THIS AVALANCHE FORECAST EXPIRED ON March 6, 2018 @ 6:59 am
Avalanche Forecast published on March 5, 2018 @ 6:59 am
Issued by Brandon Schwartz - Tahoe National Forest

Avalanche danger is CONSIDERABLE for all elevations. Large destructive Deep Slab avalanches are an ongoing problem with large human triggered avalanches occurring both Saturday and Sunday. Loose wet and wind slab avalanche problems may also be encountered today. This is not typical Sierra post storm conditions. If going into the backcountry, deliberately choose terrain and plan a route that avoids exposure to the current avalanche problems. Do not underestimate potential avalanche size or likelihood of triggering.

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.

3. Considerable

?

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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We have a deep slab problem. Four Deep Slab avalanches occurred during the past two days. The avalanches occurred at a variety of elevations on N-NE-ENE aspects. At least nine Deep Slab avalanches are known to have occurred during the recent storm and in the sunny days following. Deep Slab avalanches are a continued avalanche problem today.

The source of the problem is a weak layer of old faceted (loose, sugary) snow buried 4 to 8 feet deep in the snowpack in many locations on NW-N-NE-ENE aspects. It can be difficult for backcountry travelers to get force down to the depth of this layer where it is deeply buried. In areas where this weak layer comes closer to the snow surface such as near exposed rocks, isolated trees, cliffs, and convex slope roll overs, large avalanches can be triggered by backcountry travelers. This can allow for numerous tracks to exist on the slope prior to an avalanche as was the case in Sunday's avalanche near Lost Lakes (Carson Pass area). Tracks on a slope are not signs of stability when dealing with this avalanche problem.

Once a Deep Slab avalanche is triggered, propagation will be wide. The avalanche will likely be size D3 (could bury and destroy a car, damage a truck, destroy a wood frame house, or break a few trees). The avalanche could run well into areas of mature forest that might typically be considered "safe terrain".

This is not the typical type of avalanche dealt with on a storm to storm basis during a Sierra winter. Tactics often applied for managing the common wind slab and storm slab problems may not be effective for dealing with deep slabs. For example, slope cuts are fully inappropriate for this specific avalanche problem. The only way to deal with this avalanche problem is through avoidance. When traveling on NW-N-NE-ENE aspects at any elevation, avoid travel on slopes steeper than 30 degrees and stay out from under steeper terrain above. This type of avalanche problem could be triggered from the valley floor in some cases. Do not underestimate the potential avalanche size. Previous tracks do not equate or prove stability.

Avalanche Problem 2: Loose Wet
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Rapid warming, air temperatures climbing above freezing, and March levels of incoming solar radiation will likely create areas of loose wet avalanche problems today. Sun exposed areas on E-SE-S-SW-W aspects will become problematic as the day progresses. Avalanche size up to D2 is possible (could bury or injure a person).

Identify and avoid steep sun exposed areas where roller balls are occurring. Roller balls may occur naturally as snow falls off of rocks, cliffs, trees, or can be human triggered.

Avalanche Problem 3: Wind Slab
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There have been a few recent wind slab avalanches over the past several days. Winds have shifted from SW to NE to S so the potential exists for wind slabs to linger near and above treeline on just about any aspect. The largest wind slabs will likely exist on NW-N-NE-E-SE-S aspects.

Much of the terrain for the largest wind slabs will overlap with areas of concern for deep slab. It will be challenging to manage wind slabs in these areas while at the same time avoiding deep slabs. Use a compass to keep track of aspect and take both avalanche problems into account when dealing with NW-N-NE-ENE aspect terrain.

recent observations

* Two large Deep Slab avalanches occurred Sunday. One triggered remotely from 500' away while digging a snowpit at the head of Negro Canyon (Donner Summit area) and the other triggered by snowmobile near Lost Lakes (Carson Pass area). Two large Deep Slab avalanches occurred on Saturday. One skier triggered on Schallenberger Ridge (Donner Summit area) and one of unknown trigger on Trimmer Peak (Jobe/Freel area). Several other Deep Slab avalanches occurred naturally during the storm cycle in Pole Creek and in the Desolation Wilderness. In all cases these avalanches occurred on N-NE-ENE aspects and had crown heights of 4 to 8 feet. See observations below for photos and other specifics.

* A small skier triggered wind slab was reported Sunday morning from Hourglass Bowl on Tamarack Peak (Mount Rose area). Another larger wind slab was skier triggered on the S side of Mount Rose proper (the peak not the ski area) in a cross loaded gully.

* Significant warming effects on the snow surface were observed and reported from around the forecast area Sunday on E-SE-S-SW-W aspects.

Weather and CURRENT CONDITIONS
weather summary

After a cold start to the morning, rapid warming is forecast for today. Maximum daytime air temperatures for areas above 7,000' are forecast to reach the upper 20s to upper 30s today with fairly intense incoming solar radiation. Winds increased out of the NE last night for a few hours, gusting into the upper 20s and low 30s. Winds have shifted to the S this morning and are forecast to become light. Continued light winds and additional daytime warming are forecast for Tuesday. The next chance for precipitation is expected Wednesday night.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 12 to 19 deg. F.
Max. temperature in the last 24 hours: 25 to 33 deg. F.
Average wind direction during the last 24 hours: Variable SW and NE
Average wind speed during the last 24 hours: 10 mph
Maximum wind gust in the last 24 hours: 33 mph
New snowfall in the last 24 hours: 0 to trace inches
Total snow depth: 62 to 92 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Sunny skies. Clear skies. Partly cloudy skies, becoming mostly cloudy. High level cloud cover.
Temperatures: 32 to 38 deg. F. 9 to 14 deg. F. 39 to 44 deg. F.
Mid Slope Winds: Variable Variable Variable
Wind Speed: Light winds Light winds Light winds
Expected snowfall: 0 0 0
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Sunny skies. Clear skies. Partly cloudy skies, becoming mostly cloudy. High level cloud cover.
Temperatures: 28 to 34 deg. F. 10 to 15 deg. F. 36 to 42 deg. F.
Ridge Top Winds: S Variable Variable
Wind Speed: 10 to 15 mph with gusts to 25 mph in the morning, becoming light. Light winds Light winds
Expected snowfall: 0 0 0
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

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