Eastern Sierra Avalanche Advisory - 2/26/17

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THIS AVALANCHE ADVISORY EXPIRED ON February 27, 2017 @ 7:15 am
Avalanche Advisory published on February 26, 2017 @ 7:15 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

The primary avalanche concern for Sunday will be newly formed Wind Slabs primarily on NW-N-NE-E-SE aspects as winds increase from the SW in the middle and upper elevations. Older, less sensitive Wind Slabs that formed mid-week due to Southerly and Westerly winds. Saturday winds veered to the North forming widespread Wind Slabs throughout the upper mid to upper elevations on most aspects. Upper mid to upper elevations avalanche concerns will rise through today as winds increas through the day with – isolated natural avalanches possible, human triggered avalanches likely above treeline in steep areas that continue to wind-load. Caution on slopes steeper than about 35 degrees, natural avalanches possible, and human triggered avalanches likely in exposed terrain. Identify possible wind loaded areas such as below ridgelines, sidewalls of gullies, crossloaded shallow depressions, and near terrain features that promote loading and drifting. Carefully evaluate the snowpack and terrain before getting on steep mid to upper elevation slopes.         

 

 

How to read the advisory

Bottom Line

The primary avalanche concern for Sunday will be newly formed Wind Slabs primarily on NW-N-NE-E-SE aspects as winds increase from the SW in the middle and upper elevations. Older, less sensitive Wind Slabs that formed mid-week due to Southerly and Westerly winds. Saturday winds veered to the North forming widespread Wind Slabs throughout the upper mid to upper elevations on most aspects. Upper mid to upper elevations avalanche concerns will rise through today as winds increas through the day with – isolated natural avalanches possible, human triggered avalanches likely above treeline in steep areas that continue to wind-load. Caution on slopes steeper than about 35 degrees, natural avalanches possible, and human triggered avalanches likely in exposed terrain. Identify possible wind loaded areas such as below ridgelines, sidewalls of gullies, crossloaded shallow depressions, and near terrain features that promote loading and drifting. Carefully evaluate the snowpack and terrain before getting on steep mid to upper elevation slopes.         

 

 

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

Moderate SW to W winds earlier in the week, which then veered to the N Saturday, windloading leeward slopes, primarily at the upper mid to upper elevations. Saturday, Northerly winds resulted in significant snow transport (snow banners) over high peaks from Whitney to Bishop with recent reports of natural Wind Slab avalanche activity within the past couple of days. Moderate SW winds are forecasted to develop mid-day Sunday with gusts up to 60mph during the day, increasing to 80mph by this evening, forming isolated Wind Slabs on leeward slopes. Be wary of firm denser snow hollow sounding and recently formed drifts on slopes 35 degrees and steeper. Do your own localized stability assessments, and avoid steep slopes, especially convex areas, where suspect wind-slabs may exist. Even a small avalanche can carry a rider dangerous terrain.

Avalanche Problem 2: Loose Wet
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Decreasing Danger

Isolated small Loose Wet release are possible below ~ 8000’ on solar aspects (E-S-W) where strong warming (sheltered sunny locations) can soften and weaken the surface snow. Natural releases possible, triggered releases likely. Generally not a major hazard to riders unless unusually large, associated with a terrain trap resulting in deep burial, or where a rider is carried into hazardous terrain (cliffs).  Hazard generaly decreasing through the day with increasing clouds. 

advisory discussion

The most recent Atmospheric River event moved out of the region Wednesday (2/22). The snowpack continues to settle in sheltered locations, with snow sensors showing significant settlement over the past 48 hours. Moderate SW to W winds throughout most of the week transported snow onto Leeward aspects (NW-N-NE-E-SE-S), primarily at the upper mid to upper elevations. Saturday, winds veered to the North resulting in significant snow transport (snow banners) over high peaks from Whitney to Bishop with reports of natural Wind Slab avalanche activity, which died down through the day. Today (Sunday), moderate SW winds are forecasted to develop mid-day with gusts up to 60mph during the day, increasing to 80mph by this evening, forming isolated Wind Slabs on leeward slopes (NW-N-NE-E-SE). As this week progressed, Storm Slab instability declined as the snowpack settled and strengthened. There is a weak strom system forecasted to move into the region Sunday night with up to 3” new low density snow possible over the higher elevations, which is insufficient to produce dangerous Storm Slab instability. However, Southwesterly winds of 30 – 45 mph winds forecasted for Sunday will redistribute the low-density snow onto Leeward slopes, forming new sensitive Wind Slabs on NW-N-NE-E-SE aspects. 

weather

Sunday thru Monday - a relatively weak Low Pressure system bringing gusty winds this afternoon and evening with areas of mainly light snow tonight into Monday morning. Sunday afternoon and evening will see increasing Southwesterly winds with gusts up to 40-45 mph, Sierra Crest gusts may exceed 80 mph. Light to moderate snow developing in the Sierra by this evening. Cold temperatures aloft will help to get higher snow ratios close to 15:1 and quicker snow accumulations even with the lack of overall moisture. Snow accumulations around 1-4" are expected Mono county. Light snow showers expected to continue through the day on Monday and into Monday night with little to no additional accumulations.

Tuesday thru Wednesday - Below average temps will continue through the rest of February with highs both Monday and Tuesday mainly in the mid 30s to lower 40s, and lows mainly in the teens to lower 20s, while cooler valleys near the Sierra could drop below zero. A flat ridge will bring a period of quiet weather from midweek through the start of the weekend. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Sunday Sunday Night Monday
Weather: Partly cloudy. Mostly cloudy. Snow likely. Mostly cloudy. Chance of snow showers.
Temperatures: 26 to 32 deg. F. 12 to 17 deg. F. 22 to 28 deg. F.
Wind direction: Light winds becoming southwest Southwest West
Wind speed: 15 to 25 mph with gusts to 45 mph in the afternoon. 15 to 25 mph with gusts to 60 mph. 15 to 20 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon.
Expected snowfall: 0 in. up to 3 in. up to 2 in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: Partly cloudy. Mostly cloudy. Snow likely through the night. Mostly cloudy. Chance of snow showers.
Temperatures: 20 to 26 deg. F. 6 to 12. deg. F. 15 to 21 deg. F.
Wind direction: Southwest Southwest West
Wind speed: 10 to 15 mph with gusts to 35 mph increasing to 20 to 30 mph with gusts to 60 mph in the afternoon. 30 to 45 mph with gusts to 80 mph. 20 to 30 mph. Gusts up to 65 mph decreasing to 45 mph in the afternoon.
Expected snowfall: 0 in. up to 3" in. up to 2 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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