Avalanche Advisory: Saturday - Mar 10, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 11, 2018 @ 7:43 am
Avalanche Advisory published on March 10, 2018 @ 6:43 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche danger is MODERATE today in alpine terrain where lingering wind slabs may still be triggered in extreme terrain. Human triggered avalanches may be possible on steep slopes where drifts have most recently developed. The danger will remain MODERATE for deep slab avalanches as well. Though the likelihood of triggering these deeper layers is low, the consequences could be high. Look into the snowpack to test these weak layers before you commit to playing on northerly facing slopes.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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Strong SW winds on the 7th and 8th had ample snow to blow onto NW-N-NE-E-SE aspects near and above treeline. A natural wind slab avalanche was observed in the Virginia Lakes area on Thursday. Though most wind slabs deposited during this even have become more and more stubborn it may not be impossible to find a wind deposit sensitive enough for you to trigger today in extreme terrain. Be aware of hollow sounding pillows of dense snow just under cornices, in the top of steep chutes, or on the sidewalls of alpine gullies. Low snow amounts and light winds today are unlikely to creat new wind slabs, but be aware of localized areas of blowing snow as it may point to new wind slab development through the evening.

Avalanche Problem 2: Deep Slab
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All season we have to discussing and tracking layers of persistent, weak, sugary snow (facets) that developed in areas that had shallow snow coverage prior to the storm that began March 1st. That event finally dropped enough snow to act as a cohesive slab overlying the persistent weak layers and some very large and destructive avalanches occurred through March 4th. While the threat of failure in the buried facets seems to be improving, deeply buried weaknesses are notoriously hard to predict. Cracking on the slope around you and rapid settlement heard as a “whumph” are direct signs of instability are should scare you. Snowpack tests can help you determine how sensitive these weaker snow grains are in your area. It is important to monitor these facets because while triggering them is becoming increasingly unlikely, the consequences could be high. You will most likely find these facets on NW-N-NE facing slopes buried between 1 and 1.5m deep. We have seen these layers most between 9000' and 10,500', but large avalanches in the alpine as recently as 3/4 show that areas of deeply buried facets may exist at upper elevations as well.

advisory discussion

Wind slabs developed through mid week thanks to strong SW winds and ample snow available for it to transport onto leeward slopes. The few observations we have received show that while the wind remained strong through yesterday, less and less loose snow has been blowing. Wind slabs can take days to stabilize and there may be a few still sensitive enough to trigger if you venture into extreme terrain above treeline. Look for the most recently deposited drifts just under cornices and near the tops of steep chutes and gullies.

While the chance of triggering the persistent weak layers buried by the storm last week is increasingly unlikely, deep slab avalanches are hard to predict and have high consequences. The last reported collapse of the buried weak layers was on the 6th, but snowpack tests have continued to indicate that failure in these sugary snow grains is not impossible. A group of riders hitting just the wrong place, a large cornice drop or avalanche running down and overloading the underlying snowpack, landing a big jump, these are the types of triggers that could still affect these deeper weaknesses. The resulting avalanche could be quite large. It’s a low probability-high consequence situation. The best way to know what these layers are doing on the slopes where you want to play is to dig down and check on them. They will probably be down around 3 to 4 feet deep on northerly slopes. Areas that already had some snow prior to 3/1, and areas where that snow was deep enough to cover over brush and small trees that would otherwise anchor the slab in place are the most suspect.

The chance of snow showers will increase throughout today with light winds. An inch of new snow is unlikely to create new avalanche problems today, but more overnight may continue to keep the scales tipped slightly in favor of instability through this evening and tomorrow. Mostly cloudy skies may have allowed more snow to melt at lower elevations last night, but will most likely keep solar radiation from wreaking too much havoc today. Some rollerball activity is possible, but loose wet avalanches are pretty unlikely.

Weather and CURRENT CONDITIONS
weather summary

Moisture is expected to move through the region late this afternoon through the overnight hours. Best chances for snow and precipitation remain in Mono County after about 10AM. Snow accumulations are expected to be primarily in the 2 to 4 inch range from 7000 to 8000 feet; up to 7 inches will be possible above 8000 feet. Conditions become drier Sunday and Monday as an upper level ridge axis moves over Nevada.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Chance of snow in the afternoon. Cloudy. Snow likely. Mostly cloudy then becoming partly cloudy. Slight chance of snow showers in the morning.
Temperatures: 35 to 43 deg. F. 27 to 33 deg. F. 40 to 46 deg. F.
Mid Slope Winds: Light winds. Light winds. Light winds.
Expected snowfall: up to 1 in. 70% probability...of 2 to 4 inches. 30% probability...of 3 to 6 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Snow likely in the afternoon. Cloudy. Snow in the evening, then chance of snow after midnight. Mostly cloudy then becoming partly cloudy. Slight chance of snow showers in the morning.
Temperatures: 27 to 32 deg. F. 22 to 27 deg. F. 33 to 38 deg. F.
Ridge Top Winds: S E Light winds.
Expected snowfall: up to 1 in. 70% probability...of 2 to 4 inches. 30% probability...of 4 to 7 in. 0 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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