Avalanche Advisory: Saturday - Mar 17, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 18, 2018 @ 7:07 am
Avalanche Advisory published on March 17, 2018 @ 7:07 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Happy St. Patrick’s Day

Saturday (3/17) – The avalanche danger has decreased to CONSIDERABLE as of this morning for lingering and fresh Wind Slabs on NW-N-NE-E-SE aspects near and above tree line. With new snow amounts of 9” to 20+” being reported across the region, the Storm Slab potential will remain at CONSIDERABLE in sheltered locations near tree line and below. Loose Dry avalanches will be a concern in steep sheltered terrain, especially in areas that received the upper snow amounts.

With the addition of 1” or more water weight to the snowpack, there’s a remote possibility this will increase the sensitivity of deep persistent weaknesses in the mid and lower snowpack. Avalanches could trigger or step down into these weaker layers. 

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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The eastern Sierra got a nice shot of snow yesterday (3/16) with 9” to 20” (20cm to 50cm) + of new snow being reported throughout the region on top of the 4” to 20” that we received on Wednesday/Thursday (3/14-15). These two systems were accompanied by moderate to strong Southwesterly winds near and above tree line, which formed Wind Slabs on NW-N-NE-E-SE aspects from near tree line and above. Today, moderate Westerly to Southwesterly winds above ~10,000’ will likely form additional tender Wind Slabs on NW-N-NE-E-SE-S aspects in addition to the lingering Wind Slabs that formed previously. Dense snow deposits are a telltale sign and will likely be encountered immediately below ridgelines or corniced slopes, in crossloaded gullies, or near terrain features that encourage drifting. Shooting cracks are a strong sign of instability. Perform regular hand shear assessments and pole probing to help identify poorly bonded or suspended slabs. Remember, spatial variability within the snow can occur across a slope as well as across the region. Natural avalanches are possible, triggered releases are likely.

 

Avalanche Problem 2: Storm Slab
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The latest storm to impact the region is moving off to the east and in its wake has deposited ~9” to 20”+ in 24 hours or less. The new snow is still forming bonds to the underlying snow while settling and becoming more slab like. This will maintain the potential for Storm Slabs in sheltered locations near tree line and below through today. Storm Slab potential for today is MODERATE, natural avalanches are unlikely, triggered releases are possible. Perform regular assessments to determine how well storm slabs are bonding.

 

 

 

Avalanche Problem 3: Persistent Slab
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Persistent deep slab weakness persists throughout the region but varies in depth. The variability of depth, overlying snow structure, and distribution makes this problem a challenge for forecasting. Tests continue to show clean and energetic propagation, though becoming progressively more stubborn. The added stress of an inch or more of water weight may be enough to tip the balance and cause this layer to become more sensitive or reactive and increase the likelihood that a: small Wind or Storm Slab avalanche, snowmobile, or a large cornice failure could step down into this layer and trigger a much larger destructive release. The layer of weak facets can be found ~50 cm (~20”) south of Mammoth to ~150cm (~60”) in the Mammoth/June region, somewhere in between around Virginia Lakes. The deep facet layer is showing slow signs of strengthening and has required a relatively large trigger or a sweet spot in a shallow area of the snowpack to get this deep layer to fail.  The problem primarily exists in the mid-elevations on E-N-W aspects but may extend into the upper elevations where it may be poorly bridged by Wind Slabs. As the facet layer slowly recedes into the snowpack depths and the facets begin to round and sinter, the concern will begin to subside. Make your own assessments, especially if traveling outside of the Mammoth/ June area where the snowpack is shallower.

 

advisory discussion

The latest storm to traverse the region brought anywhere from 9” to 20”+ of new snow Thursday/Friday with the greatest amounts of new snow concentrated around the Mammoth/June area, lesser amounts north and south. Upper elevation W-SW winds remained moderate to strong in the Mammoth and June region through the day and into the evening. This scenario created new Wind Slabs in the mid to upper elevations, primarily on NW-N-NE-E-N-SE-S aspects in addition to potential lingering Wind Slabs from Thursday. In sheltered locations from near tree line and below, lingering Storm Slab potential is a concern where large amounts of new snow has fallen and has not fully bonded to the underlying snow or may have weakness within the new snow, such as a density change. Additionally, sheltered areas that have received ~10” or more new snow, there is a concern of Loose Dry avalanches in steep terrain. While these slides are not a concern for burial, they can sweep a rider into potential hazardous terrain. 

Weather and CURRENT CONDITIONS
weather summary

Sat thru Sunday - The main cold front has moved out and an upper low has moved     over the eastern Sierra. The upper low has created an unstable air mass, enough for convective snow showers to continue through this afternoon with some stronger     showers producing an inch of snow. Sunday, partly cloudy due to weak ridging aloft with some snow flurries in the Sierra. Sunday night clouds should increase with westerly flow.

Monday onward - Greatest concern is the system for the late Tuesday through Thursday time frame. AR detection tools continue to indicate the deepest moisture heading into central and southern California, bringing the greatest impacts to Mono County south. However, much of the Sierra is within the ensemble plume for the     northern edge of the AR moisture. The warmer portion of the storm could bring snow levels up to 7000-8000 feet for the southern and eastern Sierra. The period of higher snow levels is only likely to last 12-18 hours before the trough and associated cold front shift into the region Thursday morning with snow levels possibly reaching valley floors by Thursday night, questionable how much moisture will be around. A period of stronger winds also looks probable Wednesday and     especially Thursday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy. Scattered snow showers. Mostly cloudy then becoming partly cloudy. Scattered snow showers in the evening. Partly cloudy.
Temperatures: 22 to 28 deg. F. 3 to 8 deg. F. 29 to 35 deg. F.
Mid Slope Winds: Southwest Light winds. Light winds.
Expected snowfall: Up to 3 in. Up to 2 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy. Scattered snow showers. Mostly cloudy then becoming partly cloudy. Scattered snow showers in the evening. Partly cloudy.
Temperatures: 15 to 20 deg. F. Zero to 5 above zero deg. F. 22 to 27 deg. F.
Ridge Top Winds: Southwest West Northwest
Expected snowfall: Up to 3 in. Up to 2 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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