THIS AVALANCHE ADVISORY EXPIRED ON January 9, 2018 @ 7:00 am
Avalanche Advisory published on January 8, 2018 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

As more snow, rain, and wind impact the forecast area over the next 24 hours, human-triggered avalanches will be likely and natural avalanches will be possible especially later today and during the night. Avalanche problems include persistent slabs, loose wet avalanches, and wind slabs. CONSIDERABLE avalanche danger will exist at all elevations. Avalanches could involve enough snow to bury a person. Dangerous avalanche conditions will exist across the forecast area.

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: Persistent Slab
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Loose weak snow (facets) still exists below the surface on many northerly aspects across the forecast area. In some areas, the recent rain has soaked this layer and started to change it. In those areas while this layer of loose snow remains wet, it remains weak. Once it refreezes it will gain strength, but refreezing will not happen until after this storm has a chance to load that loose weak layer. In areas where the facets remain dry, they also remain weak. Regardless of whether this layer is wet or dry it is barely able to support the snowpack currently resting above it. As more precipitation adds more weight to the snowpack over the next 36 hours expect persistent slab avalanches to become more widespread. Human-triggered persistent slabs will become likely today and some natural avalanches will be possible especially later in the day and during the night. In areas where rain falls on the snowpack, the rain could make this avalanche problem easier to trigger and those persistent slabs could entrain more wet snow and have some wet slab characteristics. The persistent slab problem should grow in size as more precipitation accumulates and avalanches could involve enough snow to bury a person.

Avoid steeper slopes in areas where snow surface cracking, whumpfing, and/or unstable snowpit test results are occurring.  Quick hand pits remain an effective tool for seeing if a slab over weak loose snow exists. These kinds of avalanches can be triggered from connected slopes or may not release until a person has fully committed to the slope. 

Avalanche Problem 2: Loose Wet
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1 to 2 inches of rain falling on the snowpack in the next 24 hours will cause loose wet instabilities to become widespread. Expect roller balls, pinwheels, and wet point release avalanches during the next 24 hours in any areas that receive rain. Some of these could entrain enough snow to bury a person especially on northerly aspects where the snow has not seen rain before. Rain could also exacerbate today's other avalanche problems.

Avoiding steep slopes where rain is falling or where wet snow exists represents the easiest way to avoid loose wet avalanches.

Avalanche Problem 3: Wind Slab
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Winds strong enough to transport snow and new snow will allow new wind slabs to form on wind-loaded slopes. These wind slabs should remain limited to areas above 8500 ft. today, but they should start to become more widespread overnight as snow levels start to fall. In isolated areas, wind slabs may be on top of old near surface facets on NW-N-NE aspects.

Look for cornice formation, blowing snow, and signs of previous blowing snow, especially along ridgelines and wind exposed areas. Use these clues to identify where wind slabs may exist and avoid those steep wind-loaded slopes. 

recent observations

* On Tamarack Peak, a small human-triggered avalanche was reported on Prolateriate yesterday, and a similar small natural avalanche was reported in Broken Glass.

* Human-triggered cracking, collapsing, and whumpfing occurred on Red Lake Peak and near Woods Lake on Carson Pass. The weak loose facetted snow below the new snow represented the failure layer.

* In the Bear Valley area, on Carson Pass, and on Red Lake Peak, snowpit tests targeting the facet layer yielded unstable test results.

* Observations showed moist facets in some areas yesterday and dry facets in other areas. Whether they were dry or moist depended on elevation and on whether or not a slab existed above the facets. Regardless of whether they were moist or dry, they remained loose and weak.

* NW-N-NE aspects above 8,000' in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best snow coverage at 2 to 3+ feet. Overall less snow cover exists south of Emerald Bay. Areas of decent coverage exist above 8,500' on NW-N-NE aspects in the Carson Pass area.  Coverage decreases on all other aspects. Large areas of bare ground exist on the vast majority of southerly aspects at all elevations.

Weather and CURRENT CONDITIONS
weather summary

Precipitation has started moving into CA and should continue through tomorrow evening. Where precipitation has started west of the forecast area, sensors indicate that snow levels are hovering around 9500 ft. As the precipitation moves over the mountains this morning, some of it could fall as snow down to 7500 ft. or below due to some colder air trapped at low elevations in some areas, but snow levels should rise quickly to 8500 ft. or above. Most of today's precipitation should fall as rain at all but the highest elevations. Snow levels should remain above 8000 ft. through tonight before some colder air arrives over the region tomorrow driving snow levels down below 7500 ft. during the day tomorrow. Snow levels could fall down into the 6500 ft. range by tomorrow afternoon. The forecast calls for .25 to .7 in. of precip today, another .6 to 1.4 in. tonight, and .3 to .8 in. tomorrow. The amount of snow that accumulates in an area depends on where snow levels end up, but some areas could see up to 2 ft. of new snow by the end of the day tomorrow. The forecast calls for moderate to strong south and southwest winds with the peak winds accompanying the peak precipitation intensity tonight. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 29 to 36 deg. F.
Max. temperature in the last 24 hours: 41 to 45 deg. F.
Average wind direction during the last 24 hours: Southwest and south
Average wind speed during the last 24 hours: 30 to 35 mph
Maximum wind gust in the last 24 hours: 60 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 19 to 34 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Cloudy with a mix of rain and snow in the morning changing to rain and continuing throughout the day. Snow levels 7500 ft. in the morning increasing to 8500 ft. or higher in the afternoon. Cloudy with rain and snow. Snow levels around 8000 ft. Cloudy with a mix of rain and snow. Snow levels 7500 ft. in the morning decreasing to 6500 ft. in the afternoon.
Temperatures: 39 to 44 deg. F. 30 to 35 deg. F. 36 to 41 deg. F.
Wind Direction: South South Southeast shifting to the southwest in the afternoon
Wind Speed: 10 to 15 mph with gusts to 35 mph increasing to 45 mph in the afternoon 15 to 20 mph with gusts to 55 mph 10 to 15 mph with gusts to 40 mph decreasing to 25 mph in the afternoon
Expected snowfall: Rain: .25 to .7 in. Rain: .6 to 1.4 in. | Snow: 0 to 5 in. 3 to 7 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Cloudy with snow and rain. Snow levels 7500 ft. in the morning increasing to 8500 ft. in the afternoon. Cloudy with snow and rain. Snow levels around 8000 ft. Cloudy with some rain in the morning then snow for the rest of the day. Snow levels falling to 7500 ft. in the morning then to 6500 ft. in the afternoon
Temperatures: 37 to 42 deg. F. 30 to 35 deg. F. 33 to 38 deg. F.
Wind Direction: South South South shifting to the southwest in the afternoon
Wind Speed: 20 to 35 mph with gusts to 50 mph increasing to 70 mph in the afternoon 30 to 40 mph with gusts to 80 mph 20 to 30 mph with gusts to 60 mph decreasing to 15 to 20 mph with gusts to 30 mph in the afternoon
Expected snowfall: 3 to 6 in. depending on snow levels 4 to 8 in. with a chance of 8 to 14 in. 4 to 10 in. with a chance of 10 to 16 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