Eastern Sierra Avalanche Advisory - 4/10/17

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON April 12, 2017 @ 7:07 am
Avalanche Advisory published on April 10, 2017 @ 7:07 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The primary avalanche problems for forecast period (Monday thru Tuesday) are Wet Loose releases, isolated Winds Slabs, and potential Storm Slabs. Wet Loose can be expected on solar aspects in the mid to upper elevations, all aspects below ~8000’ where a snowpack is present. Isolated Wind Slabs may 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. Storm totals of ~20” to 37” reported throughout the forecast region from Thursday thru Saturday. The threat of Storm Slabs is decreasing as the new snow bonds to the underlying snow but large triggers (i.e. large cornice fall) may be sufficient to trigger an avalanche.

Wet Loose – Natural avalanches possible, triggered release likely on slopes of 35 degrees and steeper.

Wind Slabs – Natural avalanche unlikely, triggered releases possible 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 (Mid to upper elevations) - natural and triggered avalanches unlikely. Lower elevations insufficient snowfall.   

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Loose Wet
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Warming temperatures and the intense April solar radiation will warm the surface snow quickly. As the sun warms and softens the snow surface, the snow can become saturated  and loose internal cohesion during the day, increasing the potential for natural and human triggered Loose Wet avalanches. Mild overnight temperatures can allow the surface snow to thaw more quickly. Time of day is critical for wet point releases. Easterly aspects soften first, followed by Southerly, then Westerly in the afternoon. Extra heat from cliffs, rock features and faces, trees can result in additional localized thawing. Lower elevations – northerly (shaded) aspects will soften as well at as the day progresses. Watch for signs of decreasing stability such as large rollerballs or deep boot penetration, which indicates the snow is loosing strength and possibly becoming unstable, steep terrain should be avoided where encountered. Sticky conditions may exist as the snow softens, especially below ~ 9000.

* Loose Dry possible in shelter locations on Northwesterly thru Northeasterly aspects in the upper elevations on slopes of 35 degrees and steeper.

 

Avalanche Problem 2: Wind Slab
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Last storm system (Thursday thru Saturday) came in with strong to extreme winds with gusts exceeding 100mph, forming Wind Slabs primarily on NW-N-NE-E-SE aspect at all elevations. Due to localized channeling Wind Slabs formed in: unusual locations, in normally sheltered terrain, and lower downslope than usual. The Winds Slabs have had a couple of days to strengthen but may still be sensitive to triggered release. However, Monday, southwest winds of 10 to 15 above 10,000’ are forecasted, which is just below threshold for wind transport. If winds are stronger than forecasted, they could possibly form additional isolated Wind Slabs where leeward fetches still have snow available for transport. Tuesday SW winds will be at threshold with some drifting expected in the mid to upper elevations possibly forming isolated sensitive Wind Slabs on NW-N-NE-E-SE aspects, especially where leeward fetches still have snow available for transport. You may encounter Wind Slabs below ridgelines and near terrain features that promote drifting in the upper elevations. 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 3: Storm Slab
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The forecast region has received ~20” to 37” of new snow during this last storm cycle. The system came in “right side up” with denser snow at the bottom, 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. However, a couple of layers may exist within the new snow that needs additional time to “heal” to eliminate the threat completely. Storm Slabs may be found on sheltered terrain, especially northerly aspects, on slopes of 35 degrees or steeper, primarily in the mid to upper elevations. 

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 deposited on a well developed spring type snowpack (consolidated, melt/freeze crust at the surface). 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, as well as time, 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 continue to strengthen and bond to the underlying snow. Due to the strong winds associated with the last storm system, localized channeling Wind Slabs formed in unusual locations and in normally sheltered terrain. Southwest winds of 10 to 15 above 10,000’ are forecasted for Monday, which is just below threshold for wind transport, if winds are stronger than forecasted, they could possibly form additional isolated Wind Slabs below ridgelines and near terrain features that promote drifting in the upper elevations. Tuesday SW winds will be at threshold with some drifting expected in the mid to upper elevations, possibly forming isolated Wind Slabs on NW-N-NE-E-SE aspects where leeward fetches still have snow available for transport. As the latest storm moves off to the east, temperatures are rebounding into to seasonable and under the intense April sun, the surface snow warms quickly, loosing internal cohesion, which can result in Wet Loose releases as the snow thaws during the day. These will be primarily encountered on solar aspects in the mid to upper elevations, lower elevations the daily warming can be enough to thaw all aspects extending the concern to all aspects. 

recent observations
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4/9/17 - White Wing via June Mountain

4/9/17 - Naturals On Carson Peak

4/9/17 - Dream Peak/Yost Lake Trees, June Mountain

4/9/17 - Avalanche in Hollywood Chute 

4/8/17 - Naturals in Sherwins

4/8/17 - Hemlock Ridge to the Highway

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

Storm Precipitation Totals and Temperatures (0500)
Location                                     Snow    Water    Crnt Temp  Temp Max
VA Lakes (9445’)                        22”        2.9”                25                   36
Tioga Pass (9798’)                      19”        N/A                 N/A                 N/A
Ellery Lake (9645’)                      N/A       1.2”                 20                   42
June (9148’)                               26”       3.2”                 19                   34                  
Gem Pass (10750’)                      N/A       4.1”                 N/A                 N/A

Agnew Pass (9355’)                    25”        N/A                  12                   50
Mammoth Sesame St (9014’)       37”        6”                    19                   33
Mammoth Pass (9,500’)               N/A        2.3”                10                   49
Rock Creek (9600’)                     23”         N/A                 2                     39
Saw Mill-Big Pine (10200’)           27”         N/A                 21                   37
Big Pine Creek (10000’)              33”         N/A                 N/A                  N/A

 

Weather and CURRENT CONDITIONS
weather summary

Monday thru Tuesday – Dry conditions today (Monday) as a passing wave weakens and lifts northwards; expect winds to be light as the pressure gradient relaxes. A warm front lifts through the Sierra Tuesday bringing more a chance of light snow showers over the higher elevations. Generally, QPF totals are less than 0.15 inches for the Sierra Tuesday. Winds will begin to increase as the gradient tightens ahead of the next short wave poised to enter the region.

Wednesday thru Thursday - the primary wave approaches the region with QPF estimates ranging from 0.5-0.9" in the higher Sierra with snow levels around 6500-7500ft with snow estimates of 4-8 inches above 7500 feet with little accumulation down to 6500 feet through Wednesday night. Wednesday winds will become gusty ahead of the primary wave with up to 50 mph possible for the Sierra. Models remain in good agreement with this storm moving through Thursday. Most recent models runs seem to be pushing the storm out of the region faster reducing the total rain/snow amounts slightly for Thursday as the storm moves south and begins to lose its moisture tap. This storm looks to be a decent storm for April, with total liquid amounts of 0.50-1.5" in the Sierra. Snow levels will start out near 6000-6500 feet, but then fall to near 4000-5000 feet Thursday evening. Total snowfall Wednesday through Thursday in the Sierra looks to be around a foot or more above ~7500’. A few showers will continue into Thursday night, tapering off Friday as a flat ridging building back into the region. 

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: Partly cloudy. Partly cloudy. Mostly cloudy. Slight chance of snow in the morning, then chance of snow in the afternoon.
Temperatures: 40 to 46 deg. F. 22 to 28 deg. F. 42 to 48 deg. F.
Mid Slope Winds: Light winds. Light winds. Southwest
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Partly cloudy. Partly cloudy. Mostly cloudy. Slight chance of snow in the morning. Chance of snow in the afternoon.
Temperatures: 34 to 40 deg. F. 18 to 24 deg. F. 37 to 43 deg. F.
Ridge Top Winds: Southwest Light winds becoming south Southwest
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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