Avalanche Advisory: Saturday - Mar 24, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 25, 2018 @ 6:44 am
Avalanche Advisory published on March 24, 2018 @ 6:44 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The complexity of the wind slab problem near and above treeline will create CONSIDERABLE avalanche danger today. Recent, large wind slabs may be gaining strength, but new sensitive wind slabs may form by this afternoon – especially North of Mammoth. Cautious route finding and conservative decision making is required to asses and manage the complexity. MODERATE danger exists below treeline where it is increasingly unlikely to trigger a storm slab or the deep persistent slabs, but if triggered the resulting slide could be very large.

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Recently formed wind slabs may be gaining strength throughout today, but may be quite large. Fresh, sensitive wind slabs may form as winds continue today. North of Mammoth, a few inches of new snow today may increase to likelihood of new wind slab development. In situations like this, where wind slabs could be large, and may be found with varying levels of sensitivity, cautious route finding and conservative decision making is required to manage the hazard. Specific terrain to watch out for: leeward slopes where the slabs are newer, thinner. or softer. Below cornices, the sidewalls of chutes and gullies, and near convexities are of special concern.

Avalanche Problem 2: Deep Slab
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The weakest layers in our snowpack were buried even deeper during the storm that began on 3/20. That storm snow is settling now and may make our deeply buried facets even harder to trigger. However, if a decent sized wind slab avalanche were to run down over an area where these persistent layers are still weak and less deep there is the potential for a step down and the resulting avalanche could be huge. In general, it is very unlikely for this to happen. But the consequences are high. W-N-E facing slopes above 9,000’ and especially areas that received less snow during the last storm (S of the Mammoth Area) are a little more concerning.

advisory discussion

The last major storm exited the region on Thursday night leaving large amounts of snow available for wind transport at all elevations. And that’s exactly what the strong winds yesterday did: move snow across ridgelines and pack it into touchy wind slabs up and down the range. With cold temperatures overnight, these recently formed slabs may still be sensitive enough for human triggering today, especially in steep alpine terrain. With the wind blowing snow into the evening yesterday, some of these wind slabs are expected to be large. We have one report of a very large natural wind slab avalanche on Mt. Wood, and many small natural avalanches in the southern half of the range, that may have occurred from the wind loading yesterday. Triggering these slabs will be most likely where they are thinner or softer: below cornices, the sidewalls of chutes and gullies, and near convexities. One such slab was triggered remotely yesterday afternoon on the Chicken Wing. As the recent wind slabs begin to bond, new sensitive wind slabs may form by this evening. Especially in the northern half of the area where the potential for a few inches of snow are forecasted to fall this afternoon. That will give SW winds more ammo to continue loading leeward slopes. In situations like this, where wind slabs could be large, and may be found with varying levels of sensitivity, cautious route finding and conservative decision making is required to manage the hazard. Pay attention to where snow has recently drifted, or is drifting anew. Hollow sounding snow, or cracking around you means that you may already be standing on wind slabs touchy enough to trigger.

Snowpack tests in Mammoth and June show that the storm slabs from 3/21-22 are beginning to settle down and become less reactive in most areas. As the snow becomes more supportable and less likely to avalanche, it will also be harder to trigger the deep persistent layers buried about 2m under the surface. That’s good news. The likelihood of you triggering the deep facet layers is very low. But the possibility of a wind slab avalanche running down and stepping down into an area where these layers are still weak is not out of the question. The resulting slide would be deadly and so it is not a scenario that we’re comfortable ruling out just yet. You can use your probe to feel down into the snowpack and see if there are less resistant layers lurking under your feet. You’ll probably find them above 9,000’ on W-N-E facing slopes. They’ll be a little closer to the surface (and thus slightly easier to trigger) in areas that received less snow during the last storm cycle. Managing the wind slab problem to avoid a smaller avalanche running down over these layers, and becoming much larger, is a good way to avoid triggering the deep slabs. Other large triggers, like a snowmobile landing a jump in just the right spot, are also more likely to result in a very large slide.

Weather and CURRENT CONDITIONS
weather summary

Snow showers, mainly across the northern half of the forecast area, could bring light snowfall accumulations along with breezy conditions into Sunday morning.

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: Sunny then becoming partly cloudy. Scattered snow showers. Partly cloudy. Scattered snow showers. Partly cloudy. Isolated snow showers.
Temperatures: 26 to 34 deg. F. 9 to 15 deg. F. 24 to 30 deg. F.
Wind Direction: SW SW W
Wind Speed: 15 to 25 mph. Gusts up to 40 mph. 15 to 25 mph with gusts to 40 mph. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: Up to 1 in. 80% probability...up to 2 inches. 20% probability...of 2 to 4 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy. Scattered snow showers. Mostly cloudy then becoming partly cloudy. Scattered snow showers. Partly cloudy. Isolated snow showers.
Temperatures: 20 to 26 deg. F. 3 to 8 deg. F. 16 to 22 deg. F.
Wind Direction: SW SW SW
Wind Speed: 30 to 45 mph with gusts to 55 mph decreasing to 20 to 30 mph with gusts to 45 mph in the afternoon. 25 to 35 mph. Gusts up to 55 mph decreasing to 45 mph after midnight. 20 to 30 mph becoming west 10 to 15 mph with gusts to 25 mph in the afternoon.
Expected snowfall: Up to 1 in. 70% probability...up to 2 inches. 30% probability...of 2 to 4 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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