THIS AVALANCHE FORECAST EXPIRED ON April 8, 2018 @ 6:59 am
Avalanche Forecast published on April 7, 2018 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

CONSIDERABLE avalanche danger exists at all elevations today. Natural avalanches are possible and human triggered avalanches are likely. Today's avalanche problems include widespread loose wet avalanches, possible deep slab avalanches, and maybe some wind slab avalanches at the higher elevations.  Avalanches that occur today could be large and destructive and could have severe consequences.

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: Loose Wet
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Deep loose wet snow exists on all aspects and at all elevations due to significant rainfall over the last 24 hours saturating the snowpack in many places. The rain will continue today and it will continue to weaken the snowpack. Loose wet avalanches will be likely today on any steep slopes. Northerly aspects and higher elevation slopes that have experienced less robust melting during the warm weather will be more likely to hold larger loose wet instabilities. Wet snow instabilities could include pinwheels, rollerballs, and loose wet avalanches. Some of the loose wet avalanches could involve large amounts of wet snow and could be large enough to bury or injure a person or maybe even a car.

Avalanche Problem 2: Deep Slab
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The rain has added significant load to the existing snowpack while melting bonds and significantly weakening the snowpack at the same time. The decreased strength should make it easier to trigger the deeply buried Feb. facet weak layer and the added weight may be enough overload that weak layer. As a result, large deep slab avalanche activity is possible today on NW-N-NE aspects especially in areas that did not avalanche during the previous deep slab avalanche cycle. In addition rain saturated weak snow may allow some deep wet slab avalanches to occur on other aspects especially in areas where the snowpack rests on surfaces like granite slabs. Natural deep slab avalanches are possible today. Any slab avalanches that occur today could be large and destructive and would involve heavy wet snow.

This kind of avalanche problem is difficult to predict and a high amount of uncertainty is associated with it. What is certain is that if one of these avalanches does occur and a person is invovled, the consquences would be very serious. Due to the combination of severe consequences and high uncertainty backcountry travel in avalanche terrain or in the runout zones of steep slopes is not recommended today.

Avalanche Problem 3: Wind Slab
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If snow level drops before the precipitation stops, some higher elevation areas could recieve enough snow for small wind slabs to form near ridgelines on leeward aspects by this afternoon. Leeward slopes above 9000 ft. hold the best postential for some wind slab formation. If wind slabs do form, some human-triggered wind slab avalanches may be possible in the most heavily wind-loaded areas. 

Identify where wind slabs may exist using clues like active wind-loading, cornices above slopes, drifted snow, ripples in the snow, and other wind created textures then avoid the wind slabs in favor of more sheltered terrain. 

recent observations
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* Knee-deep, unconsolidated, unsupportable wet snow existed on Andesite Peak by 10:30 am yesterday. Ski cuts on small test slopes produced small loose wet instabilities at this time as well. 

* Observations targetting the Feb. facet layer on Silver Peak, Jakes Peak, and on Elephants Hump this week have shown that this layer remained loose and that fractures could still travel along the layer if it breaks. Data has also indicated that it would take a very large trigger (like 2 to 5 inches of rain?!) to break this layer. 

Weather and CURRENT CONDITIONS
weather summary

So far 1 to 3 inches of rain has fallen on the forecast area in most places during the last 24 hours. Some areas on the west side of the crest like Bear Valley have reported more than 3.5 inches of rain. Heavy rain should continue this morning with another 1 to 2 inches of rain possible by noon. The high snow levels (still above 11000 ft at 6 am this morning) should start to fall as a cold front moves into the area. By this afternoon snow levels could drop to 7000 ft. As the front passes through the area, precipitation should decrease dramatically, but enough instability may linger for 2 to 5 inches of snow to accumulate above 8000 ft. by this evening. Precipitation should come to an end tonight and the forecast calls for a sunny spring day tomorrow. Strong southwest winds will continue through today before they start decreasing tonight and tomorrow. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 41 to 43 deg. F.
Max. temperature: 43 to 45 deg. F.
Average ridgetop wind direction: Southwest
Average ridgetop wind speed: 50 to 55 mph
Maximum ridgetop wind gust: 86 mph
New snowfall: Rain: 1.30 to 3.1 inches
Total snow depth: 54 to 92 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with heavy rain this morning, then a chance of rain and snow showers in the afternoon. Snow levels dropping from 12000 ft. this morning to 7000 ft. by the end of the day. Mostly cloudy becoming partly cloudy. Slight chance of snow showers in the evening. Snow levels falling below 7000 ft. Sunny
Temperatures: 41 to 46 deg. F. 23 to 28 deg. F. 47 to 53 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Wind Speed: 20 to 35 mph with gusts to 65 mph 15 to 25 mph with gusts to 60 mph 10 to 15 mph with gusts to 45 mph in the morning becoming light in the afternoon
Expected snowfall: Rain: .85 to 1.80 in. | Snow: up to 1 trace to 1 0
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with heavy rain this morning, then a chance of rain and snow showers in the afternoon. Snow levels dropping from 12000 ft. this morning to 7000 ft. by the end of the day. Mostly cloudy becoming partly cloudy. Slight chance of snow showers in the evening. Snow levels falling below 7000 ft. Sunny
Temperatures: 37 to 42 deg. F. 21 to 26 deg. F. 42 to 48 deg. F.
Ridge Top Winds: Southwest Southwest Southwest
Wind Speed: 40 to 55 mph with gusts to 115 mph decreasing to 30 to 50 mph with gusts to 90 mph in the afternoon 25 to 35 mph with gusts to 70 mph increasing to 30 to 45 mph with gusts to 85 mph after midnight 25 to 35 mph with gusts to 65 mph decreasing to 15 to 25 mph with gusts to 55 mph in the afternoon
Expected snowfall: Rain: .9 to 1.80 in. | Snow: 2 to 5 Trace to 1 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