Avalanche Advisory: Friday - Mar 16, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON March 17, 2018 @ 7:07 am
Avalanche Advisory published on March 16, 2018 @ 7:07 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Friday (3/16) – The avalanche danger remains at CONSIDERABLE as of early this morning for lingering and fresh Wind Slabs on NW-N-NE-E-SE aspects near and above treeline. However, it will begin to rise quickly toward HIGH by mid-day with the onset of moderate to heavy snowfall forecasted for the region with accumulations of ~ 15” to 30+” forecasted by Saturday morning.

As the new snowfall accumulates (~8” or greater), danger for Storm Slabs will rise from Moderate to CONSIDERABLE in sheltered areas from treeline and below. Remote possibility the additional snowload will increase the sensitivity of deep persistent weaknesses in the mid and lower snowpack. Avalanches could trigger  or step down into these weaker layers. 

4. High

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Above Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Near Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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The region got a nice shot of snow beginning late Tuesday thru Wednesday with 4”to 20” of new snow along with moderate Southwesterly winds. Thursday, strong Southwesterly winds aloft continued throughout the day and into the night, loading NW-N-NE-E-SE aspects from near tree line and above. 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 a region. Natural avalanches will rise from possible to likely as snow begins to accumulate by mid-day and the winds form new sensitive Wind Slabs, human triggered avalanches will become very likely by mid-day.    

 

Avalanche Problem 2: Storm Slab
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Generally, the recent snow from (3/13-3/14) has generally bonded well to the underlying snow with the exception of June Lake and the Crestview area where there is significant amount of graupel that fell mid-storm (3/13-3/14). Snowpits near June Mountain Wednesday found a thick loose graupel layer that poured out of the pit wall with Storm Slab above failing easily during stability tests. Observations from Chicken Wing on Thursday showed some improvement but still sensitive. With the approaching storm and the added snow weight, this could become an active weak layer in this area. With the forecasted ~15 to 30+” of snow by Saturday AM, the threat of Storm Slabs will rise to CONSIDERSBLE for near and treeline and below. Natural avalanches are possible, triggered releases likely. Perform regular assessments to determine how well storm slabs are bonding. 

Avalanche Problem 3: Persistent Slab
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Persistent deep slab weakness … well persists. 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. With this approach storm system, the added stress of the new snow may be enough to tip the balance and cause this layer to become more sensitive or reactive. The layer can be found ~50 cm (~20”) south of Mammoth to ~150cm (~60”) in the Mammoth/June region, somewhere in between around Virginia Lakes. Recently, the layers relative stubbornness would require a large trigger or a sweet spot in a shallow area of the snowpack to get this deep layer to fail. The new snow from the approaching storm may add over an 1” of water weight by Saturday morning will likely increase the sensitivity of this layer and increase the likelihood that: a small wind or storm slab avalanche, or a snowmobile, or a large cornice failure could step down into this layer and trigger a much larger destructive release. The problem primarily exists in the mid-elevations on E-N-W aspects but may extend into the upper elevations but is poorly bridged by Wind Slabs. The problem is slowly receding into the snowpack depths as it becomes thoroughly buried and the facets begin to round, 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 last storm to traverse the region brought anywhere from 4” to 20” of new snow Tuesday night – Wednesday with the greatest amounts of new snow concentrated around the Mammoth/June area, lesser amounts north and south. Upper elevation SW winds were moderate and rose to strong Thursday with large snow banners evident from Mammoth south with strong SW winds continuing through the night, even at the lower elevations. This scenario created Wind Slabs during and post-frontal passage in the mid to upper elevations, primarily on NW-N-NE-E-N-SE aspects. Today will bring more snow and wind, which will generate a new round of tender Wind Slabs along with the lingering Wind Slabs from the previous day. 

Weather and CURRENT CONDITIONS
weather summary

...WINTER STORM WARNING REMAINS IN EFFECT UNTIL 11 AM PDT  SATURDAY...

Fri thru Sunday – moisture will continue to stream into the Sierra today and tonight, shifting southward through the day. Mono County will see conditions worsen during the day today, with peak snowfall rates most likely there between 10 am and 10 pm. System will end from north to south tonight, but with potential for some narrow bands of heavy snow to persist across Mono Saturday morning. Once the main storm clears, snow showers will hang around through Saturday afternoon.A warming and drying trend starts Sunday.

Monday onward…  Greatest concern in the long-term period is a warmer and wet system with an atmospheric river tap. The moisture is headed toward southern California based on GEFS ensemble mean guidance, makes its way toward the Sierra Early Wednesday morning into Thursday. The region will be on the warm side of the jet, with snow levels likely to be around the 7000 ft. Upper elevations of the Sierra have the potential for another round of heavy snow. As an upper level trough swings into the west Thursday, it will push a cold front through the region, with falling snow levels. The final piece of the upper low moves overhead on Friday keeping chances for snow showers in the forecast Friday into Saturday.    

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Snow through the day. Slight chance of thunderstorms in the afternoon. Cloudy. Slight chance of thunderstorms in the evening. Snow through the night. Mostly cloudy. Snow showers likely.
Temperatures: 22 to 30 deg. F. 8 to 13 deg. F. 22 to 28 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 20 to 30 mph with gusts to 55 mph. 15 to 20 mph. Gusts up to 45 mph decreasing to 35 mph after midnight. 10 to 15 mph with gusts to 35 mph in the morning becoming light.
Expected snowfall: 8 to 16 in. 7 to 14 in. 1 to 3 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Snow. Cloudy. Snow. Mostly cloudy. Snow showers likely.
Temperatures: 15 to 20 deg. F. 3 to 9 deg. F. 14 to 19 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 30 to 50 mph with gusts to 75 mph. 25 to 35 mph. Gusts up to 65 mph decreasing to 55 mph after midnight. 20 to 30 mph with gusts to 45 mph decreasing to 10 to 15 mph in the afternoon.
Expected snowfall: 8 to 16 in. 8 to 16 in. 1 to 3 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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