Eastern Sierra Avalanche Advisory - 1/7/18

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THIS AVALANCHE ADVISORY EXPIRED ON January 9, 2018 @ 6:47 am
Avalanche Advisory published on January 7, 2018 @ 6:47 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche picture for Sunday and Monday is complex with cautious route-finding and conservative decision-making essential. As the danger increases Monday, travel in avalanche terrain will not be recommended.

Sunday- Natural avalanches will be possible and human triggered avalanches likely. Expect wind slabs below ridgelines and on cross-loaded slopes and middle and upper elevations. Deeper, persistent weak layers may be triggered under the weight of new snow and a rider’s weight on northerly slopes.

Monday- As more precipitation falls and winds increase throughout the forecast area, the avalanche danger will rise and become dangerous. Natural avalanches will become likely; human triggered avalanches very likely.

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Avalanche Problem 1: Wind Slab
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Yesterday, 3-11” of new snow fell across the forecast zone and was redistributed by moderate Southwest winds. Winds can create slabs 3 to 5 times as deep as snow that just falls from the sky, so some of these new wind slabs could be large. And observations from yesterday show that they are sensitive to trigger. These slabs could take a day or two to stabilize, but before that can happen a new storm on Monday-Tuesday will create newer, and deeper, wind slabs at middle and upper elevations. Avoid round, pillow shaped drifts on the leeward sides of ridges, on cross-loaded slopes, and the sides of chutes and gullies. Taking note of where snow is blowing will help you recognize terrain to avoid. Shooting cracks and hollow sounding snow are that wind slabs may already be under your feet and could be triggered by your weight.

Avalanche Problem 2: Persistent Slab
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The dry weather and shallow snowpack until now has created layers of weak, sugary snow throughout the snowpack. Stability tests prior to, and during Saturday’s storm have shown that these layers can fail under stress. With the new snow, and even more to come, these layers may be triggered and release avalanches that are quite large. Persistent weak layers are hard to predict, but likely trigger points include places where the snowpack is shallower and on steeper, shady slopes. Be wary of hollow sounding snow. Take the time to do your own detective work and take a close look at the snow buried under your feet.

Avalanche Problem 3: Storm Slab
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Skiers intentionally triggered small storm slabs on Saturday, and more new snow is likely starting Monday morning. In sheltered areas where the new snow does not bond as well to the underlying surface, or where new snow is thicker than what is underneath, it will be likely you could trigger storm slabs. Chances of new storm snow avalanches will increase throughout Monday.

advisory discussion

Like a long lost friend from far away, winter weather has finally returned to the east side. And everything seems rosy until you realize that your buddy comes bearing some baggage that you’d forgotten she had. And now you have to deal with it too.

All the welcome new snow coming into our region is sitting on top of a weak, complex snowpack that we are usually unaccustomed to here in the Sierra. The Ridiculously Resilient Ridge that had parked itself over California until recently had kept things dry and the snow cover thin, with few exceptions. So as new precipitation rolls in we will likely see the usual avalanche problems of wind and storm slabs, but weak layers persist underneath and could fail under the new load creating avalanches that are quite large and difficult to predict.

The storm on Saturday left 11” of new snow around Mammoth, and between 3 and 5” elsewhere. This snow was light and dry from June Mountain north, but dense and heavy further south. As the snowfall tapered off mid day, Southwest winds increased and sensitive wind slabs developed on northerly slopes. Snow levels were relatively high between 8,100’ and 8,500’ with rain below. This added little in the way of new snow to the already meager snow depth at that elevation. The new snow sits atop a variety of surfaces, from rain crust, to near surface facets, to melt-freeze. This means that bonding between old and new snow will be variable as well.

Significant new snowfall and stronger southerly winds are expected beginning Monday morning. And though it will be a welcome change to the high and dry, it will likely add to an already complicated avalanche danger. Pay close attention to changing conditions and make conservative terrain choices.

Early season obstacles are not yet completely buried. Be wary of rocks and trees just under the surface.

Weather and CURRENT CONDITIONS
weather summary

...WINTER STORM WATCH NOW IN EFFECT FROM MONDAY EVENING THROUGH TUESDAY EVENING ABOVE 7000 FEET…

Sunday will be a brief break in precipitation with light to moderate southwest winds and well above seasonal temperatures. A potent low pressure system, favoring the eastern Sierra in Mono County will bring another round of significant precipitation to the region Monday and Tuesday. Travel in the Sierra could become difficult at times due to heavy rain and high elevation snow early Monday morning through Tuesday. Dry and warm conditions will return for the end of the week and through next weekend, with a return to wet weather probable by mid-January.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Slight chance of snow after midnight. Cloudy. Chance of snow and rain.
Temperatures: 43 to 49 deg. F. 29 to 35 deg. F. 39 to 45 deg. F.
Wind Direction: Light winds becoming SW. Light winds becoming S. S
Wind Speed: Light winds. Gusts up to 25 mph in the afternoon. Light winds. Gusts up to 25 mph increasing to 35 mph after midnight. South 10 to 15 mph. Gusts up to 25 mph increasing to 45 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 70% probability...of 2 to 5 inches. 30% probability...of 6 to 9 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Slight chance of snow after midnight. Cloudy. Chance of snow in the morning, then snow in the afternoon.
Temperatures: 38 to 43 deg. F. 24 to 29 deg. F. 33 to 38 deg. F.
Wind Direction: SW SW S
Wind Speed: 15 to 20 mph. Gusts up to 30 mph in the afternoon. 15 to 20 mph with gusts to 30 mph increasing to 20 to 30 mph with gusts to 40 mph after midnight. 15 to 25 mph with gusts to 35 mph increasing to 30 to 40 mph with gusts to 55 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 70% probability...of 3 to 6 inches. 30% probability...of 7 to 10 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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