Avalanche Advisory: Thursday - Mar 1, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 2, 2018 @ 6:48 am
Avalanche Advisory published on March 1, 2018 @ 6:48 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche danger will be CONSIDERABLE for much of today near and above treeline, but as 12” or more of new snow accumulates and strong SW winds redistribute it into dense wind slabs the avalanche danger may climb to HIGH. Wind slabs in exposed areas, storm slabs in sheltered areas, and even deeper triggered persistent slabs are all problems you may encounter if you choose to play in or below avalanche terrain by this evening. Whiteout conditions with very poor visibilty and cold wind chill are also expected.

4. High

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Above Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Near Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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S winds began picking up yesterday and depositing recent storm snow onto isolated leeward features near and above treeline. A strong winter storm will begin to impact our area today bringing sustained strong SW winds and up to a foot or more of snow by late afternoon, with more to come tonight. The potential for wind slab avalanches will increase in likelihood and size throughout the day. The most likely aspects for new wind slab formation are NW-N-E-SE today, but recent wind slabs have been found on S and W aspects as well. Look for cornice formation and avoid slopes where snow is being deposited. The wind is very efficient at redistributing snow, and wind slabs will become sensitive to triggering, and larger, earlier in the day than other avalanche problems.

Avalanche Problem 2: Storm Slab
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By early evening snowfall totals for our area are expected to be upwards of 12”. That new load in that short of a time period is enough to cause storm slab avalanches on what was previously stable snow. By the time we get 12” of new snow, or if snowfall rates reach 2” per hour for any sustained period we should start to see storm slab avalanches on steep sheltered slopes near and below treeline.

Avalanche Problem 3: Persistent Slab
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The persistent layers of weak, sugary, faceted snow that has been found in the Mammoth and Virginia Lakes regions could result in some very large avalanches as the additional load of new storm snow piles up quickly today and tonight. These facets have been found between 25-60cm down in the snowpack on northerly slopes between ~9000’ and ~10,000’ and have been reactive to stress in isolated snowpack tests. Smaller wind slab or storm slab avalanches that overload this layer may cause sudden failure and the resulting avalanche may be much larger. You will have to dig down and do your own stability tests to know if these layers can be found under your feet. The extra load on these facets may cause the snow to “whumph” and collapse suddenly which is a sign that they can fail and cause a big avalanche.

advisory discussion

If the storm that will affect our area lives up to the hype it will be the strongest of the season by far. New snow totals could reach between 7” and 18” by evening. There is a winter storm warning and a blizzard warning in effect from the National Weather Service. Travel in the mountains will be especially hazardous by late afternoon. The strong southwesterly flow accompanying the cold front will have no trouble building large wind slabs across leeward slopes. New snow loads that exceed 12” will very likely cause storm slab avalanches. Both of these avalanche problems will develop on top of soft snow that could become overloaded and fail as the scales tip. Smaller avalanches that run in just the right place may trigger deeper persistent weak layers. The resulting avalanches could be very large.

With cold temperatures, snow levels are forecast to be quite low which means that lower elevation slopes that were previously well anchored may see some avalanche activity for the first time tonight. Obstacles that were easy to spot before today may become covered with low density snow and become hidden just under the surface.

Whiteout conditions could make travel in the mountains dangerous as strong winds combine with heavy snowfall rates by this afternoon.

Weather and CURRENT CONDITIONS
weather summary

...WINTER STORM WARNING IN EFFECT FROM 4 AM THURSDAY TO 10 AM PST SATURDAY… ...BLIZZARD WARNING IN EFFECT FROM 10 AM THURSDAY TO 1 PM PST FRIDAY…

A strong winter storm will bring periods of snow with travel impacts in the Sierra today through Saturday. Expect blizzard conditions at times today through early Friday morning along Sierra passes and in Mono County. Strong winds are expected to combine with snowfall rates of 2+ inches per hour by evening.

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: Cloudy. Snow. Cloudy. Snow. Cloudy. Snow.
Temperatures: 28 to 34 deg. F. 18 to 24 deg. F. 23 to 29 deg. F.
Wind Direction: SW S SW
Wind Speed: 20 to 30 mph with gusts to 75 mph increasing to 30 to 45 mph with gusts to 85 mph in the afternoon. 30 to 45 mph with gusts to 80 mph. 20 to 30 mph. Gusts up to 70 mph decreasing to 60 mph in the afternoon.
Expected snowfall: 70% probability...of 7 to 13 inches. 30% probability...of 13 to 18 in. 70% probability...of 12 to 20 inches. 30% probability...of 20 to 28 in. 80% probability...of 5 to 11 inches. 20% probability...of 11 to 16 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Cloudy. Snow. Cloudy. Snow. Cloudy. Snow.
Temperatures: 21 to 27 deg. F. 11 to 16 deg. F. 16 to 22 deg. F.
Wind Direction: SW SW SW
Wind Speed: 45 to 65 mph with gusts to 100 mph. 50 to 70 mph with gusts to 105 mph. 40 to 60 mph. Gusts up to 100 mph decreasing to 85 mph in the afternoon.
Expected snowfall: 70% probability...of 7 to 15 inches. 30% probability...of 15 to 20 in. 70% probability...of 16 to 24 inches. 30% probability...of 24 to 32 in. 80% probability...of 6 to 12 inches. 20% probability...of 12 to 18 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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