THIS AVALANCHE ADVISORY EXPIRED ON March 2, 2015 @ 7:00 am
Avalanche Advisory published on March 1, 2015 @ 7:00 am
Issued by Brandon Schwartz - Tahoe National Forest

Areas of MODERATE avalanche danger exist on all aspects at all elevations on slopes 35 degrees and steeper due to a combination of wind slab, persistent slab, and loose wet avalanche problems. Small to large human triggered avalanches remain possible today. Don't loose sight of how human factors may positively or negatively affect decision making in avalanche 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.

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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With the shift and speed increase of ridgetop winds from the SW to NE, and fresh round of wind slab formation is expected to have occurred last night. With light winds during nearly all of the snowfall event, significant amounts of new snow were available for transport and redistribution in near and above treeline areas. New wind slabs will exist today on SE-S-SW-W-NW aspects near and above treeline. The strongest NE winds are expected to focus along the Sierra Crest. New wind slab development may be minimal along the east side of Lake Tahoe.

Avalanche Problem 2: Persistent Slab
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Snowpit data collected yesterday indicated that the slab of storm snow deposited on top of near surface facets lacked sufficient cohesion to show instability issues. As the new snow settles and gains strength, it is possible that the area of greatest relative weakness within the snowpack may shift down to the base of the storm snow. In near treeline and below treeline areas above 8,000' on NW-N-NE aspects where near surface facets have been observed at the old/new snow interface, this may actually lead to a decrease in stability over the coming days as compared to what was observed yesterday. This also means that previous tracks on a slope from earlier in the storm cycle will have no bearing on current slope stability. Handpits will easily detect near surface facets at the base of the storm snow. Snowpit tests will indicate if/when this layer interface is becoming problematic. Glossary of terms

Avalanche Problem 3: Loose Wet
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As air temperatures approach freezing and the increasingly strong solar input from the early March sun shines through breaks in cloud cover, small areas of loose wet instability are expected to occur. Roller balls and pinwheels are expected as the vast majority of activity as snow falls off of rocks and trees. In an isolated sun exposed area, an actual loose wet avalanche may occur on a slope steeper than 37 degrees.

recent observations

Small natural and skier triggered avalanches occurred yesterday near Grouse Rocks (Ward Canyon area) and on the East face of Mt. Judah (Donner Summit area). In all cases, avalanche activity occurred on N-NE-E aspects both above and below treeline on slopes steeper than 35 degrees. The snowpack was noted as failing on a subtle change in density within the storm snow. All crown heights were observed at less than 1 foot.

Snowpit data collected on Elephant's Hump (Carson Pass area), Grouse Rocks (Ward Canyon area), and Andesite Ridge (Donner Summit area) indicated that the overlying slab of storm snow was very soft, had low cohesion, and was therefore difficult to test using compression based techniques. Bonding of new snow to old snow surfaces of crust or near surface facets was moderate in strength. A number of very small loose dry skier triggered avalanches were observed and reported from around the forecast area with snowpack failure occurring within the storm snow. Wind slab formation was minimal yesterday with one 20' long skier triggered shooting crack observed yesterday afternoon near Grouse Rocks in a lee area along the summit ridge. Some concern exists that as the storm snow settles and gains strength, failure on near surface facets at the base of the storm snow will become more of an issue and avalanche size will increase. Glossary of terms

Weather and CURRENT CONDITIONS
weather summary

Storm snow totals since Friday have exceeded forecast amounts in most areas. Most portions of the forecast area received 8 to 12 inches of new snow with standouts of 18 to 25 inches in the Donner Summit and Ward Canyon areas. Snow showers are forecast to continue today and tomorrow with little to no additional accumulation. Mostly cloudy skies will allow for sun breaks to occur periodically throughout the day. Ridgetop winds which remained light out of the southwest for most of the snowfall event have shifted to the northeast and increased to moderate to strong in speed over the Sierra Crest. Gusts to 54 mph observed overnight are expected to decrease to gusts to 30 mph today. Light to moderate speed northeast winds are forecast for tonight and tomorrow. Remote sensors are reporting air temperatures this morning in the low to mid 20s for areas 8,000' to 9,000'. Maximum daytime air temperatures are forecast to reach the mid 20s to low 30s today for areas above 7,000'. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 21 to 27 deg. F.
Max. temperature in the last 24 hours: 27 to 30 deg. F.
Average wind direction during the last 24 hours: Southwest shifting to northeast
Average wind speed during the last 24 hours: SW 15 mph | NE 31 mph
Maximum wind gust in the last 24 hours: SW 27 mph | NE 54 mph
New snowfall in the last 24 hours: 6 to 10 inches
Total snow depth: 40 to 52 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 with periods of snow showers, mainly south of Hwy 50. Partly cloudy skies. Mostly cloudy skies with a chance of snow showers.
Temperatures: 27 to 32 deg. F. 13 to 20 deg. F. 27 to 34 deg. F.
Wind Direction: NE Variable NW
Wind Speed: 15 to 20 mph, gusts up to 30 mph in the morning. Light winds Light winds increasing to 10 to 15 mph in the afternoon.
Expected snowfall: 0 to 1 in. 0 in. Trace to 1 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy skies with periods of snow showers, mainly south of Hwy 50. Partly cloudy skies. Mostly cloudy skies with a chance of snow showers.
Temperatures: 23 to 29 deg. F. 11 to 18 deg. F. 26 to 32 deg. F.
Wind Direction: NE E NW
Wind Speed: 15 to 25 mph with gusts to 45 mph, focused over the Sierra Crest. 10 to 15 mph with gusts to 25 mph in the evening, becoming light. Light winds increasing to 10 to 15 mph with gusts to 25 mph in the afternoon.
Expected snowfall: 0 to 1 in. 0 in. Trace to 2 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.