to 50-70mph, with gusts over 100mph over ridge-tops.  Natural avalanches will be possible and human triggered avalanches very likely in wind loaded terrain >35 degrees at mid to upper elevations on NW-N-NE-E-SE facing slopes.  Be on the lookout for these fresh dense wind slabs especially below ridgelines and cross-loaded slopes.  These wind slabs will be fairly obvious with denser, hollow sounding snow.  Nearby cornice formation and rippled snow surface can be clues to where these wind slabs may exist.  While exposed slopes will be of most obvious concern, high winds can also deposit snow into deeper soft wind slabs that could be sensitive to human triggering in open forested terrain.  These deposits in more protected terrain may only be slightly more dense than snow in un-wind affected areas.  Shooting cracks are an obvious warning sign that these softer wind deposits exist and may be unstable.  These avalanches will likely range from small slabs that could knock someone off balance, to carrying a person into undesirable terrain, to avalanches that could bury a person.

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

As snow accumulates throughout the day from this relatively warm storm, avalanche danger will increase even in wind protected areas on slopes steeper than 35 deg. at all elevations and aspects.  It is unknown how well this new, realtively dense snow will bond to the uderlying snow surface.  Make your own localized observations and do your own shallow tests to help you make safe decisions.        

Avalanche Character 3: Deep Slab
Deep Slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to Storm or Wind Slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

A faceted basal snowpack still exists throughout much of the forecast area due to shallow early season snow conditions and cold temperatures.  While it is unlikely for a human to trigger an avalanche at this level, the resulting slide would likely be large with bad consequences.  This layer is becoming less and less of a concern as the snowpack deepens.  However, it is still something to be aware of, especially where shallower snowpack exists outside of the Mammoth area.  Regularly reevaluate the local snow stability by digging snow pits and performing stability tests to help you make informed decisions.  This layer will likely become a greater concern during and right after a significant new snow load.

Snowpack Discussion

Avalanche danger will increase throughout the day today as snowfall continues (2-7” of new expected) and high winds out of the southwest increase up to 50-70mph, with gusts over 100mph over ridge-tops.  Natural avalanches will be possible and human triggered avalanches very likely in wind loaded terrain >35 degrees at mid to upper elevations on NW-N-NE-E-SE facing slopes.  Be on the lookout for fresh dense wind slabs especially below ridgelines and cross-loaded slopes.  While exposed slopes will be of most obvious concern, high winds can also deposit snow into soft wind slabs that could be sensitive to human triggering in open forested terrain.  These avalanches will likely range from small slabs that could knock someone off balance and carry him or her into undesirable terrain, to avalanches that could bury a person.

recent observations

A party yesterday morning (Monday) reported significant shooting cracks (shooting out farther than a ski length), about 5-6” deep, in steep exposed terrain near Mammoth Rock in the Sherwins.  Snow was wind deposited into soft slabs where these cracks were observed.  The party took caution to avoid these steep open areas and to stay in denser trees.

A search for buried surface hoar in openings amongst protected forested areas below the perch yesterday (Monday) were unsuccessful.  It may still exist in isolated areas, but we did not find any today.  Hand shear tests in this area did not fail easily, and compression / ECT tests showed consistent non-propagating fractures with moderate force about 25cm down from the surface along a subtle density change.  The bottom third of the 1m deep snowpack at this pit location was developed facets, but they were not reactive in stability tests.

An avalanche course in the Sherwins on Sunday made the following observations:  Evidence of small wet loose avalanches from the warm/wet air mass (and possibly some light drizzle) on Saturday up to 8,700’ in elevation (northerly facing slopes); a variety of test results failing and not failing at a snow interface 30cm down; They remarked that basal facets in areas with snow pack greater than 140cm seem to be strengthening from when they were observed 2 weeks ago.

Skiers on Chicken Wing on Sunday saw signs of wet point releases from rock bands that occurred on Saturday, and had some roller ball activity today.  They reported wind affected snow and thin wind slabs forming on westerly aspects, and high winds in the afternoon transporting snow up high and down low in exposed areas.  Pit dug on ENE aspect 75ft below ridge top revealed a 115cm deep snowpack, with the bottom 40cm consisting of basal facets, which a compression test failed at the top of after 22 taps, Q1. 

Mammoth Mountain Ski Patrol performed avalanche control work yesterday (Monday) including hand charging and ski cutting.  They reported moderate avalanche results, from surface sloughing to 1-2’ crowns in the newly wind deposited snow. 

weather

Another in a series of warm winter storms is upon us this morning and is expected to continue throughout the day and into the evening.  Snowfall accumulation is expected to be between 2-7” today, with another couple inches possible this evening.  High winds out of the southwest will increase throughout the day, reaching 50-70mph in the afternoon with gusts over 100mph over ridge-tops.  Temperatures should reach the high 20s to mid 30s over 10,000’.         

High pressure will move in from the coast tonight (Tuesday) and keep conditions dry for Wednesday and Thursday.  For Wednesday expect sunny skies, high temperatures in the mid to upper 30s around 10,000’, and breezy southwest winds.  There is potential for a stronger and wetter storm to move into our region Thursday night with windy conditions again bringing snow fall for Friday and Saturday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: CLOUDY. CHANCE OF SNOW IN THE MORNING...THEN SNOW IN THE AFTERNOON. PARTLY CLOUDY. SNOW SHOWERS IN THE EVENING. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: HIGHS IN THE UPPER 20S TO MID 30S deg. F. LOWS IN THE LOW TO MID 20S deg. F. HIGHS IN THE LOW 30S TO 40 deg. F.
Wind direction: SOUTHWEST BECOMING SOUTH IN THE AFTERNOON SOUTHWEST SOUTHWEST
Wind speed: 30 TO 50 MPH, INCREASING TO 40 TO 60 MPH IN THE AFTERNOON. GUSTS 60 TO 80 MPH. 35 TO 555 MPH WITH GUSTS TO 75 MPH. 15 TO 35 MPH WITH GUSTS 35 TO 55 MPH DECREASING TO 15 TO 20 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON.
Expected snowfall: 2-5 in. UP TO 1 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: CLOUDY, SNOW. MOSTLY CLOUDY, SNOW SHOWERS IN THE EVENING PARTLY CLOUDY
Temperatures: HIGHS IN THE MID 20S TO 30 deg. F. LOWS IN THE UPPER TEENS TO MID 20S deg. F. HIGHS IN THE UPPE 20S TO MID 30S deg. F.
Wind direction: SOUTHWEST WEST WEST
Wind speed: 50 TO 55 MPH WITH GUSTS TO 85 MPH INCREASING TO 65 TO 70 MPH WITH GUSTS TO 95 MPH IN THE AFTERNOON. 70 TO 75 MPH WITH GUSTS TO 100 MPH DECREASING TO 55 TO 60 MPH WITH GUSTS TO 90 MPH AFTER MIDNIGHT. 40 TO 45 MPH WITH GUSTS TO 65 MPH DECREASING TO 25 TO 30 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON.
Expected snowfall: 3-8 in. UP TO 1 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. 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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