Avalanche Advisory: Tuesday - Jan 8, 2019

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THIS AVALANCHE ADVISORY EXPIRED ON January 9, 2019 @ 6:51 am
Avalanche Advisory published on January 8, 2019 @ 6:51 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Considerable avalanche danger exists in the form of Wind Slabs and Persistent Slabs due to significant snowfall over the weekend and the most extreme SW winds that we have seen in years.  NW-N-E-SE facing slopes at all elevations are the most concerning, even in typically sheltered locations. 

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Large to very large human triggered wind slab avalanches are still possible to likely today at all elevations, even in typically sheltered areas.  The most extreme SW winds that we have seen in years left no terrain untouched Sunday night, leading to a heavy upside-down slabby snowpack everywhere that snow was deposited.  Observations from VA Lakes, Mammoth and Bishop creek yesterday all reported signs of whoomphing and shooting cracks extending long distances, and evidence of natural avalanche activity that occurred during the storm, which are all red flags. Due to the snow being very challenging to travel thru, and the high danger, we haven’t received any back-country observations from upper elevations, making forecaster confidence low at upper elevations.  NW-N-E-SE facing slopes where snow has been deposited will be of greatest concern.  Carefully evaluate the snow and keep your decisions conservative by sticking to lower angle or wind scoured terrain until these slabs have more time to stabilize.

To add to the problem, as snow begins accumulating again tonight with strong SW winds, small new sensitive wind slabs are likely to form.

Avalanche Problem 2: Persistent Slab
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    Certain
    Very Likely
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Before this past weekend’s storms which dropped up to 3-4ft+ of new snow, widespread weak faceted snow had been found at the surface of the old snow or near the surface just underneath crusts.  It is now possible that avalanches will fail on these now-deeper buried weak layers, resulting in much larger avalanches than might be otherwise expected.  Avalanches that fail on these deeper layers could propagate great distances across slopes, run much farther down slope, and be much more deadly.  Avalanches could fail directly on this deeper layer, or smaller wind slab avalanches could act as the trigger.  This problem and concern could last for some time after the recent wind slabs stabilize, and become more stubborn to trigger.  Dig down into the snowpack to evaluate these weak layers in safe locations, and realize that they could be quite variable across a slope.  With the uncertainty that exists it is best to stick to lower angle terrain or areas that have been scoured by the wind.

advisory discussion

While 125mph ridge-top gusts is a lot, the most notable thing about Sunday night’s winds was how strong they were even at low elevations.  Some large trees blew down in the town of Mammoth Lakes.  Typically we include storm slab problems in the avalanche advisory with significant snowfall events for areas that are sheltered from the wind, but this time sheltered areas just didn’t exist.  Everywhere was either wind scoured or was left with a dense layer of wind deposit on top.  Warming temperatures during Sunday night’s storm didn’t help with this upside-down layering.  Add widespread underlying old faceted snow, and you get a very worrisome snowpack structure at the moment anywhere that wind deposited snow exits.      

Weather and CURRENT CONDITIONS
weather summary

3-4ft+ of snow fell over the weekend with the most extreme SW winds (even at lower elevations) than we have seen in years.

A break in stormy weather will continue thru most of today before another weaker system moves in tonight thru tomorrow, bringing strong South winds and 4-8” of snow possible along the Sierra crest.  

Expect mostly cloudy skies today, warm temperatures reaching the mid 30s around 10,000’, and relatively light South winds increasing in the afternoon with gusts up to 75mph over ridge-tops.  Another less potent system moves in tonight, bringing some light flurries in the afternoon, a few inches of snow along the crest tonight with south winds up to 90mph over ridge-tops, and a few more inches of snow tomorrow.  

Unsettles weather will continue thru the weekend into next week, but precipitation amounts are looking very light at this point. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Slight chance of snow showers in the afternoon. Snow levels 8000 feet. Chance of precipitation is 10%. Mostly cloudy. Chance of snow through the night. Snow levels 7000 feet. Chance of precipitation is 35%. Cloudy. Snow likely. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 70%.
Temperatures: 35 to 40. deg. F. 24 to 29. deg. F. 34 to 40. deg. F.
Mid Slope Winds: South 15 to 20 mph. Gusts up to 35 mph increasing to 50 mph in the afternoon. South 15 to 20 mph increasing to 20 to 30 mph after midnight. Gusts up to 75 mph. South 25 to 35 mph decreasing to 15 to 30 mph in the afternoon. Gusts up to 75 mph.
Expected snowfall: 0 in. 90% probability up to 2 inches. 10% probability of 3 to 6 inches. | SWE = less than 0.10 inch. in. 80% probability of 2 to 4 inches. 20% probability of 3 to 6 inches. | SWE = up to 0.40 inch. in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Slight chance of snow showers in the afternoon. Snow levels 8000 feet. Chance of precipitation is 15%. Cloudy. Chance of snow through the night. Snow levels 7500 feet decreasing to below 7000 feet after midnight. Chance of precipitation is 40%. Cloudy. Snow likely. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 70%.
Temperatures: 29 to 34. deg. F. 18 to 23. deg. F. 28 to 34. deg. F.
Ridge Top Winds: South 15 to 25 mph with gusts to 40 mph increasing to 25 to 35 mph with gusts to 65 mph in the afternoon. South 30 to 45 mph. Gusts up to 70 mph increasing to 90 mph after midnight. South 40 to 60 mph with gusts to 90 mph becoming southwest and decreasing to 25 to 35 mph with gusts to 60 mph in the afternoon.
Expected snowfall: trace in. 90% probability of 1 to 3 inches. 10% probability of 3 to 6 inches. | SWE = less than 0.10 inch. in. 80% probability of 2 to 4 inches. 20% probability of 3 to 6 inches. | SWE = 0.15-0.40 inch. 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 Avalanche Advisory is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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