Eastern Sierra Avalanche Advisory - 4/9/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 10, 2017 @ 7:07 am
Avalanche Advisory published on April 9, 2017 @ 7:07 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

The primary avalanche problem for today (Sunday) is potentially tender Winds Slabs and possible Storm Slabs. Wind Slabs will likely be encountered primarily on NW-N-NE-E-SE aspects in the mid to upper elevations and below ~8000’ on slopes that have extensive fetches and where a snowpack is present. ~20” to 37” of new snow reported throughout the forecast region has fallen in the past 72 hours, which has elevated the Storm Slab risk. The threat of Storm Slabs in the mid to upper elevations is decreasing as the new snow begins to sinter and bond to the underlying snow and weaknesses within the new snow begin to strengthen but additional time is needed for further consolidation and strengthening to eliminate the concern.   

Wind Slabs – Natural avalanche possible, triggered releases likely on slopes of 35 degrees and steeper. Due to the strong winds and local variability, wind slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual.

Storm Slab – natural release unlikely, triggered releases possible in terrain 35 degrees and steeper. 

How to read the advisory

Bottom Line

The primary avalanche problem for today (Sunday) is potentially tender Winds Slabs and possible Storm Slabs. Wind Slabs will likely be encountered primarily on NW-N-NE-E-SE aspects in the mid to upper elevations and below ~8000’ on slopes that have extensive fetches and where a snowpack is present. ~20” to 37” of new snow reported throughout the forecast region has fallen in the past 72 hours, which has elevated the Storm Slab risk. The threat of Storm Slabs in the mid to upper elevations is decreasing as the new snow begins to sinter and bond to the underlying snow and weaknesses within the new snow begin to strengthen but additional time is needed for further consolidation and strengthening to eliminate the concern.   

Wind Slabs – Natural avalanche possible, triggered releases likely on slopes of 35 degrees and steeper. Due to the strong winds and local variability, wind slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual.

Storm Slab – natural release unlikely, triggered releases possible in terrain 35 degrees and steeper. 

Avalanche Problem 1: Wind Slab
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Winds during the storm across the region were strong to extreme with gusts exceeding 100mph, which formed Wind Slabs primarily on NW-N-NE-E-SE aspect at all elevations. Due to localized channeling Wind Slabs may have formed in unusual locations, in normally sheltered terrain, and may extend possibly extend lower downslope than usual. Winds of 10 to 15 above 10,000’ are forecasted for today (Sunday), which is just below threshold for wind transport. If winds are stronger than forecasted, new sensitive Wind Slabs could begin to form and become an additional concern. Be wary of hollow, drum-like sounding snow or shooting cracks. Caution – this last storm formed potentially large tender cornices, which can break unexpectedly and further back from the edge than anticipated. 

Avalanche Problem 2: Storm Slab
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The forecast region has received ~20” to 37” of new snow during this last cycle. The system came in “right side up” with denser snow at the bottom and lighter snow toward the surface, which helps to limit weaknesses within the storm snow. Stability test indicate that the new snow is bonding to the well to the underlying snow but additional time is needed until this threat completely subsides. Possible weaknesses within the new snow is still a concern for today (Sunday)while the new snow sinteres and strengthens. Storm Slabs may be found in sheltered terrain on slopes of 35 degrees or steeper, primarily in the mid to upper elevations. 

Avalanche Problem 3: Loose Wet
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Solar aspects will begin to heat up today as temperatures begin to rebound. The new snow has yet to consolidate and is prone to Wet Loose failure as the snow begins to heat-up. Caution on SE-S-W aspects as air temperatures climb under the April sun. 

advisory discussion

An unusual late season (3’rd since 2000) Atmospheric River moved into the region late Thursday into Saturday with heavy wet snow above ~7500’ upper elevations. As per usual with these storms, it came in warm and wet then cooling and depositing lighter density snow as the system exits the region. This storm dropped 1.2” to 6” of liquid water equivalent (Mammoth Mountain) along the eastern Sierra since late Thursday evening. Storm totals for this system ranged from 19” at Tioga Pass to 37” at Mammoth Mountain. Snowfall was heaviest from June Mountain south. The system came in with moderate to strong Southwesterly flow (typical of these Atmospheric River storms) with winds gusting over 100 at ridgetops, which formed Wind Slabs throughout the region above ~7000’ in exposed areas. The new snow has begun to bond to the old/new snow interface and shows signs that it was deposited right side up (denser snow towards the bottom, lighter snow toward the surface). This has helped to lower the threat of Storm Slabs but the strong SW winds during the storm formed sensitive wind slabs primarily on NW-N-NE-E-SW aspects. The Wind Slabs that formed during the storm are still tender and need more time to strengthen and bond to the underlying snow. Due to localized channeling Wind Slabs may have formed in unusual locations, in normally sheltered terrain, and may extend possibly extend lower downslope than usual. Winds of 10 to 15 above 10,000’ are forecasted for today (Sunday), which is just below threshold for wind transport. If winds are stronger than forecasted, new sensitive Wind Slabs could being to form and become a concern. 

recent observations

4/8/17 - Naturals in Sherwins

4/8/17 - Hemlock Ridge to the Highway

4/8/17 - Loose Wet Slides Below Carson Peak

4/7/17 - Mammoth Mtn Ski Patrol Results, Lower Mtn

4/7- Wet New Snow, Snowpack Test Results On Punta Bardini

Storm Precipitation Totals and Temperatures (4/7 thru 4/8)
Location                                     Snow    Water    Current Temp
VA Lakes (9445’)                         22”        2.9”         9
Tioga Pass (9798’)                      19”        N/A        N/A
Ellery Lake (9645’)                      N/A        1.2”        -9
June (9148’)                                 26”        3.2”        7
Gem Pass (10750’)                      N/A        4.1”       N/A

Agnew Pass                                 25”        N/A        7
Mammoth Sesame St (9014’)     37”          6”         4
Mammoth Pass (9,500’)              N/A        2.3”       -2
Rock Creek (9600’)                     23”         N/A        -5
Saw Mill-Big Pine (10200’)         27”         N/A        0
Big Pine Creek (10000’)             33”          N/A       N/A

weather

Sunday thru Tuesday – A cold morning (Sunday) followed by a cool spring day with light winds. Two more weak shortwaves will transit the region move into the Pacific Northwest Monday thru Tuesday with precipitation will remain north of the forecast region. Temperatures will continue their warming trend back to near average for Tuesday. Winds will increase a bit Tuesday as the pressure gradient tightens over the region ahead of the next storm system with winds of 20-30 mph range.  

Wednesday thru Friday - Low-pressure system deepens off the Pacific Coast and starts to move inland on Wednesday, bringing another round of gusty winds, valley rains and mountain snow for the middle part of the week. Models are in pretty good agreement with very high chances for precipitation in the forecast in the Sierra. Subtropical moisture will get pulled up into this system and push into the Sierra Wednesday into Thursday. Total precipitation has been a little less certain, but this still look to be rather significant for an April storm. Storm total precipitation in the Sierra looks to exceed one inch, with over a foot of snow likely for the high Sierra during this time. The storm system moves out of the area on Thursday night with some lingering light showers continuing in the Sierra into Friday.

 

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: Sunny then becoming partly cloudy. Partly cloudy. Partly cloudy.
Temperatures: 33 to 39 deg. F. 17 to 23 deg. F. 37 to 43 deg. F.
Wind direction: Light winds Light winds becoming southwest Light winds
Wind speed: 10 to 15 mph after midnight. Gusts up to 25 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: Partly cloudy Partly cloudy Partly cloudy
Temperatures: 27 to 33 deg. F. 12 to 18 deg. F. 31 to 37 deg. F.
Wind direction: Southwest Light winds becoming southwest Southwest
Wind speed: 10 to 15 mph. Gusts up to 25 mph in the morning. 10 to 15 mph after midnight. Gusts up to 35 mph. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: 0 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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