THIS AVALANCHE FORECAST EXPIRED ON March 19, 2018 @ 6:58 am
Avalanche Forecast published on March 18, 2018 @ 6:58 am
Issued by Brandon Schwartz - Tahoe National Forest

MODERATE avalanche danger exists today at all elevations due to a combination of wind slab and loose wet avalanche problems. Backcountry travelers should not be surprised by the occurrence of a human triggered avalanche today. Areas of unstable snow will exist within surrounding greater areas of seemingly stable snow. Take active measures to avoid the specific terrain features where unstable snow is most likely to exist.

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    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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Expect to find lingering wind slabs today near treeline and above treeline in wind loaded areas on NW-N-NE-E-SE aspects. Avalanche size could be very large with sufficient destructive potential to bury and destroy a car or break a few trees (up to size D3).

These wind slabs may be difficult to trigger (stubborn) until fully committed to the slope. Signs of instability may be difficult to find today prior to an avalanche occurrence. Identify and avoid areas of potentially unstable wind slab. The most likely areas of unstable snow will exist below cornice features and on slopes with wind pillows across the upper portion of the slope.

Rapid warming today may exacerbate the wind slab problem on sun exposed E-SE aspects.

Avalanche Problem 2: Loose Wet
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Rapid warming is expected today. Mid March levels of incoming solar radiation combined with a higher spring-like sun angle will allow for surface wet snow formation and the possibility of loose wet avalanches. Loose wet instability is possible on all aspects today. Avalanche size could be large enough to bury or injure a person (up to size D2).

Roller balls will occur naturally today as snow falls off of rocks and trees. This will be the first clue for potential loose wet instability. Human triggered roller balls are an additional sign of wet snow instability. Avoid steep slopes in areas where roller balls are occurring.

 

Forecast discussion

Triggering a Deep Slab avalanche has become very unlikely. The vast majority of observations and data targeting the once faceted persistent weak layer have indicated that strength gains are sufficient to handle the new snow load. There have been a few outlying pieces of data to suggest that while a deep slab avalanche is very unlikely, it is not impossible. If a deep slab avalanche were to occur it would be huge, likely unsurvivable, and could run very long distances into mature forest (size D4+). The only way to manage this type of avalanche problem is through avoidance of NW-N-NE aspect avalanche terrain.

recent observations
Submit Observations

* Observations made yesterday on Andesite Peak (Donner Summit area) revealed reactive wind slabs below cornice features in near treeline and above treeline terrain. Wind slab avalanches were intentionally skier triggered on test slopes with crowns up to 2 feet deep. Higher density wind transported snow on top of lower density storm snow was the problematic snowpack structure. Problematic storm slabs were not seen in below treeline terrain in this area. Snowpit tests indicated that non-persistent weak layers within the storm snow were gaining strength. Observations made Friday in this same area revealed widespread wind slab and storm slab avalanches.

* No additional field observations were received from Saturday to aid in the creation of today's advisory.

* The forecast area picked up 4 to 6 feet of snow at and above 7,000' this past week. The vast majority of that snow fell Tuesday afternoon through Saturday morning (March 13-17).

Weather and CURRENT CONDITIONS
weather summary

Short lived high pressure is over the forecast area today and tomorrow before the next storm system begins to impact the region on Tuesday. For today expect sunny skies, light winds, and a fairly cold early morning start giving way to significant warming of air temperatures as the day progresses. Additional warming is expected on Monday with light to moderate speed S winds.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 5 to 15 deg. F.
Max. temperature: 21 to 28 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 20 mph
Maximum ridgetop wind gust: 60 mph
New snowfall: 2 to 5 inches
Total snow depth: 78 to 109 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Sunny skies, becoming partly cloudy. Mostly cloudy skies. Mostly cloudy skies, becoming partly cloudy.
Temperatures: 31 to 36 deg. F. 15 to 20 deg. F. 36 to 41 deg. F.
Mid Slope Winds: Variable Variable S
Wind Speed: Light winds Light winds Light winds
Expected snowfall: 0 0 0
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Sunny skies, becoming partly cloudy. Mostly cloudy skies. Mostly cloudy skies, becoming partly cloudy.
Temperatures: 26 to 32 deg. F. 14 to 19 deg. F. 32 to 38 deg. F.
Ridge Top Winds: Variable S S
Wind Speed: Light winds Light winds becoming 10 to 15 mph with gusts to 30 mph after midnight. 10 to 15 mph with gusts to 25 mph.
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