Eastern Sierra Avalanche Advisory - 3/5/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 6, 2017 @ 6:02 am
Avalanche Advisory published on March 5, 2017 @ 6:02 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

The primary avalanche concern for Sunday will be small to large, Wind Slab avalanches (NW-N-NE-E-SE-S) and developing Storm Slab avalanches as snowfall accumulates through the day.

Wind Slab – Moderate to Strong Southwesterly (Sunday) to Westerly (Sunday night) winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on W-NW-N-NE-E-SE-S aspects, above ~8000’ in favored locations (gullies, shallow depressions, rock outcroppings, below ridgelines, etc.) However, due to the strong gusty winds and localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Throughout the day, the avalanche hazard will rise as more snow accumulates and winds persist.

In the AM - Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

By mid-day: Natural avalanches likely; human- triggered avalanches very likely, as snow accumulates and winds persist, dangerous avalanche conditions will increase. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Storm Slab - One to two feet of new snow is forecasted for the region above ~7000’ over the next 24 hours, which will elevate the risk of Storm Slab avalanches for terrain above ~7000. The rapid loading will produce potentially large destructive avalanches running well into the lower elevations. Do your own localized assessment of wind slab sensitivity before committing to steep terrain, and if traveling near ridges beware of cornices!

In the AM - Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route finding and conservative decision-making essential.

By mid-day - Natural avalanches likely; human- triggered avalanches very likely, as snow accumulates, dangerous avalanche conditions will increase. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

How to read the advisory

Bottom Line

The primary avalanche concern for Sunday will be small to large, Wind Slab avalanches (NW-N-NE-E-SE-S) and developing Storm Slab avalanches as snowfall accumulates through the day.

Wind Slab – Moderate to Strong Southwesterly (Sunday) to Westerly (Sunday night) winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on W-NW-N-NE-E-SE-S aspects, above ~8000’ in favored locations (gullies, shallow depressions, rock outcroppings, below ridgelines, etc.) However, due to the strong gusty winds and localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Throughout the day, the avalanche hazard will rise as more snow accumulates and winds persist.

In the AM - Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

By mid-day: Natural avalanches likely; human- triggered avalanches very likely, as snow accumulates and winds persist, dangerous avalanche conditions will increase. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Storm Slab - One to two feet of new snow is forecasted for the region above ~7000’ over the next 24 hours, which will elevate the risk of Storm Slab avalanches for terrain above ~7000. The rapid loading will produce potentially large destructive avalanches running well into the lower elevations. Do your own localized assessment of wind slab sensitivity before committing to steep terrain, and if traveling near ridges beware of cornices!

In the AM - Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route finding and conservative decision-making essential.

By mid-day - Natural avalanches likely; human- triggered avalanches very likely, as snow accumulates, dangerous avalanche conditions will increase. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

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 to strong Southwesterly (Sunday) to Westerly (Sunday night) winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on NW-N-NE-E-SE-S aspects, above ~8000. Strong gusty winds or localized wind channeling, may result in Wind Slabs being encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Wind Slabs will likely be encountered along ridgelines, crossloaded gullies and depressions, in and around terrain features that promote drifting and loading. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet.

In the AM - Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

By mid-day – Natural avalanches likely; human- triggered avalanches very likely, as snow accumulates and winds persist, dangerous avalanche conditions will increase. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

 

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

Approaching winter storm is forecasted to produce 1 to 2 feet of snow above ~7000 with a interface of either Melt/freeze crust on solar aspects and windpack on Northerly aspects in the alpine may potentially form an ideal bedsurface for Storm Slabs above ~7000’. The rapid loading will likely produce potentially large destructive avalanches running into the lower elevations. 

In the AM - Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route finding and conservative decision-making essential.

By mid-day: trending toward – Natural avalanches likely; human- triggered avalanches very likely, as snow accumulates, dangerous avalanche conditions will increase. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

advisory discussion

The last storm to impact the region was Monday (2/27) with 1 to 7” of new snow (greatest amounts new Mammoth and June Lakes) and strong SW winds, which resulted in variety of wind effected snow, from soft to hard slabs, with protected areas offering good skiing. As skies cleared, high pressure moved in and dominated most of the week with slowly warming temperatures and light winds being the theme, allowing newly formed Wind Slabs to strengthen and stabilize. As temperatures rebounded through the week small Loose Wet releases in and around rock-bands on solar aspects became an issue, primarily in the low to mid elevations. Saturday, pre-frontal southwest winds began to increase as the next system prepared to enter region Sunday with gusts of over 120mph along the Crest and sustained winds 20 to 30 below ~ 10,000’, forming isolated, sensitive Wind Slabs on exposed mid to upper elevations on NW-N-NE-E-SE aspects. As the storm system moves into the region today (Sunday) snowfall will increase with 1 to 2’ forecasted for the higher elevations. Forecasted moderate to strong SW winds during the storm produce widespread sensitive Wind Slabs on NW-N-NE-E-SE aspects throughout the mid to upper elevations.  Wind loading may extend further down slope than anticipated. As snow begins to accumulate during the day, the potential for Storm Slab avalanches will increase throughout the mid and upper elevations. Cooler temperatures and cloud cover today will eliminate any wet-loose instability issues. 

weather

...WINTER STORM WARNING REMAINS IN EFFECT THRU 10 PM PST SUNDAY...

 * Timing: Snow began to develop Saturday PM heavy snow likely into Sunday. Snow showers to continue through the day Sunday.

 * Snow Accumulations: 1 to 2 feet above 7000 feet (including Mammoth Lakes), with 3 to 6 inches for communities along Highway 395. 1 to 4 inches for eastern Mono County.

 * Winds: Southwest 15 to 25 mph with gusts to 50 mph. Ridge wind gusts between 100 to 115 mph.

 * Impacts: Snow and gusty winds are likely to produce significant reductions to visibility and difficult travel, especially across Sierra passes, in addition to possible chain controls.

Sunday thru Monday – An advancing winter storm and approaching cold front will produce moderate to heavy snow across the Sierra. The heaviest bands are expected by10 am Sunday, shifting over that time. Large dendrites are likely and snow rates will reach 2 inches per hour in the heavy bands. Post-frontal convective snow showers will continue Sunday with much colder temperatures and a few more inches possible along the Sierra. Off and on snow showers will continue across the Sierra today as instability increases within the post-frontal cold air aloft with a lull in snowfall by late this afternoon and into the evening. The strong surface gradient will continue to yield brisk west winds today (Sunday) into Monday, which will create low visibility problems in areas of blowing snow and low wind chill. Instability-driven snow showers will persist mainly across the Sierra through Monday evening with a few additional inches of accumulation possible.

Tuesday thru Wednesday – much drier with temperatures warming up and approaching freezing. Winds will also diminish with just the ridges being gusty at times. Storm track retreating north into the Pacific Northwest midweek, supporting relatively mild conditions with slightly above average afternoon temperatures (55-65 lower valleys, 44-55 Sierra valleys) Wednesday.

 

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: Cloudy. snow showers. Mostly cloudy. Scattered snow showers through the night. Mostly cloudy. Slight chance of snow showers.
Temperatures: 23 to 33 deg. F. 7 to 13 deg. F. 29 to 35 deg. F.
Wind direction: Southwest West West
Wind speed: 20 to 30 mph. Gusts up to 70 mph. 20 to 30 mph with gusts to 65 mph shifting to the southwest 15 to 20 mph with gusts to 55 mph after midnight. 15 to 20 mph. Gusts up to 60 mph decreasing to 45 mph in the afternoon.
Expected snowfall: 12 to 18 in. up to 4 in. 0 in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: Cloudy. snow showers. Mostly cloudy. Scattered snow showers through the night. Mostly cloudy. Slight chance of snow showers.
Temperatures: 18 to 24 deg. F. 1 below to 9 above deg. F. 25 to 30 deg. F.
Wind direction: Southwest West West
Wind speed: 35 to 55 mph. Gusts up to 110 mph decreasing to 95 mph in the afternoon. 30 to 50 mph with gusts to 80 mph decreasing to 30 to 40 mph with gusts to 70 mph after midnight. 30 to 45 mph with gusts to 75 mph decreasing to 25 to 35 mph with gusts to 65 mph in the afternoon.
Expected snowfall: 12 to 18 in. 1 to 4 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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