Eastern Sierra Snowpack Summary - 2016-02-22 07:32

THIS ADVISORY EXPIRED ON February 25, 2016 @ 7:32 am
Avalanche Advisory published on February 22, 2016 @ 7:32 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

High pressure continues to be the dominant player through the weekend with clear skies and above normal temperatures forecast with high’s of 40’s to 50’s with melt-freeze crusts on SE-S-SW-W aspects. As the sun advances around the compass rose, the melt-freeze crust will thaw and becoming less supportive during the day. Wet-Loose avalanches are possible, which has the potential to trigger larger Wet Slabs on steep slopes (>35 degrees). Plan to be off of these slopes early before they thaw and become unsupportive.  Take note of Rollerball (Pin Wheels) activity, which is an indicator of weakening snow grain bonds and a rapidly warming snow surface. As the week progresses, natural and human triggered wet loose avalanches are possible, on southeast slopes in the morning, southerly aspects by mid morning to mid-day, and southwesterly by afternoon.  Use extra caution on large mountain faces with complex terrain and multiple aspects. Lower elevations (below 8000’), the snowpack is disappearing rapidly but the remaining snow will warm rapidly on all aspects with risk of wet loose activity increasing on slopes >35 degrees by the afternoon. Be especially cautious in complex terrain with multiple aspects, which can result in unequal heating and thawing.

Avalanche Character 2: 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.

Recent strong post-storm winds formed extensive windslabs on steep exposed terrain throughout the mid to upper elevations on a variety of leeward aspects. The warm temperatures have aided bonding and strengthening with little indications of instability. However, NW winds of 30 to 40 mph (gusts in the 50’s) are forecasted for Monday, which will form new windslabs on NE-SE-SW aspects. Given the limited snow available for transport, these will be mostly isolated in nature. Favored areas are below ridgelines and adjacent to terrain features.   

Snowpack Discussion

The main avalanche concern for this period is warming temperatures and the potential for wet releases on southerly/sunny aspects (SE-S-SW-W) as the snow thaws. Additional avalanche concern is for fresh isolated windslabs in the mid to upper elevations below ridgelines and adjacent to terrain features.

High pressure continues to dominate the weather picture through Thursday, with sunny skies and scattered clouds occasionally filtering through the Eastern Sierra Front. On Southerly aspects, a weak melt-freeze crust has formed, becoming more established as the week progresses. Temperatures are forecast to rise over the next few days with highs well above seasonable with temperatures reaching 40’s to 50’s with lows in upper teens to upper 20’s, possible as warm as low 30’s. As a result, mid to upper elevation, a maturing melt-freeze crust has formed with varying degrees of supportiveness has formed on aspects E-SE-S-SW. As the temperatures warm the crust will begin to breakdown and becoming less supportive depending on aspect, and time of day. The forecasted winds for Monday will help keep temperatures down a bit, which will slow the thawing and limit the wet loose concern. However, in more sheltered locations wet loose slides maybe encountered on steep (>35 degrees) slopes with sun exposure, which could possibly trigger a wet slab release, especially Tuesday as temperatures are forecasted to climb again with winds easing, which will help to thaw the melt-freeze crust much more quickly. If planning to travel in the backcountry, anticipate being off of sunny aspects prior to thawing beyond the first few inches. Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes in the morning and moving around the compass toward southwesterly by afternoon. Watch for signs of instability, such as Rollerball (Pin Wheels) activity. This indicates a weakening of snow grain bonds and a rapidly warming snow surface with an increased potential for natural and human triggered wet loose avalanches, which can lead to larger slope failures on southeast slopes in the early AM, southerly mid-morning to mid-day, and southwesterly by afternoon. At elevations below 8000’,the snowpack is disappearing rapidly but the remaining snow will warm rapidly on all aspects with risk of wet loose activity increasing on slopes >35 degrees by the afternoon.

The mid to upper elevations, the recent warm temps have helped settle and strengthen previously formed windslabs and storm slabs. However, the very strong winds that developed as the storm exited the region formed widespread windslabs throughout the mid to upper elevations, except in the most shelter areas. These windslabs have mostly settled and strengthened but are exceedingly firm and quite widely distributed.  Moderately strong NW winds (20 to 30 mph, with gusts in the 40’s) forecasted for Monday has the potential to form isolated windslab pockets on exposed mid to upper elevations below ridgelines and adjacent to terrain features where there is sufficient transportable snow upwind. These will most likely be found on leeward slopes (NE-SE-SW), below ridgelines, and cross-loaded faces and gullies. 

Keep in mind as you travel in steeper more complex terrain that with the arrival of spring like conditions and the presence of windslabs throughout much of the mid to upper elevations, that snow conditions can be exceedingly firm and falls can result in very dangerous slides, possibly into or over objective hazards with severe consequences. Sunday, a report of a skier falling and taking a severe slide, which resulted traumatic injury and a helicopter evacuation should be an added note of caution. If electing to travel in exposed steep terrain, make sure you have the proper equipment (ice axe, self-arrest grips, crampons, etc.) and are well practiced in self-arrest.


recent observations

Gilcrest (2/20) -Lookers left main chute had debris in runout zone that occurred during last storm naturally, (D3, R3 - could destroy a car, small building, medium size relative to path).  Soft slab, unknown crown.  Pit dug at 8500ft, NE aspect, clean failure just above old snow surface (melt-freeze crust) with hard force. Variable snow surface conditions from breakable crust to firm windboard, to soft snow. No visible wet slide activity on southern slopes.

Mammoth Basin (2/21) - moderate amount of sizeable rollerball activity on some steep slopes on all aspects including north below 9000'. No evidence of loose wet slides above tree line.  Evidence of old windslab avalanche with crown still visible below rock band of crest, likely released during the recent storm.

Sherwins, Bardini Chute (2/21, Elevation: 8839’, Aspect: NW) – Stability tests failed at old/new interface. Avalanche debris present on lower third; light to moderate sluffing.

June Lakes, Negatives (2/21) – Stability tests hard failures at the old/new snow interface. Windslabs unreactive, Solar aspects, melt-freeze crust, not fully formed. Northerly aspects mixed soft, windbuff, and windslab. Shelter trees soft facets. Large crown visible under summit rock band, south side of the bowl. Failed during storm cycle. 


Mon-Tues: Most of the area will remain dry for the next few days. A weak short wave will move over the far northern California late tonight. Behind the wave northwest flow develops at the surface and aloft as a high-pressure ridge builds just off the west coast Monday. This should result in slightly cooler valleys temperatures Monday and Tuesday, though still near to above average. Temperatures will begin to warm further by Tuesday with light low level winds.

Wed-Thursday: the ridge axis slides just east of the region with temps aloft begin to cool slightly. Surface temperatures will warm into the mid to upper 50's in the Sierra valleys. There is the hint of a weak wave in the mid/upper levels Wednesday over the far northern California bringing increased clouds locally.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Temperatures: 44 TO 50 deg. F. 24 TO 29 deg. F. 46 TO 52 deg. F.
Wind speed: 10 TO 20 MPH. 10 TO 15 MPH. AROUND 10 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Temperatures: 34 TO 44 deg. F. 29 TO 34 deg. F. 38 TO 46 deg. F.
Expected snowfall: 0 in. 0 in. 0 in.

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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