Avalanche Advisory: Wednesday - Mar 7, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 8, 2018 @ 6:56 am
Avalanche Advisory published on March 7, 2018 @ 6:56 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Wednesday (3/7) - The avalanche danger for Wednesday (3/7) is MODERATE on NW-N-NE slopes at tree line and below due to deeply buried weak layers and the potential for large triggered avalanches. Natural avalanches are unlikely, isolated areas exist where triggered avalanches are 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. 

Caution – 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.

 

 

2. Moderate

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

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.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Deep Slab
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Deeply buried weak loose facets in the snowpack is the biggest concern today. 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”).  Remotely triggered avalanches, large crowns were observed Sunday, with whoomphing and large scale slope collapses being reported as recently as Tuesday (3/6). As the new snow settles and strengthens with time, it will become more difficult to trigger an avalanche and the danger will subside. Until then, potentially thin areas or large triggers (cornice fall, groups of people, etc.) may trigger these deep layers.  Areas of greatest concern are NW-N-NE facing slopes near and below treeline 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 could exist. Sheltered areas, convex rolls, unsupported slopes, and slopes with few or deeply buried anchors are most suspect.  Whoomphing and shooting cracks are critical clues and should not be ignored. Carefully evaluate your slopes of interest, especially if considering riding in steep or complex terrain. Be aware that triggered releases are possible and the resulting avalanche could be large and destructive.     

Avalanche Problem 2: Wind Slab
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Today, with plenty of transportable snow and forecasted southerly winds of 15 to 25 MPH in upper elevations, there may be enough snow transported to form relatively shallow localized Wind Slabs on W-NW-N-NE-E aspects primarily in the Alpine regions (above ~10,000’). Caution is recommended when approaching freshly formed drifts, hollow sounding slabs, or obvious Leeward slopes with large fetches.

 

Avalanche Problem 3: Loose Wet
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Mostly cloudy skies and light winds with warming temperatures are forecasted for today (Wednesday). Despite the cloud cover, some southerly aspects may warm enough to become unstable in the mid to lower elevations, especially near rock bands that heat up.  Depending on the cloud cover and thickness, the concern is greatest on SE-E-SW facing slopes. 

advisory discussion

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 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,  less so in the Bishop and Virginia Lakes regions where snow lines were previously much higher and sheltered snow coverage much more narrow.   

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

Wed thru Thursday - As ridge axis shifts east to today, thicker cloud cover will spread across CA-NV. Temperatures will trend warmer with highs in the 30’s and mid 40s. From tonight through Thursday, an upper level low pressure approaches the coast near the US-Canada border by Thursday, bringing mainly increasing clouds and southerly winds. Gusts could reach near 75 mph for the Sierra.

Fri - A couple more weak disturbances will spread more cloud cover over the region through Friday night, with light rain and higher elevation snow showers possible. There are timing and track variances with these disturbances (slightly more favorable potential Friday afternoon-evening), but associated precip amounts are expected to be light. Temperatures remaining above average (although a few degrees cooler than Thursday) snow levels 6500-7000 feet in the Sierra. Moderate southwest breezes are expected Friday.

Sat and Beyond - Models continue to show a split and weakening further for the late Saturday into Saturday night wave with a break before the main trough moves inland beginning late Tuesday. Saturday into Saturday night, the initial splitting system moves in with quite a bit of subtropical moisture, the forcing is weak. Snow levels look to be around 7000 feet, precip will be light, up to 1/4" of liquid and maybe 2" of snow in the mountains. Then ridging develops as the low in the East Pacific deepens before it moves in. Temperatures look a lot warmer now with 700 mb temps rising above the freezing mark and less cloud cover. Highs in 50s Sierra valleys. Then the trough slowly moves onshore late Tuesday into Wednesday.

 

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Partly cloudy then becoming clear. Partly cloudy.
Temperatures: 39 to 46 deg. F. 23 to 28 deg. F. 42 to 50 deg. F.
Wind Direction: Light winds becoming southwest Southwest Southwest
Wind Speed: 10 to 15 mph with gusts to 25 mph in the afternoon. 10 to 20 mph. Gusts up to 30 mph increasing to 45 mph after midnight. 15 to 20 mph with gusts to 45 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Partly cloudy. Partly cloudy.
Temperatures: 32 to 38 deg. F. 19 to 24 deg. F. 34 to 41 deg. F.
Wind Direction: South Southwest Southwest
Wind Speed: 15 to 25 mph. Gusts up to 25 mph increasing to 35 mph in the afternoon. 15 to 25 mph with gusts to 35 mph increasing to 30 to 45 mph with gusts to 70 mph after midnight. 30 to 45 mph with gusts to 70 mph.
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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