Avalanche Advisory: Friday - Mar 9, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 10, 2018 @ 6:41 am
Avalanche Advisory published on March 9, 2018 @ 6:41 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The avalanche danger for Friday (3/9) is CONSIDERABLE for Wind Slabs on NW-N-NE-E-SE aspects 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. 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.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

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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Strong to extreme southwesterly winds were recorded overnight at Mammoth Mountain (35 – 60+ mph) above 10,000’.  Today’s forecast calls for continued SW winds of 20 to 35 mph with gusts of 50 mph (above 10,000’). The combination of strong winds and plenty of transportable snow in the upper elevations will maintain the threat for 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 remain a lingering concern. The storm from 3/1 – 3/3 deposited substantial snow throughout the region, which has settled and formed a slab over the weak facets from earlier in the season. This weak layer can be found ~ 50cm to ~ 150cm (~20” to ~ 60”) down from the surface primarily between 9000' to 10500', higher in some locations. Remotely triggered avalanches, large crowns were observed Sunday (3/4), with whoomphing and large-scale slope collapses being reported as recently as Tuesday (3/6). The new snow continues to settle/strengthen with the deep facet layer slowly becoming less reactive and more difficult to trigger (stubborn). However the problem has not gone away altogether. Potentially, deep and large avalanches may be triggered from adjacent thin areas or large triggers (cornice fall, Wind Slab avalanche, groups of people, etc.).  Areas of greatest concern are NW-N-NE facing slopes near and below tree line that had prior snow coverage.  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. Natural avalanches are unlikely, triggered releases are possible. 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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Today’s forecast is calling for mostly cloudy skies and temperatures mostly in the 40’s below 10,000’ today (Friday). With overnight lows struggling to fall much below freezing in many locations and despite the cloud cover, enough warming may occur in lower and possible the mid elevations for 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. The presence of numerous 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 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 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 stubbornly 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 remains 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 colder 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 is 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

Fri thru Sunday – today, a weak wave traverses through northwestern California with mostly cloudy skies expected for the region. Another relatively weak wave will move into the central and southern Sierra Saturday night and Sunday. Snow levels are projected to increase to around 7000-7500 feet early Sunday morning adding a little uncertainty to Mammoth Lakes’ forecast for precipitation. Snow levels will start low enough that it will be primary all-snow, but cannot rule out some mixed    in rain if precip slacks off during the daylight hours. Accumulations of heavy wet snow for Mono County will generally be light with up to 4 inches down to 7000 feet, 2 to 6 inches around 8000 feet, and 5 to 9 inches towards the Sierra Crest.

Mon thru Thursday - The operational GFS and ECMWF move the main upper low closer to the region by early Tuesday and better chances for precipitation into the area. Snow levels remain rather high early on but begin to fall late Tuesday into     Wednesday. Differences in the model solutions become more amplified by     Wednesday/Thursday. The ECMWF is a little faster bringing heavier QPF into the area Wednesday while the GFS has its greatest QPF Thursday. The GEFS ensemble members vary from a progressive trough moving through the region by Thursday to a closed/cut off low offshore. If the ECMWF solution verifies, there would likely be more precipitation and cooler overall temperatures in the Sierra. 

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: Mostly Cloudy. Partly cloudy then becoming mostly cloudy. Mostly cloudy. Chance of snow through the day.
Temperatures: 39 to 47 deg. F. 21 to 29 deg. F. 39 to 45 deg. F.
Mid Slope Winds: Southwest Southwest Light winds.
Expected snowfall: 0 in. 0 in. up to 1 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy. Partly cloudy then becoming mostly cloudy. Mostly cloudy. Chance of snow.
Temperatures: 32 to 38 deg. F. 22 to 27 deg. F. 30 to 36 deg. F.
Ridge Top Winds: Southwest Southwest South
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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