Eastern Sierra Avalanche Advisory

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Avalanche Advisory published on December 16, 2017 @ 6:58 am
This Avalanche Advisory expires in 1 day, 11 hours, 44 minutes
This advisory is valid for 72 hours
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

The primary avalanche concern thru Monday will focus on newly formed and likely sensitive Wind Slabs near and above treeline on W-S-E aspects as weak storm moves through the region with moderate to strong Northerly winds. Secondary concern continues to be isolated stubborn Wind Slabs resting on weak faceted grains near and above treeline on W-N-E aspects that formed previously. Isolated natural avalanches are possible, human triggered avalanches are likely on steep terrain or where wind slabs overlay weak faceted (sugar-like) snow. Typically, wind slabs may be encountered: below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. The wind slabs may not be big enough to result in burial, they can entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Below ~9,000’, natural and triggered releases are unlikely due to little or no snow (below threshold).​ 

How to read the advisory

Bottom Line

The primary avalanche concern thru Monday will focus on newly formed and likely sensitive Wind Slabs near and above treeline on W-S-E aspects as weak storm moves through the region with moderate to strong Northerly winds. Secondary concern continues to be isolated stubborn Wind Slabs resting on weak faceted grains near and above treeline on W-N-E aspects that formed previously. Isolated natural avalanches are possible, human triggered avalanches are likely on steep terrain or where wind slabs overlay weak faceted (sugar-like) snow. Typically, wind slabs may be encountered: below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. The wind slabs may not be big enough to result in burial, they can entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Below ~9,000’, natural and triggered releases are unlikely due to little or no snow (below threshold).​ 

Avalanche Problem 1: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Primary concern is freshly formed Wind Slabs as a weak storm system moves through the region Saturday into Sunday with moderate to strong Northerly winds and T to 2” of new snow forecasted for the higher elevations. This will likely form fresh (likely sensitive) Wind Slabs on NW-W-S-SE-E aspects, near and above treeline. Winds will begin to ease Sunday and by Monday newly formed Wind Slabs should strengthen and begin bonding to the underlying snow, which will ease concern for triggered releases from likely to possible. Though Wind Slabs are expected to be somewhat shallow in nature, a slide is conceivable and could entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Typically, wind slabs may be encountered: below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. To date, observations have been limited, so it’s important to do your own stability assessments of wind slabs encountered and realize stability can vary tremendously over short distances. Below ~9,000’, natural and triggered releases are unlikely due to little or no snow (below threshold).​

 

 

 

Avalanche Problem 2: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Secondary concern is for older isolated stubborn Wind Slabs that formed in late November and early December that are resting on weak faceted snow on W-N-E aspects, near and above treeline. Normally, Wind Slabs tend to bond to the underlying snow within a few hours to few days. However, the recent extended period of cold clear nights and a shallow snowpack have formed weak faceted snow in the upper snowpack, especially on northerly facing aspects, which slows the normal bonding and strengthening processes, which has resulted in isolated pockets of wind slab overlaying faceted (sugar) snow. While the wind slabs may be somewhat isolated, a slide is conceivable and could entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Typically, wind slabs may be encountered: below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. To date, observations have been limited, so it’s important to do your own stability assessments of wind slabs encountered and realize stability can vary tremendously over short distances. Below ~9,000’, natural and triggered releases are unlikely due to little or no snow (below threshold).​

 

advisory discussion

The snowpack in the Sierras remains thin and mostly confined to elevations above ~9,000 to 9,500’ around Mammoth, possibly higher elsewhere. Dense early season storms in November deposited enough snow for good coverage, however early season conditions exist with numerous obstacles and many hazards just under the snow surface. The recent cold overnight temperatures have created a moderate to strong temperature gradient in the upper snowpack, producing weak sugary faceted snow in the upper snowpack in sheltered areas. Previous wind events formed pockets of dense wind slabs, near and above treeline that are resting on weak faceted snow. Normally, enough time has elapsed for the Wind Slabs to strengthen and stabilize but the facets in the upper snowpack are inhibiting bonding and slowing the normal strengthening processes. As a result lingering Wind Slab instabilities have been noted on northerly aspects, near and above treeline. Below treeline, the snowpack continues to be exceptionally thin. The snowpack near and above treeline alternates between soft facts in sheltered areas, wind stripped, wind deposited areas, and melt / freeze patches. Good skiing can be found on sheltered Northerly aspects or on Southerly aspects with enough coverage that are reportedly offering good spring-like skiing in the limited areas. As the winter progresses, the facets in the upper snowpack may form a weak layer, once buried, the melt-freeze crusts may act as a prime bed surface for future avalanches. 

weather

Saturday - Fast moving cold front and associated band of light precipitation is currently moving southeasterly through western Nevada with breezy northeast winds, light precipitation, and below average temperatures Saturday. Upslope flow is projected to occur along the eastern Sierra, which could result in slightly higher precipitation and lingering showers into the morning hours in Alpine, northern Mono, and southern Lyon counties. Easterly winds along the crest will strengthen today and last through tonight. Winds gusts of 75-100 mph are not out of the question along the ridgetops as very strong temperature gradient sets up along the Sierra crest.

Sun thru Monday - Generally light winds can be expected Sunday into Monday with daytime temperatures quickly rebounding Sunday into Monday and overnight temperature inversions quickly redeveloping.

Tues thru Thursday - A weak ridge will begin to shift east on Tuesday ahead of the next trough due in Wednesday with valley inversions persisting into Tuesday as mid-level flow will not increase substantially until after sundown Tuesday. A strong cold front along with strong winds look to accompany a slider-style system passing through the northern Sierra and western Nevada on Wednesday. The system shows good instability and frontal forcing along with more moisture. Based on the persistent track of this low and its fast movement, snowfall amounts are expected to remain generally light with a trace to two inches possible. As this system departs, gusty northeast winds will develop across the ridges by Thursday. These winds may persist into next weekend as upper level ridge builds across the west coast.

 

 

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: Mostly cloudy then becoming sunny. Scattered snow showers in the morning. Clear Sunny
Temperatures: 27 to 33 deg. F. deg. F. 20 to 25 deg. F. 31 to 37 deg. F.
Wind direction: North North North
Wind speed: 15 to 20 mph. Gusts up to 45 mph increasing to 60 mph in the afternoon. 15 to 25 mph with gusts to 65 mph. 10 to 15 mph with gusts to 55 mph.
Expected snowfall: up to 1 inch in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Mostly cloudy then becoming sunny. Scattered snow showers in the morning. Clear Sunny
Temperatures: 24 to 29 deg. F. 19 to 24 deg. F. 26 to 31 deg. F.
Wind direction: North North North
Wind speed: 25 to 35 mph with gusts to 50 mph increasing to 30 to 50 mph with gusts to 75 mph in the afternoon. 45 to 60 mph with gusts to 85 mph decreasing to 35 to 50 mph with gusts to 75 mph after midnight. 30 to 45 mph with gusts to 75 mph.
Expected snowfall: 1 to 2 in. 0 in. 0 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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