Avalanche Advisory: Friday - Mar 23, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 24, 2018 @ 6:53 am
Avalanche Advisory published on March 23, 2018 @ 6:53 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche danger near and above treeline today is HIGH. Continuous loading by strong southerly winds will make travel in avalanche terrain not recommended. Below treeline the danger will still be CONSIDERABLE where human triggering of large storm slab avalanches will be likely. Very large and dangerous deep slab avalanches are also a concern. Sunny skies and excitement about the new snow could lead to poor decision making today.

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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Moderate to Strong southerly winds have continued overnight with up to 6” of new snow since 5pm yesterday evening. Winds are forecasted to increase throughout today ahead of another small storm moving through the area tonight. Wind slabs will continue to build in exposed areas at all elevations. Even though new loading from snowfall has stopped, remember that the wind can overload the snowpack at 3-5 times the rate that flakes can fall from the sky. Any slopes where wind has deposited new snow in the past 24hrs are very dangerous today. Stay out of terrain with cornices overhead or where you can see blowing snow.

Avalanche Problem 2: Storm Slab
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4’ or more of heavy new snow has fallen since Wednesday afternoon with loading rates exceeding 2”/hr at times. The new load was upside-down in structure (dense snow over less dense) and many avalanches yesterday entrained the full depth of the storm snow. While little has fallen overnight (up to 6” in the mountains outside Bishop, and less further north) storm slabs will most likely still be sensitive enough for human triggering today and could still be quite large. Steep sheltered slopes are not safe from storm slab avalanches today. Very cautious route finding and conservative decision making is required to manage the hazard. Stay off of, and out from under, slopes that are 30 degrees and steeper.

Avalanche Problem 3: Deep Slab
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The snowpack structure this season has been weak. Poorly bonded sugar snow still exists under all the new snow above 9000’ on W-N-E aspects. With the heavy new load above it’s like building a concrete structure on top of sand, it doesn’t inspire confidence. The fear is that a large trigger, like an avalanche running down from above, could overload these weak layers with huge consequences. The result could be deadly.

advisory discussion

Widespread avalanches, both natural and human triggered, were reported throughout the forecast area yesterday. Many were very large. But with 4’ or more of new snow and a little over 4” of new water added to the snowpack over such a short period what did you expect? The important thing for today is to remember that it will take some time for the snowpack to adjust to the load. In exposed areas, and especially near and above treeline today the wind will continue to make the snowpack top heavy. But even below treeline it it likely that a rider could trigger the recently formed storm slabs. And perhaps the spookiest, hardest to predict avalanche problem today will be the deep persistent layers that could be triggered by a wind or storm slab avalanche running down and overloading them.

 

Weather and CURRENT CONDITIONS
weather summary

 A break in precipitation is expected for most of today. Gusty winds develop this afternoon ahead of a couple fast-moving and colder storms that will move through with some snow for northeast California into northwest Nevada this weekend.

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: Sunny. Clear then becoming partly cloudy. Isolated snow showers after midnight. Partly cloudy. Scattered snow showers.
Temperatures: 33 to 41 deg. F. 17 to 22 deg. F. 25 to 33 deg. F.
Wind Direction: SW SW SW
Wind Speed: 15 to 25 mph with gusts to 40 mph. 20 to 30 mph with gusts to 45 mph 15 to 25 mph with gusts to 45 mph.
Expected snowfall: 0 in. Less than 1 in. 70% probability...less than 1 inch. 30% probability...of 1 to 3 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Sunny. Clear then becoming partly cloudy. Isolated snow showers after midnight. Mostly cloudy. Scattered snow showers.
Temperatures: 26 to 31 deg. F. 11 to 16 deg. F. 19 to 25 deg. F.
Wind Direction: SW SW SW
Wind Speed: 30 to 45 mph with gusts to 60 mph. 30 to 50 mph with gusts to 60 mph. 30 to 40 mph with gusts to 55 mph decreasing to 20 to 30 mph with gusts to 40 mph in the afternoon.
Expected snowfall: 0 in. Up to 1 in. 60% probability...up to 1 inch. 40% probability...of 1 to 3 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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