THIS AVALANCHE ADVISORY EXPIRED ON April 16, 2017 @ 7:00 am
Avalanche Advisory published on April 15, 2017 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

The avalanche danger will increase to MODERATE today as sun and warmer temperatures cause more rapid warming. Human-triggered loose wet avalanches will be possible. Some natural loose wet avalanches may also be possible in a few areas. Human-triggered wind slab avalanches may remain possible in complex or extreme terrain or on wind-loaded slopes where rapid warming weakens wind slabs.

Avoid slopes where signs of warming or wet snow exist and slopes where wind slabs remain especially if they receive sunshine.   

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Loose Wet
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Warmer daytime highs and strong April sun will quickly melt through last night's refreeze leaving wet unconsolidated snow sitting on top of rain crusts. Once this wet snow forms, loose wet avalanches will be possible on any steep slopes where wet snow exists. The E-SE-S-SW-W aspects that receive the most direct sunshine and low elevation northerly aspects hold the best potential for the largest wet snow instabilities, but wet snow could form on all aspects and at most elevations today. These loose wet avalanches could entrain all of the new snow and could have serious consequences for people recreating in the backcountry. While loose wet avalanches should comprise most of the wet snow instabilities, some isolated wet slab avalanches are not impossible on some sun-exposed slopes. 

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 and human-triggered wet snow instabilities will become possible if people venture onto slopes where wet snow exists. Seek out shaded, sheltered slopes where the snow remains cold and non-wind-affected or start and finish early before the snow warms up for better and safer recreation. 

Avalanche Problem 2: Wind Slab
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In many areas, the wind slabs that formed during the storm should have become more difficult to trigger; however, human-triggered wind slab avalanches may still remain possible in some places. The most fragile lingering wind slabs will exist in couloirs, in gullies, on unsupported slopes, or in other complex or extreme terrain. Today's sunshine and warming temperatures could also weaken the wind slabs and cornices on sun-exposed slopes 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.

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.

recent observations

Yesterday several wind slab avalanches were reported including a skier triggered one East of Frog Lake near Carson Pass and cornice triggered ones in Polaris Bowl (near Blue Lakes), on Echo Peak, and on Tamarack Peak. These all had crowns about 1 ft deep and entrained enough snow to have serious consequences for backcountry travelers. No one was caught in these avalanches. The one in Polaris Bowl and the one near Frog Lake both occurred between 9:30 and 10:00 am. The others likely occurred around that time as well but the exact time they occurred remains uncertain. It is possible but uncertain that sun and rapid warming could have caused the cornices to weaken and fail yesterday morning. In addition to these wind slabs, ski cuts released a small loose wet avalanche yesterday afternoon on sun-exposed SE-S aspects on the Fireplug (in the Mt. Rose backcountry), and a larger natural loose wet avalanche also occurred on a low elevation ENE aspect at the start of Shirley Canyon.

 

Weather and CURRENT CONDITIONS
weather summary

The high pressure over the region will bring warmer temperatures and more sunshine to the forecast area today. Expect daytime highs to climb 8 to 10 degrees above yesterday with temperatures in the mid to upper 40's above 7000 ft. today. Some thin high cloud cover should start to move into the region ahead of a small spring storm expected to arrive tomorrow. The cloud cover and winds should increase this afternoon and thicken into the evening and overnight. By tomorrow the forecast calls for mostly cloudy skies and stronger southwest winds along with a chance of precipitation starting in the afternoon. Snow levels should start between 7000 and 8000 ft. and fall to near lake level by Monday morning. For more details on this storm check in with the Reno NWS.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 23 to 33 deg. F.
Max. temperature in the last 24 hours: 30 to 40 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 10 mph
Maximum wind gust in the last 24 hours: 25 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 130 to 200 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Mostly sunny with some thin high clouds for most of the day becoming partly cloudy later this afternoon and evening Mostly cloudy Mostly cloudy with increasing clouds and a chance of rain and snow in the afternoon
Temperatures: 45 to 51 deg. F. 29 to 34 deg. F. 44 to 50 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 10 to 15 mph 15 to 25 mph 15 to 20 mph with gusts to 45 mph increasing to 20 to 30 mph with gusts to 75 mph in the afternoon
Expected snowfall: 0 in. 0 in. up to 1 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Mostly sunny with some thin high clouds for most of the day becoming partly cloudy later this afternoon and increasing clouds in the evening Mostly cloudy Mostly cloudy with increasing clouds and a chance of snow in the afternoon
Temperatures: 41 to 47 deg. F. 27 to 32 deg. F. 40 to 46 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 15 to 25 mph with gusts to 30 mph increasing to 45 mph in the afternoon 20 to 30 mph with gusts to 45 mph increasing to 60 mph after midnight 25 to 35 mph with gusts to 70 mph increasing to 30 to 50 mph with gusts to 100 mph in the afternoon
Expected snowfall: 0 in. 0 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