THIS AVALANCHE ADVISORY EXPIRED ON April 20, 2017 @ 7:00 am
Avalanche Advisory published on April 19, 2017 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

As sun and warm temperatures cause rapid warming today, the avalanche danger will increase to CONSIDERABLE once last night's refreeze melts. Large human-triggered loose wet avalanches are likely today if people venture onto steep slopes where wet snow exists. Natural loose wet avalanches are possible. Some wet slab avalanches may also be possible today. The warming will weaken any lingering wind slabs keeping human-triggered wind slab avalanches possible today as well. Avoid slopes where wet snow or wind slabs exist.

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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Strong April sunshine and warm temperatures will lead to rapid warming of the snow today. This rapid warming will quickly melt through last night's refreeze and weaken the snow. Human-triggered loose wet avalanche activity will become likely on any slopes that receive sunshine today. Some natural loose wet activity will be possible as well. These loose wet avalanches could entrain all of the new snow and could involve enough snow to pose problems for backcountry travelers. The largest loose wet instabilities will exist on the slopes where the most new snow exists and where the most warming occurs.

Avoid slopes where signs of rapid warming like sticky wet surface snow, roller balls, pinwheels, or other signs of wet snow exist. Once the sun hits the slopes today, the snow will quickly become unstable and less fun to recreate on. Seek out shaded sheltered slopes where the snow remains cold and non-wind-affected for better and safer recreation.

Avalanche Problem 2: Wind Slab
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Fragile wind slabs and cornices may still linger on or above wind-loaded slopes. Today's rapid warming could also weaken the bonds holding these wind slabs and cornices in place and make them more fragile and easier to trigger as the day warms up. Wind slabs could exist on wind-loaded N-NE-E aspects as well as on cross-loaded NW and SE aspects. Any wind slab avalanches that occur today could involve enough snow to bury or injure a person. The largest and most fragile wind slabs will exist in near and above treeline terrain in areas where the most new snow has accumulated on slopes that receive sunshine.

Clues like cornices above slopes, drifted snow, ripples in the snow surface, and other wind created textures can help identify where fragile wind slabs may exist. Use this information to avoid the wind slabs and to find sheltered non-wind-affected snow to travel on.

Avalanche Problem 3: Wet Slab
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Loose wet snow instabilities should comprise most of the wet snow issues today, but some wet slab avalanches may become possible as the day warms up. Wet slab avalanche problems could develop in sun-exposed areas on any slopes where more than a few inches of new snow exists. Any wet slab avalanches that occur today could involve enough snow to bury or injure a person.

recent observations

Yesterday observations from Hidden Peak found 8 to 10 inches of new snow at the upper elevations and 1 to 2 inches of new snow below 7300 ft. This new snow rested on top of deep wet snow. Ski cuts on test slopes at all elevations triggered large pinwheels and ski-width-wide loose wet avalanches that entrained whatever new snow existed. Near the summit of Hidden Peak where wind-loading had formed wind slabs, small cornice pieces dropped on the slopes also triggered similar loose wet instabilities. 

Weather and CURRENT CONDITIONS
weather summary

A short-lived high-pressure ridge will bring warmer temperatures and mostly sunny skies to the forecast area this morning. A weak spring storm should begin to push some cloud cover and increased southwest winds into the area this afternoon and evening. The forecast calls for some light snow showers with up to 2 inches of new snow accumulation during the night. The showers should taper off tomorrow and skies should begin to clear again. Expect moderate to strong southwest winds during the storm tonight.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 18 to 26 deg. F.
Max. temperature in the last 24 hours: 33 to 40 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 15 to 20 mph
Maximum wind gust in the last 24 hours: 43 mph
New snowfall in the last 24 hours: up to 1 inches
Total snow depth: 131 to 193 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny with a few clouds developing this afternoon and evening Cloudy with isolated snow showers in the evening and more widespread snow showers after midnight Mostly cloudy with a chance isolated snow showers in the morning becoming partly cloudy to mostly sunny in the afternoon
Temperatures: 38 to 48 deg. F. 26 to 31 deg. F. 33 to 43 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: Light in the morning increasing to 15 to 20 mph with gusts to 40 mph in the afternoon 15 to 25 mph with gusts to 50 mph 10 to 15 mph with gusts to 40 mph decreasing to gusts to 30 mph in the afternoon
Expected snowfall: 0 in. up to 2 in. up to 1 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny with a few clouds developing this afternoon and evening Cloudy with isolated snow showers in the evening and more widespread snow showers after midnight Mostly cloudy with a chance isolated snow showers in the morning becoming partly cloudy to mostly sunny in the afternoon
Temperatures: 34 to 44 deg. F. 19 to 27 deg. F. 33 to 41 deg. F.
Wind Direction: Southwest Southwest Southwest shifting to the west in the afternoon
Wind Speed: 20 to 30 mph with gusts to 55 mph increasing to 70 mph in the afternoon 30 to 45 mph with gusts to 85 mph 20 to 35 mph with gusts to 65 mph decreasing to 15 to 25 mph with gusts to 50 mph in the afternoon
Expected snowfall: 0 in. up to 2 in. up to 1 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