THIS AVALANCHE ADVISORY EXPIRED ON March 7, 2018 @ 6:55 am
Avalanche Advisory published on March 6, 2018 @ 6:55 am
Issued by Steve Reynaud - Tahoe National Forest

Dangerous avalanche conditions continue to exist throughout the forecast region due to a deep slab avalanche problem at all elevations.  Large destructive avalanches have occurred in our area and continue to be possible today.  Loose wet avalanches will be likely today as daytime warming occurs.  Make a plan with your group that avoids terrain and exposure to the current avalanche problems.  Cautious route finding and conservative decision making are essential.

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

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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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More avalanche reports have come in that show that a widespread deep slab avalanche cycle has been occurring throughout the forecast area on NW-N-NE-ENE aspects at all elevations.  Many of these deep slabs occurred during or immediately after the storm and released naturally.  There have been at least 3 that have been human triggered post storm with 2 occurring on Sunday.  Deep slab avalanches continue to be an ongoing avalanche problem today.

The issue is a layer of faceted snow that exists 4 to 8' deep below the snow surface.  This can be difficult for a backcountry user to get force down to this weak layer where it is deeply buried.  Likely trigger points could be areas where the weak layer is closer to the snow surface like near: rocks, trees, cliffs, convex rollovers, and areas with a shallower snowpack. 

All of the deep slab avalanches that have occurred so far have had wide propagation and have been large avalanches (D3 or bigger-large enough to destroy a house and break trees up to taking out large swaths of mature forest).  Tracks on a slope will not indicate stability as was shown from Sunday's avalanche at Lost Lake (Carson Pass area) where the slope was tracked out before the avalanche was triggered.  These deep slabs have the potential to be remotely triggered as was shown by the Negro Canyon avalanche ( Donner Summit area) where the avalanche occurred 500' away from the trigger point in lower angle terrain with shooting cracks traveling up to 1000' away. 

Deep slabs are unpredictable and signs of instability may not be present.  The only effective management option is avoidance.  Avoid slopes steeper than 30 degrees and slopes connected to steeper terrain on NW-N-NE-E aspects.  Good recreation conditions currently exist on Northerly aspects that are below 30 degrees in steepness with no steeper avalanche terrain connected. 

Avalanche Problem 2: Loose Wet
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As daytime warming occurs today, loose wet avalanches will become likely on E-SE-S-SW-W aspects at all elevations.  Warming temperatures that are well above freezing with March levels of solar radiation will cause an increase in loose wet activity today.  Most of this activity should be in the form of roller balls and pin wheels but some larger loose wet avalanches are possible and could be large enough to injure or bury a backcountry user.

Avoid steep sun exposed aspects when daytime warming occurs. 

recent observations

* 3 human triggered Deep Slab avalanches have occurred over the last 4 days.  On Saturday, a skier triggered deep slab occurred on Schallenberger Ridge (Donner Summit area).  On Sunday, 2 deep slab avalanches occurred.  One in Negro Canyon (Donner Summit area) while digging a snowpit in lower angle terrain, and one at Lost Lakes (Carson Pass area) triggered by a snowmobiler.  All of these deep slab avalanches were at least D3 or above, all had wide propagation, with crowns ranging from 4 to 8' with one reported up to 15 to 20' deep in spots.

* Many more reports have come in that show a widespread Deep Slab Avalanche cycle has been occurring.  Natural avalanches from Mt. Tallac, Pole Creek, Desolation Wilderness, 2 from Trimmer Peak, and National Geographic Bowl.  In all cases these avalanches occurred on N-NE-ENE aspects and had crown heights of 4 to 8 feet plus.  See observations below for photos and other specifics.

* Warming effects have been reported throughout the forecast area on E-SE-S-SW-W aspects. 

Weather and CURRENT CONDITIONS
weather summary

A high pressure ridge will be over our area for today into Wednesday bringing warming temperatures with light winds.  Increasing clouds today may limit some warming potential into the afternoon.  Winds will increase on Wednesday night through the weekend with periods of light precipitation possible.  Confidence is growing in a larger storm moving into our area early next week.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 17 to 33 deg. F.
Max. temperature in the last 24 hours: 37 deg. F.
Average wind direction during the last 24 hours: NE
Average wind speed during the last 24 hours: 20 to 25 mph
Maximum wind gust in the last 24 hours: 29 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 59 to 88 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Mostly cloudy.
Temperatures: 39 to 45 deg. F. 19 to 24 deg. F. 44 to 49 deg. F.
Wind Direction: S
Wind Speed: Light winds Light winds Light winds becoming S 10 to 15mph with gusts to 35mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Mostly cloudy.
Temperatures: 36 to 42 deg. F. 17 to 22 deg. F. 39 to 45 deg. F.
Wind Direction: S S
Wind Speed: Light winds Light winds becoming S 10 to 15mph with gusts to 25mph after midnight. 15 to 25mph. Gusts to 30mph increasing to 45mph in the afternoon.
Expected snowfall: 0 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