Avalanche Advisory: Thursday - Mar 8, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 9, 2018 @ 6:34 am
Avalanche Advisory published on March 8, 2018 @ 6:34 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The avalanche danger for Thursday (3/8) is CONSIDERABLE for Wind Slabs on NW-N-NE-E-SE aspect near and above tree line. Natural avalanches are possible, triggered avalanches are likely where Wind Slabs have formed. Deeply buried weak layers and the potential for large triggered avalanches continue to linger on NW-N-NE aspects primarily between 9000’ and 10500’, possibly higher in some locations. For Deep Slab avalanches, the hazard is MODERATE. Natural releases are unlikely, triggered releases possible. Remotely triggered avalanches reported on Sunday, with slope collapses noted as recently as Tuesday. This is complex snowpack for the Sierra.  Evaluate snowpack and terrain carefully, identify potential features of concern, avoid being on or under steep slopes where this dangerous snowpack structure exists, and be conservative in your decision-making. 

 

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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Overnight, strong to extreme southwesterly winds were recorded at Mammoth Mountain (25mph – 60+ mph).  Today’s forecast calls for SW winds of 30 to 40 mph with gusts of 70 mph (above 10,000’). The combination of strong winds and plentiful transportable snow in the upper elevations will form Wind Slabs on NW-N-NE-E-SE aspects primarily in the Alpine regions (above ~10,000’). However, it is possible that isolated Wind Slabs may form in favored locations (rocks, crossloaded gullies and chutes, leeward slopes, etc.) near tree line. Natural avalanches are possible, triggered releases are likely near tree line and above. Caution is recommended when approaching freshly formed drifts, hollow sounding slabs, or obvious Leeward slopes with significant fetches. Additionally, Wind Slab avalanches may trigger deeper weaknesses within the snowpack and the resulting avalanche could be large and destructive.     

Avalanche Problem 2: Deep Slab
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Deeply buried weak loose facets in the snowpack remains a concern. The storm (3/1 – 3/3) deposited substantial snow throughout the region, forming a slab over the weak facets from earlier in the season. This weak layer can be found ~ 50cm to ~ 150cm deep  (~20” to ~ 60”), primarily between 9000' to 10500'.  Remotely triggered avalanches, large crowns were observed Sunday, with whoomphing and large scale slope collapses being reported as recently as Tuesday (3/6). The new snow continues to settle and is strengthening with time and it seems that the deep facet layer is slowly becoming less reactive and more difficult to trigger (stubborn). However the problem has not gone away altogether. Potentially thin areas or large triggers (cornice fall, Wind Slab avalanche, groups of people, etc.) may still trigger these deep layers.  Areas of greatest concern are NW-N-NE facing slopes near and below tree line that had snow coverage prior to this last storm.  However, above tree line, thick hard wind deposits that are weakly bridging over the deep weak layers likely exists. Sheltered areas, convex rolls, unsupported slopes, and slopes with few or deeply buried anchors are most suspect.  Whoomphing and shooting cracks are critical clues of instability and should not be ignored. Carefully evaluate your slopes of interest, especially if considering riding in or around steep or complex terrain. Be aware that triggered releases are possible and the resulting avalanche could be large and destructive.     

Avalanche Problem 3: Loose Wet
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The forecast calls for sunny skies and light to moderate winds (below 10,000’) with temperatures in the 40’s for elevations below 10,000’ today (Thursday), which may result in enough warming in low to mid elevations to produce isolated Loose Wet releases, especially in sheltered terrain or near rock bands & cliffs that heat up the surrounding snow.  The concern is greatest on SE-E-SW facing slopes during the day as the snow softens. To avoid Loose Wet avalanches, time your travels to be off solar slopes before they thaw excessively. Rollerballs and Pinwheels are signs the surface snow is becoming saturated and loosing cohesiveness. If you encounter this, it’s time to find another slope that hasn’t thawed to the same degree. 

advisory discussion

With strong southwesterly winds overnight, which are forecasted to continue through today into tomorrow in the Alpine regions, and plenty of transportable snow, Wind Slabs will likely form on NW-N-NE-SE aspects near and above tree line, especially in favored locations (rocks, crossloaded gullies and chutes, leeward slopes, etc.) near tree line. Natural avalanches are possible, triggered releases are likely near tree line and above. There is the potential that a Wind Slab avalanche could trigger the deep weaknesses, discussed below, within the snowpack and the resulting avalanche could be large and destructive.     

The latest discussion among local riders and snow nerds is the complexity of the snowpack, which isn’t typically for the Sierra, especially this late in the season. The snowpack complexity is a result of thin snow cover for a good portion of the winter, variable snow distribution, and cold temperatures, which allowed strong facet growth in the little snow cover available.

The wind slabs and storm slabs from last storm (3/1 – 3/3) have settled, but weak faceted layers that formed earlier in the season remain reactive deep in the snowpack. This is primarily confined to a narrow elevation band between ~9000’ to ~10500’, higher in some locations, and mostly on protected or sheltered NW-N-NE aspects where there has been enough snow all season to cover up bushes and other anchors that has the greatest potential for a large deep slab avalanches. With warmer temperatures, this deep weakness will gradually strengthen, but will likely remain a concern for at least the near future. In the upper elevations, deeper snow deposits and thick hard wind-slabs have helped to bridge over this weakness.  In the Mammoth and June region the problem is most widespread due to greater snow coverage early season and more abundant protected and shelter slopes, less so in the Bishop and Virginia Lakes regions where snow lines were previously higher and sheltered snow coverage much more limited in distribution and depth.   

* It’s that time of year where the snowpack is warming on most aspects but still cold on northerly shaded slopes, which can result in skins icing up in the soft snow after becoming saturated where the snow has become wet. Make sure to pack your skin wax or better yet pre-treat your skins before leaving for the field.

Caution – the lower elevation snow coverage remains thin with plenty of hazards lurking just below the snow surface (i.e. rocks, logs, and stumps). The snow is hiding plenty of hazards while providing little protection.

 

 

Weather and CURRENT CONDITIONS
weather summary

Thurs thru Friday- Winds aloft will continue to increase this morning with breezy conditions expected in the Sierra and along the Sierra Front today. Winds will diminish this afternoon and evening as gradients aloft weaken. Temperatures will rise to the 30’s in the Alpine and 40’s lower elevations during the day, lows upper teens and 20’s.   

Sat thru Sunday - GFS/NAM/EC continue to project a weak, fast-moving wave moving through the central/southern Sierra Saturday night into early Sunday. Snow levels will be relatively high, around 7000ft. This appears to be a lower snow ratio event compared to last week`s system with heavier, wetter snow expected.

Long term - Wet pattern forecasted to continue through next week. Timing and strength of individual pieces of Pacific energy and moisture remains uncertain, but precipitation should occur on Sunday and again Tuesday-Wednesday. The Tuesday-Wednesday system is of higher concern due to the potential influence of an Atmospheric River and a slug of colder air with the core of the system to boost instability.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Sunny Partly cloudy then becoming mostly cloudy. Mostly cloudy. Slight chance of snow showers.
Temperatures: 41 to 49 deg. F. 24 to 29 deg. F. 37 to 47 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Expected snowfall: 0 in. 0 in. up to 1 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Sunny Partly cloudy then becoming mostly cloudy. Mostly cloudy. Chance of snow showers.
Temperatures: 30 to 40 deg. F. 19 to 24 deg. F. 29 to 39 deg. F.
Ridge Top Winds: Southwest Southwest Southwest
Expected snowfall: 0 in. 0 in. up to 1 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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