THIS AVALANCHE ADVISORY EXPIRED ON January 20, 2018 @ 6:59 am
Avalanche Advisory published on January 19, 2018 @ 6:59 am
Issued by Brandon Schwartz - Tahoe National Forest

CONSIDERABLE avalanche danger exists at all elevations in areas that held snow cover prior to last night's snowfall. A combination of wind slab, storm slab, and persistent slab avalanche problems exist. Very large avalanches could occur below treeline. Complex and dangerous avalanche conditions exist. Powder fever could easily induce poor decision making today.

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: Wind Slab
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New snow last night combined with SW winds is expected to have created new wind slabs in near treeline and above treeline terrain on NW-N-NE-E-SE aspects. Avalanche size to D2 is possible. Look for and avoid areas of recently drifted snow. Uses clues such as cornice features, wind pillows, snow surface sculpting, and snow surface cracking to identify likely areas of unstable wind slab.

Avalanche Problem 2: Storm Slab
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New storm slabs may exist in wind protected areas near treeline or below treeline on any aspect that held previous snow cover. Avalanche size to D2 is possible. Snowpack failure could occur at the old/new snow interface or within the new snow. Use clues such as snow surface cracking while breaking trail to identify areas of potential storm slab instability.

Avalanche Problem 3: Persistent Slab
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New snow load will add significant stress to the loose grain (sugary) mid snowpack facet layer that is the weak layer of concern for the persistent slab avalanche problem. This weak layer exists in isolated areas near treeline and below treeline on NW-N-NE aspects. Avalanche size to D3 is possible. Snowpit tests performed yesterday in the Carson Pass and Mount Rose areas indicated that this weak layer was in poor condition to handle new snow loading in isolated areas.

This avalanche problem is often found in the lower to middle portions of a slope and not at the top. Human triggering may occur while either approaching a slope from the bottom or part way down on the descent. This will be quite different from triggering a wind slab avalanche near the top of a slope. The snowpack may fail 1 to 2+ feet deep into the old snow, creating an avalanche that could be very difficult to survive.

Avoid avalanche terrain in areas where snowpack collapse with a possible whumpfing sound is occurring. Snowpits dug near the top of the avalanche start zone may not give indications of problematic instability mid slope. Significant spatial variability exists. Adjust thinking and travel planning to anticipate the possibility of high consequence avalanches in less common areas near and below treeline.

recent observations

* Snowpit test results targeting mid pack faceted snow associated with the persistent slab avalanche problem vary between unstable and stable around the forecast area. There is no clear explaination for these variations in stability between similar terrain at different locations.

* Old snow surfaces beneath the new storm snow range from crust to near suface facets, largly dependent on elevation and aspect.

* NW-N-NE aspects above 8,000' in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best coverage at 2 to 4+ feet. Overall less snow cover exists south of Emerald Bay. Areas of decent coverage exist above 8,300' on NW-N-NE aspects in the Carson Pass area.  Coverage decreases dramatically on all other aspects.

Weather and CURRENT CONDITIONS
weather summary

The current storm system is passing through the region and will exit later today. New snow accumulation above 8,000' ranges from 5 to 12 inches. The greatest accumulations have occurred along the Sierra Crest in the northern portion of the forecast area. Snow showers will continue into this evening with snowfall rates of 1 to 2 inches per hour, generally lasting an hour or less. Short lived high pressure will build over the forecast area tonight before the next weather system arrives on Sunday. Ridgetop winds are decreasing this morning with moderate to strong SW winds becoming moderate W winds this afternoon. Light to moderate speed N winds are expected tonight.

Note - Ridgetop wind data from along the Sierra Crest for the past 24 hours was not available this morning. Winds speed numbers listed below were taken from the summit of Slide Mt., NV.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 22 to 24 deg. F.
Max. temperature in the last 24 hours: 37 to 41 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: See note above - 45 mph
Maximum wind gust in the last 24 hours: See note above - 82 mph
New snowfall in the last 24 hours: 5 to 12 inches
Total snow depth: 24 to 48 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 skies with widespread snow showers. Short duration periods of high intensity snowfall likely. Mostly cloudy skies with scattered snow showers. Partly cloudy skies.
Temperatures: 26 to 31 deg. F. 15 to 20 deg. F. 25 to 30 deg. F.
Wind Direction: SW NW N
Wind Speed: 15 to 20 mph. Gusts to 50 mph decreasing to 30 mph in the afternoon. 10 to 15 mph in the evening, becoming light. Gusts up to 25 mph. Light winds
Expected snowfall: Likely up to 2 in. | Small chance 2 to 5 in. Likely up to 2 in. | Small chance 2 to 5 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy skies with widespread snow showers. Short duration periods of high intensity snowfall likely. Mostly cloudy skies with scattered snow showers. Partly cloudy skies.
Temperatures: 22 to 27 deg. F. 12 to 17 deg. F. 20 to 25 deg. F.
Wind Direction: SW shifting to W N N
Wind Speed: 20 to 35 mph with gusts to 65 mph, shifting and decreasing to 15 to 25 mph with gusts to 45 mph in the afternoon. 15 to 20 mph with gusts to 35 mph. 10 to 15 mph with gusts to 25 mph.
Expected snowfall: Likely 1 to 3 in. | Small chance 3 to 6 in. Likely up to 2 in. | Small chance 2 to 5 in. 0 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