THIS AVALANCHE FORECAST EXPIRED ON February 27, 2018 @ 6:59 am
Avalanche Forecast published on February 26, 2018 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest

As new snow accumulates today, the avalanche danger should rise to CONSIDERABLE in near and above treeline terrain and MODERATE in below treeline terrain. Expect fragile wind slabs near and above treeline and possible storm slabs in more sheltered areas near and below treeline. Human triggered avalanches will become likely today.

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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New snow and strong southwest winds today means another round of wind slab formation on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in near and above treeline terrain. The size of the wind slabs that form on these leeward slopes will depend on the amount of new snow that accumulates. Wind slab avalanches could involve enough snow to bury or injure a person and some may entrain more snow in the most heavily wind-loaded areas. These wind slabs may form faster than they can bond to the existing snow surfaces and human-triggered wind slab avalanches should become likely today with some natural wind slab avalanches becoming possible.

Use clues like recent avalanches, shooting cracks, cornices above slopes, blowing snow, ripples, drifted snow, hollow sounding snow, or other wind created surface textures to identify where wind slabs may exist and avoid those areas.   

Avalanche Problem 2: Storm Slab
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Rapid new snow accumulation and changing conditions during the storm may allow for some human triggered storm slab avalanches to become possible on steep slopes in sheltered near and below treeline terrain today. Most of these should remain small, but some could involve enough snow to bury or injure a person. In some isolated areas on NW-N-NE aspects, this new snow may provide enough of a slab layer for a storm slab avalanche to step down to the weak layer (facets) buried in the snowpack. In this case, these avalanches may behave more like a persistent slab and could fail after a person committed to the slope or propagate farther than expected.  

Recent avalanche activity, human-triggered cracking, collapsing, or whumping, and/or snowpit tests can help determine where storm slabs may exist. Avoiding steep slopes with signs of instability represents a prudent choice today. Low angle sheltered slopes should provide great recreation opportunities today.  

recent observations

* Observations around the forecast area have shown that a layer of loose weak snow (facets) exists below the recent snow. Yesterday on Trimmer Peak (South Lake Tahoe area) some skier-triggered cracking occurred on this layer. On Miess Ridge (Carson Pass area), Becker Ridge (Echo Summit area),  and Castle Peak (Donner Summit area), snowpit tests targeting this layer yielded inconsistent results with a mix of unstable results and other results. Snowpit tests on Relay Peak (Mt. Rose backcountry) did not yield unstable results.

* Small stiff wind slabs that did not extend very far downslope existed along Miess Ridge. Ski kicks on these triggered small shooting cracks. 

Weather and CURRENT CONDITIONS
weather summary

Snow started falling in the northern part of the forecast area around 4 am this morning. As the storm moves southward, snow should become widespread over the region. The forecast calls for the another 4 to 11 inches of new snow today with periods of snowfall rates at 1 to 2 inches per hour in some places. Snowfall should continue this evening before starting to diminish after midnight. At some point, this storm should stall, which would allow the area it stalls over to pick up significantly more snow than the surrounding areas. Predicting where and when this will happen is difficult and a high degree of uncertainty remains as to the location and timing of this event. The southwest winds have returned and increased and should remain strong through today. By tonight they should start to decrease and shift to the northeast. Expect northeast winds to continue tomorrow. The forecast calls for clearing skies and cold temperatures tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 13 to 16 deg. F.
Max. temperature in the last 24 hours: 26 to 35 deg. F.
Average wind direction during the last 24 hours: NE shifting to SW
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 75 to 85 mph
New snowfall in the last 24 hours: trace to 3 inches
Total snow depth: 32 to 57 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Snow Cloudy with snow showers in the evening becoming mostly cloudy with a chance of snow showers after midnight Partly cloudy with a chance of snow showers in the morning becoming sunny by the afternoon.
Temperatures: 23 to 28 deg. F. 6 to 11 deg. F. 24 to 29 deg. F.
Mid Slope Winds: Southwest Northeast Northeast
Wind Speed: 20 to 30 mph with gusts to 65 mph decreasing to 10 to 15 mph with gusts to 40 mph in the aftenoon Light increasing to 10 to 15 mph with gusts to 35 mph after midnight 10 to 15 mph with gusts to 30 mph
Expected snowfall: 4 to 10 2 to 6 trace
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Snow Cloudy with snow showers in the evening becoming mostly cloudy with a chance of snow showers after midnight Partly cloudy with a chance of snow showers in the morning becoming sunny by the afternoon.
Temperatures: 19 to 25 deg. F. 3 to 9 deg. F. 20 to 26 deg. F.
Ridge Top Winds: Southwest shifting to the south in the afternoon West shifting to the northeast after midnight Northeast
Wind Speed: 30 to 45 mph with gusts to 95 mph decreasing to 20 to 30 mph with gusts to 70 mph in the afternoon 10 to 15 mph increasing to 15 to 25 mph with gusts to 35 mph after midnight 15 to 25 mph with gusts to 60 mph decreasing to 50 mph in the afternoon
Expected snowfall: 5 to 11 2 to 6 trace
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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