THIS AVALANCHE ADVISORY EXPIRED ON February 7, 2015 @ 6:59 am
Avalanche Advisory published on February 6, 2015 @ 6:59 am
Issued by Brandon Schwartz - Tahoe National Forest

For much of this morning, avalanche danger remains LOW for all elevations and aspects. Following the onset of precipitation, avalanche danger will rise to MODERATE danger on all aspects on slopes 35 degrees and steeper during the afternoon and evening hours. Avalanche danger will further increase to CONSIDERABLE danger in these same areas during the overnight hours. Human triggered avalanches will become likely and natural avalanches possible as high intensity precipitation occurs tonight and tomorrow morning.

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: Wind Slab
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For areas above 8,000', the combination of new snow and gale force winds will create new wind slabs in wind loaded areas both above and below treeline. Slab development will be focused on NW-N-NE-E-SE aspects, but possible in isolated areas other aspects as well due to swirling winds. Areas of wind slab instability are expected to form late this afternoon and become larger and more widespread during the evening and overnight hours. 

Avalanche Problem 2: Loose Wet
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In areas where rain falls on snow, loose wet avalanches will become possible. In most areas below 8,000', free water drainage from the snowpack is well established and wet loose instability is expected only in areas where temporarily lower snow levels deposit a few inches of new snow that then receives rain as snow levels rise back towards 8,000'. This same type of instability scenario is expected in areas above 8,000' where snow levels temporarily rise and deposit rain on top of either new snow or old low density near surface snow layers. Natural or human triggered wet loose avalanches will become possible this afternoon through tomorrow on all aspects, but generally require steeper slopes with slope angles of 37 degrees or greater.

Avalanche Problem 3: Wet Slab
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Wet slab avalanches will become possible as the storm progresses mainly above 8,000' in areas where rising snow levels deposit rain on top of new snow that sits on top of an old snow bed surface. Whether a loose wet or wet slab avalanche occurs in this case will depend on the presence or absence of weak layers within the new snow. If no significant weak layers exist within the new snow, loose wet is more likely. If a significant weak layer forms within the new snow or bonding to the old snow surface is poor, then wet slab avalanches will be possible.

recent observations

Observations made yesterday on Red Lake Peak (Carson Pass area) and on Mt. Judah (Donner Summit area) fell inline with other recent observations from around the forecast area indicating that the existing snowpack is generally in good condition to handle new loading from rain and snow. Free water drainage from the snowpack is well established in the vast majority of areas below 8,000'. Near surface layers consist of a mix of well established melt-freeze on SE-S-SW aspects and a well bonded mix of crusts, rounds and rounding facets on W-NW-N-NE-E aspects. On some of these more northerly aspects rain and melt-freeze crusts are at the snow surface and in other areas they are buried several inches down. Some NW-N-NE aspect locations hold a notable layer of lower density rounding facets below the thickest rain crust in the upper third of the snowpack. This layer has been a focus for monitoring during the month of January and it has not shown any signs that it will be problematic during the upcoming storm cycle.

A poor to decent snow surface refreeze is expected to have occurred last night due to cloud cover and above freezing air temperatures.

Weather and CURRENT CONDITIONS
weather summary

Gale force winds are buffeting the forecast area as an atmospheric river type Pacific storm system moves into the region. Southwest winds are gusting 110 to 115 mph over ridgetops. Air temperatures remained above freezing again last night with mid 30s to low 40s reported this morning between 8,000' and 9,600'. Air temperatures on the mountain valley floors are in the mid to upper 40s. Maximum daytime air temperatures are forecast to reach the low 40s to low 50s today for areas above 7,000'. Precipitation is expected to spread through the forecast area during the mid afternoon and evening hours today. Rain and rain water equivalent amounts of 1.5 to 3 inches are expected this evening through tomorrow morning. Precipitation is expected to focus along the Sierra Crest with lesser amounts along the Carson Range on the east side of Lake Tahoe. Snow level is expected to hold around 8,000' to 8,500' with periodic fluctuations between 6,500' and 10,000' possible. New snow amounts of 6 to 20 inches are possible above 8,000' over the next 24 hours. Snowfall and rain are expected to taper to showers tomorrow afternoon with continued strong southwest winds. Maximum daytime air temperatures are forecast to decrease 5 to 10 degrees tomorrow vs. today. A second storm is expected to impact the region Sunday/Monday.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 37 to 42 deg. F.
Max. temperature in the last 24 hours: 42 to 47 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 55 mph
Maximum wind gust in the last 24 hours: 114 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 20 to 31 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy skies with a slight chance of rain and snow in the morning. A chance of rain and snow in the afternoon. Cloudy skies with rain and snow. Cloudy skies with rain and snow.
Temperatures: 45 to 51 deg. F. 33 to 40 deg. F. 40 to 46 deg. F.
Wind Direction: SW SW SW
Wind Speed: 35 to 40 mph with gusts to 70 mph. 35 to 40 mph with gusts to 70 mph. 30 to 35 mph with gusts to 55 mph, decreasing to 15 to 20 mph with gusts to 30 mph in the afternoon.
Expected snowfall: 1 to 3 in. Likely 2 to 6 in. with possible 6 to 10 in. 2 to 4 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy skies with a slight chance of snow in the morning. A chance of snow and rain in the afternoon. Cloudy skies with rain and snow. Cloudy skies with rain and snow.
Temperatures: 41 to 47 deg. F. 29 to 36 deg. F. 31 to 38 deg. F.
Wind Direction: SW SW SW
Wind Speed: 70 to 80 mph with gust to 125 mph. 70 to 80 mph with gusts to 125 mph. Gusts decreasing to 110 mph after midnight. 40 to 50 mph with gusts to 75 mph.
Expected snowfall: 1 to 3 in. Likely 6 to 12 in. with possible 12 to 18 in. 3 to 6 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.