THIS AVALANCHE ADVISORY EXPIRED ON March 5, 2018 @ 7:11 am
Avalanche Advisory published on March 4, 2018 @ 12:11 pm
Issued by Andy Anderson - Tahoe National Forest

UPDATE @ 12:00 pm. Danger increased to CONSIDERABLE. Large, dangerous, human-triggered deep slab avalanches are possible. Some already occurred today. These avalanches can be triggered remotely and would be almost impossible to survive. Wind slab and loose wet problems may also exist. Instability may lurk within areas of seemingly stable snow. If traveling in an area where deep slabs may exist avoid this problem by staying on slopes less steep than 30 degrees that are not connected to steeper terrain.

2. Moderate

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

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Deep Slab
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At least two human-triggered deep slab avalanches have occurred in the last 24 hours: one this morning at 11:30 am and one yesterday at 2:45 in the afternoon. Both were remotely triggered and both propagated across large areas and involved enough snow to destroy a car, a wood framed house, or maybe even a train (D3 to D4 in size). Human triggered deep slab avalanches are possible on near and below treeline NW-N-NE aspects where a facet layer (loose weak snow) is buried 4+ ft. deep in the snowpack. This layer and therefore deep slabs may exist in above treeline terrain as well, but some uncertainty surrounds whether or not the problem exists at those upper elevations.

These avalanches would have dire consequences and likely be unsurvivable. While the weak layer is buried deeply in most areas and may be difficult to trigger in some places, people can still cause this layer to fail especially around trigger points such as single trees, exposed rocks, and convex slope rollovers where the weak layer may be closer to the snow surface and easier to trigger. Large triggers such as multiple people on the same slope at once have a greater chance of triggering deep slab avalanches.

Due to the dire consequences of these deep slab avalanches and a reasonable possibility of triggering them, avoidance represents the best course of action. If traveling in an area where this weak layer may exist avoid this problem by staying on slopes less steep than 30 degrees without steeper terrain above. 

Avalanche Problem 2: Wind Slab
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A few more inches of new snow fell last night combined with moderate speed SW winds. This will have created new wind slabs near treeline and above treeline on NW-N-NE-E-SE aspects. Expected avalanche size up to D2 is possible (large enough to bury or injure a person).

Identify and avoid areas of potentially unstable wind slab. Slopes below cornice features and wind pillows are suspect today. Snow surface cracking in wind exposed areas is an additional clue of unstable snow in the immediate area.

Avalanche Problem 3: Loose Wet
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Warming this afternoon will be borderline for creating small areas of loose wet avalanche problems on lower elevation SE-S-SW aspects. This avalanche problem is expected to become more likely tomorrow. Avoid steep sun exposed slopes where roller balls are occurring.

recent observations

* 11:25 am addition: Another skier triggered Deep Slab avalanche was triggered in Negro Canyon. This slide was triggered from lower angle slopes 1000 ft. away and broke with a 400 ft. wide and up to 8 ft. deep crown. It ran downslope for 500 ft. and into dense trees. 

* 8:15 am addition: Skier triggered Deep Slab avalanche on Schallenberger Ridge (Donner Summit area) yesterday. More info below. Click through all of the tabs in the observation for all photos and info.

* Numerous natural wind slab and storm slab avalanches occurred during this past storm cycle. In many areas evidence of these avalanches is nearly completely covered by new snow.

* A small skier triggered wind slab avalanche occurred yesterday on Andesite Peak on a NE aspect above treeline (Donner Summit area).

* Widespread shooting cracks and whumpfing were reported on a SW aspect 34 degree slope between 7,500' to 7,825' in the Wood Creek Drainage (Mount Rose area).

* A deeply buried facet layer is the weak layer of concern for the deep slab problem. 

Weather and CURRENT CONDITIONS
weather summary

Snow showers last night into this morning will diminish as the day progresses. The most intense snow showers this morning are expected along the E side of Lake Tahoe with the possibility of a few quick inches of accumulation. Otherwise, decreasing cloud cover and decreasing SW to W ridgetop winds are forecast for today. Clearing skies and warming air temperatures are expected over the next few days.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 10 to 16 deg. F.
Max. temperature in the last 24 hours: 20 to 28 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 30 mph
Maximum wind gust in the last 24 hours: 52 mph
New snowfall in the last 24 hours: 2 to 6 inches
Total snow depth: 65 to 96 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy skies becoming party cloudy. Isolated snow showers in the morning. Partly cloudy skies, becoming clear. Sunny skies.
Temperatures: 25 to 30 deg. F. 5 to 10 deg. F. 32 to 37 deg. F.
Wind Direction: SW to W Variable Variable
Wind Speed: 10 to 15 mph with gusts to 25 mph in the morning, becoming light. Light winds Light winds
Expected snowfall: Up to 1 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy skies becoming party cloudy. Isolated snow showers in the morning. Partly cloudy skies, becoming clear. Sunny skies.
Temperatures: 19 to 27 deg. F. 6 to 11 deg. F. 28 to 34 deg. F.
Wind Direction: SW to W Variable SW
Wind Speed: 15 to 20 mph. Gusts to 45 mph decreasing to 35 mph in the afternoon. Light winds 10 to 15 mph with gusts to 25 mph in the morning, becoming light.
Expected snowfall: Up to 1 in. 0 in. 0 in.
Disclaimer

This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258