Eastern Sierra Avalanche Advisory - 3/6/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 8, 2017 @ 6:57 am
Avalanche Advisory published on March 6, 2017 @ 6:57 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

The primary avalanche concern for Monday thru Tuesday will be: Wind Slab avalanches and isolated Storm Slab avalanches.

Wind Slab – Moderate to Strong Southwesterly to Westerly winds and plenty of transportable snow have formed sensitive Wind Slabs forming primarily on NW-N-NE-E-SE-S aspects, above ~8000’. Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual.

Monday thru Tuesday – Westerly to Southwesterly winds will continue to form sensitive Wind Slabs on NW-N-NE-E-SE-S aspects in the mid to upper elevations (~above 9000'). Dangerous avalanche conditions exist. Careful snowpack evaluation, cautious route finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely. Destructive avalanches running into the lower elevations are possible.

Storm Slab - One to two feet of new snow fell in the forecast region above ~8000’ in less than 24 hours, which has elevated the risk of Storm Slab avalanches for terrain above ~8000.

Monday - Careful snowpack evaluation, cautious route finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely on slopes of 30 degrees and steeper. Destructive avalanches running into the lower elevations are possible. Exercise caution when travel in or under avalanche terrain.

Tuesday - Heightened avalanche conditions on shelter slopes of 35 degrees and steeper. Evaluate snow and terrain carefully; identify features of concern. Natural avalanches unlikely; human- triggered avalanches possible. 

 

How to read the advisory

Bottom Line

The primary avalanche concern for Monday thru Tuesday will be: Wind Slab avalanches and isolated Storm Slab avalanches.

Wind Slab – Moderate to Strong Southwesterly to Westerly winds and plenty of transportable snow have formed sensitive Wind Slabs forming primarily on NW-N-NE-E-SE-S aspects, above ~8000’. Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual.

Monday thru Tuesday – Westerly to Southwesterly winds will continue to form sensitive Wind Slabs on NW-N-NE-E-SE-S aspects in the mid to upper elevations (~above 9000'). Dangerous avalanche conditions exist. Careful snowpack evaluation, cautious route finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely. Destructive avalanches running into the lower elevations are possible.

Storm Slab - One to two feet of new snow fell in the forecast region above ~8000’ in less than 24 hours, which has elevated the risk of Storm Slab avalanches for terrain above ~8000.

Monday - Careful snowpack evaluation, cautious route finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely on slopes of 30 degrees and steeper. Destructive avalanches running into the lower elevations are possible. Exercise caution when travel in or under avalanche terrain.

Tuesday - Heightened avalanche conditions on shelter slopes of 35 degrees and steeper. Evaluate snow and terrain carefully; identify features of concern. Natural avalanches unlikely; human- triggered avalanches possible. 

 

Avalanche Problem 1: Wind Slab
  • Character ?
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Moderate Southwesterly to Westerly winds at the upper elevations thru Tuesday and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on NW-N-NE-E-SE-S aspects, ~above 9000'. Previously strong gusty winds or localized wind channeling, may have formed Wind Slabs 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. Mid to Lower elevations will see significantly less wind thru Tuesday and as a result, Wind Slabs will be more isolated.

Monday thru Tuesday – Westerly to Southwesterly winds will continue to from sensitive Wind Slabs in the mid to upper elevations (~above 9000') on NW-N-NE-E-SE-S aspects. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely. Destructive avalanches running into the lower elevations are possible.

Avalanche Problem 2: Storm Slab
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  • Size ?
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    Decreasing Danger

Departing winter storm deposited 1 to 2 feet of snow (above ~8000) on to a snow interface of either Melt/Freeze crusts on solar aspects or a wind buffed/firm snow surface on exposed northerly slopes in the mid to upper elevations. The different snow interfaces are good bed surfaces for potential avalanches and will require some time for the new snow to bond.

Monday - Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely on slopes of 30 degrees and steeper. Destructive avalanches running into the lower elevations are possible. Exercise caution when travel in or under avalanche terrain.

Tuesday - Heightened avalanche conditions on shelter slopes of 35 degrees and steeper. Evaluate snow and terrain carefully; identify features of concern. Natural avalanches unlikely; human- triggered avalanches possible. 

Avalanche Problem 3: Loose Wet
  • Character ?
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    Very Likely
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  • Size ?
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    Increasing Danger

Loose Wet releases possible in the Low to Mid elevations on solar aspects Tuesday was temperatures rise into the 40's. These maybe encountered in, around, and under rock bands, cliffs, and large rock faces. 

advisory discussion

A weak Atmospheric River storm system moved into the region Monday (2/27/17) with 1 to 7” of new snow (greatest amounts new Mammoth and June Lakes) and strong SW winds forming Wind Slabs on NW-N-NE-E-SE aspects. High pressure moved into the region and dominated most of the week with slowly warming temperatures and light winds being the theme, allowing newly formed Wind Slabs time 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.

The latest storm to impact the Sierra Front began to move into the region late Saturday (3/4/17) with moderate to strong Southwesterly pre-frontal winds (sustained winds of 20 to 30 MPH below ~ 10,000’) with gusts of over 120 MPH along the Crest. With little transportable snow available, isolated sensitive Wind Slabs formed on NW-N-NE-E-SE aspects in the mid to upper elevations on exposed slopes. Sunday (3/5/17), the storm system moved into the region early morning with snowfall rates of 1 to 2” an hour and snow amounts of 1 to 2’ of low-density snow at higher elevations by early Sunday evening. Moderate to strong SW winds during the storm produced extensive sensitive Wind Slabs on NW-N-NE-E-SE aspects throughout the mid to upper elevations with localized eddies and channeling producing Wind Slabs on unusual aspects, potentially extending further down slope than normal, and into typically sheltered areas. Storm Slab potential increased during the height of the storm due to the rapid loading overwhelming the bond between the new snow and the underlying snowpack. Snowfall began to tapper off Sunday evening but Southwest to Westerly winds will continue to transport snow onto Leeward aspects forming sensitive Wind Slabs throughout the mid to upper elevations. Warming temperatures and clearing skies will likely produce Loose Wet releases on solar aspects Tuesday.

Precipitation Totals and Temperatures (4:00 AM, 3/6/17)
Location                              Snow    Water    Current Temps
VA Lakes (9445’)                  13”       1”           8.6
Tioga Pass (9798’)                12”       N/A         N/A
Ellery Lake (9645’)                N/A       .3”           4
June (9148’)                         8”         .3”           6
Gem Pass (10750’)               N/A       .6”          N/A
Mammoth Sesame St (9014’) 10”       .8”           6
Mammoth Pass (9,500’)         N/A      N/A          7
Rock Creek (9600’)               10”       N/A          7
Saw Mill-Big Pine (10200’)     10”       N/A         14
Big Pine Creek (10000’)         N/A      N/A         N/A

 

weather

Monday – Snow showers and areas of blowing snow will continue through the Sierra this morning before shower activity begins to dissipate this afternoon. Brisk west winds persist today brisk winds will continue with potential gusts 30-40 mph in most areas, decreasing winds Monday night.

Tuesday thru Thursday – drying trend thru mid-week 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 with lighter winds and warming temperatures expected into midweek. A steady warming trend will ensue with highs reaching into the upper 40s to low 50s. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy. Scattered snow showers in the morning. Isolated snow showers in the afternoon. Mostly cloudy then becoming partly cloudy. Mostly cloudy.
Temperatures: 29 to 37 deg. F. 17 to 23 deg. F. 40 to 46 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15 to 20 mph. Gusts up to 45 mph. 10 to 15 mph with gusts to 35 mph. Southwest 10 to 15 mph with gusts to 30 mph.
Expected snowfall: up to 1 inch in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: Partly cloudy then becoming mostly cloudy. Scattered snow showers in the morning. Isolated snow showers in the afternoon. Partly cloudy. Partly cloudy then becoming mostly cloudy.
Temperatures: 26 to 34 deg. F. 14 to 19 deg. F. 35 to 41 deg. F.
Wind direction: West West Southwest
Wind speed: 10 to 20 mph with gusts to 45 mph. 10 to 20 mph with gusts to 40 mph. 20 to 30 mph. Gusts up to 50 mph decreasing to 45 mph in the afternoon.
Expected snowfall: up to 1 inch 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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