Avalanche Advisory: Friday - Mar 2, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 3, 2018 @ 6:22 am
Avalanche Advisory published on March 2, 2018 @ 6:22 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The additional new snow and wind overnight will create very dangerous avalanche conditions today. The avalanche danger is expected to be HIGH at all elevations. Strong winds will create poor visibility at times. If you must play in the mountains today stay off of slopes steeper than 30 degrees – and out from under them. Excellent route planning will be required. Travel near avalanche terrain is NOT recommended.

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.

4. High

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Below Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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Winds increased yesterday afternoon and a Blizzard Warning went into effect for our area. At upper elevations and high points the general flow has been from the SW creating large and sensitive wind slabs on NW-N-NE-E-SE aspects through the night, but channeling has lead to localized crossloading of slopes facing S-SW also. Winds have also been stronger at lower elevations affecting open areas down to the bottom of slopes, especially where there was previously snow coverage. Avoid slopes where snow has been blowing and forming round dense drifts.

Avalanche Problem 2: Storm Slab
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The past 24hrs has brought up to 25” of new snow and up to 2.2” of new water to the east side. Since yesterday morning snow water content has increased, meaning heavier snow on top of lighter snow. New amounts were more from Mammoth and north, but there is enough new snow loading to cause large storm slabs across the entire forecast area, and in some areas down to lower elevations. Sheltered slopes with less wind effect are not safe from avalanches today. Storm slab avalanches are expected in many areas where denser precipitation sits atop lighter snow.

Avalanche Problem 3: Deep Slab
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The weak layers of faceted snow that were found in the Mammoth and Virginia Lakes regions prior to the storm are now buried even deeper. They may fail under the sudden new load and the resulting avalanches could be very large. Smaller storm slab and wind slab avalanches that run down to near treeline on N facing slopes may trigger these deeper layers. Little is known about the distribution of these weaknesses outside of Mammoth and Virginia Lakes, but with the shallow snowpack and cold temperatures of the past few weeks these layers are expected to be a problem across the range.

advisory discussion

So far the winter storm affecting our region has come through with up to 20” of new snow in the mountains near Bishop and up to 2’ or more around Mammoth since yesterday morning. Up to another 24” is forecast for today and tonight. The mountains are starting to gain some coverage down to ~8000’. All that is good news for our seasonal snowpack, but the rapid loading is bad news for the risk of avalanches in the short term. There has been enough new loading of strong over weak snow in the past 48hrs that avalanches can be expected in many areas and they could be very large and destructive.

And it’s not just new snowfall that has raised the danger level. Strong and variable winds have been building slabs at all elevations. In some cases, new storm and wind slabs are sitting on top of deeply buried, but very weak layers of sugary faceted snow. If these layers fail, from the weight of new snow or from a smaller avalanche suddenly over-running them, the resulting slide could be very large.

In many cases, lower elevation slopes are seeing snow coverage for the fist time in a while. In places where there was snow down lower, like around Mammoth and north to Virginia Lakes, these lower elevations may have enough new snow for avalanches to run to the bottom of the slope. In any case, obstacles that were previously obvious may now be hidden under just enough new snow that you won’t see them coming. In fact, the strong winds may make it hard to see at all. Playing on or near steeper or challenging terrain in the mountains is not recommended today. Even if the weather eases to slightly lighter winds and showery snowfall, avalanche hazards that increased overnight will take time to stabilize.

recent observations
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3/1- Mammoth to Rock Creek: Lower Elevation Snow and Wind

New Snow (past 24hrs):

Virginia Lakes – 23”
Tioga Pass – 21”
June Mtn Weather Site – 16”
Sesame Snow Study – 25”
Sawmill Near Big Pine – 19”

2/28- Mammoth Crest: Persistent Weak Layers, Wind Slab Formation

 

Weather and CURRENT CONDITIONS
weather summary

...BLIZZARD WARNING IN EFFECT UNTIL 1 PM PST FRIDAY… ...WINTER STORM WARNING IN EFFECT UNTIL 10 AM PST SATURDAY...

A strong winter storm will continue to bring periods of heavy snow to the Sierra with travel impacts through Saturday. Snowfall rates may decrease by mid-morning, but this storm is not over--this afternoon through tonight another shortwave wrapping around the main low off the Pacific NW coast will bring increasing snowfall rates to the Sierra and northeast CA. For Mono County, the Blizzard Warning will stay in place as winds will be slower to decrease.

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: Cloudy. Snow. Cloudy. Snow. Cloudy. Snow showers likely.
Temperatures: 23 to 31 deg. F. 9 to 15 deg. F. 17 to 23 deg. F.
Mid Slope Winds: SW SW SW
Expected snowfall: 80% probability...of 6 to 12 inches. 20% probability...of 12 to 16 in. 80% probability...of 5 to 11 inches. 20% probability...of 3 to 5 in. 90% probability...of 1 to 5 inches. 10% probability...of 5 to 8 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Snow. Cloudy. Snow. Cloudy. Snow showers likely.
Temperatures: 16 to 22 deg. F. 2 to 7 deg. F. 10 to 16 deg. F.
Ridge Top Winds: SW SW SW
Expected snowfall: 80% probability...of 7 to 13 inches. 20% probability...of 13 to 18 in. 80% probability...of 6 to 12 inches. 20% probability...of 3 to 6 in. 90% probability...of 2 to 6 inches. 10% probability...of 6 to 8 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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