THIS AVALANCHE FORECAST EXPIRED ON April 7, 2018 @ 6:55 am
Avalanche Forecast published on April 6, 2018 @ 6:55 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

MODERATE avalanche danger today will rise to CONSIDERABLE avalanche danger this afternoon and during the night as heavy rain impacts the forecast area. Loose wet avalanche problems will go from possible to likely today and some of these could involve large amounts of snow. In addtion, deep slab avalanches that would be very large and destructive and would have severe consequences may become possible tonight and tomorrow.

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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Significant amounts of rain on snow during this storm will saturate the snowpack. The rain will melt the bonds holding the snowpack together and deep loose wet snow will exist on all aspects and at all elevations. Loose wet avalanches will become likely today and remain likely throughout the storm 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 injure a person.

Avalanche Problem 2: Deep Slab
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Large deep slab avalanche activity may become possible tonight or tomorrow due to the significant loading expected during this storm. The rain will add load to the snowpack, weaken and melt the bonds holding the snowpack together, and could help lubricate bed surfaces. This added liquid could possibly be enough to overload the deeply buried Feb. facet weak layer and allow some deep slab avalanche activity to become possible on NW-N-NE aspects in areas that did not avalanche during the previous deep slab avalanche cycle. Some deep wet slab avalanches may also become possible tonight or tomorrow on other aspects especially in areas where the snowpack rests on surfaces like granite slabs. Any slab avalanches that occur during this storm will involve 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 during this storm.

recent observations
Submit Observations

* Spring snow conditions with a few inches of wet corn snow resting on top of a supportable melt-freeze crust existed on S-SE aspects of Red Lake Peak by 11 am yesterday. At the lower elevations (6400 to 7400 ft) of Johnson Canyon (Donner Summit area) punchy, unsupportable boot top deep wet snow existed on NW-N-NE-E-SE-S aspects by noon yesterday. 

* Observations targetting the Feb. facet layer on Silver Peak, Jakes Peak, and on Elephants Hump this week have shown that this layer remains 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 maybe 2.5 to 4 inches of rain?) to get enough force through the strong snow above it to break this layer. This layer was responsible for the widespread deep slab avalanche cycles in March.

Weather and CURRENT CONDITIONS
weather summary

Rain has moved into the area and should continue through tomorrow. Sensors show snow levels hovering around 10000 ft. this morning and the forecast calls for them to stay above 10000 ft and even rise to 11500 ft tonight. The more intense rain should fall between midnight tonight and late morning on Saturday. By tomorrow afternoon snow levels should begin to fall and by the end of the day they could drop below 7500 ft. This drop in snow levels also coincides with a decrease in precipitation and the end of the storm so only 2 to 6 inches of new snow is expected above 8000 ft. and only up to an inch below 8000 ft. By the end of the storm, 2.5 to 4 inches of water could have accumulated along the Sierra Crest. Strong southwest winds will accompany this warm atmospheric river type storm.  

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 35 to 39 deg. F.
Max. temperature: 43 to 49 deg. F.
Average ridgetop wind direction: Southwest
Average ridgetop wind speed: 35 to 45 mph
Maximum ridgetop wind gust: 78 mph
New snowfall: Rain: .1 to .3 inches
Total snow depth: 68 to 94 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Cloudy with rain. Snow level 10500 ft. Cloudy with rain. Snow level 11500 ft. Cloudy with rain in the morning, then a chance of a mix of rain and snow showers in the afternoon. Snow level 9500 ft. decreasing to 7500 ft. in the afternoon.
Temperatures: 44 to 50 deg. F. 36 to 41 deg. F. 42 to 47 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Wind Speed: 15 to 20 mph with gusts to 50 mph 15 to 20 mph increasing to 20 to 30 mph with gusts to 60 mph after midnight 20 to 35 mph with gusts to 70 mph decreasing to 60 mph in the afternoon
Expected snowfall: Rain: .5 to .75 Rain: up to 1.15 Rain: .75 to 1.25 in. | Snow: up to 1
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy with rain. Snow level 10500 ft. Cloudy with rain. Snow level 11500 ft. Cloudy with rain and snow in the morning, then a chance of snow showers in the afternoon. Snow level 9500 ft. decreasing to 7500 ft. in the afternoon.
Temperatures: 41 to 46 deg. F. 33 to 38 deg. F. 38 to 43 deg. F.
Ridge Top Winds: Southwest Southwest Southwest
Wind Speed: 30 to 45 mph with gusts to 85 mph 30 to 50 mph with gusts to 105 mph 35 to 55 mph with gusts to 110 mph decreasing to 100 mph in the afternoon
Expected snowfall: Rain: .6 to .85 Rain: .75 to 1.25 Rain: .75 to 1.25 in. | Snow: 2 to 6
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