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.


This Avalanche Advisory was published on March 30, 2010:


March 30, 2010 at 6:57 am

Near and above treeline, avalanche danger will rise to CONSIDERABLE as the day progresses on NW-N-NE-E-SE aspects, 35 degrees and steeper. Below treeline, avalanche danger will rise to MODERATE with pockets of CONSIDERABLE danger in open wind affected areas on NW-N-NE-E-SE aspects, 35 degrees and steeper.


Forecast Discussion:


A powerful storm system is impacting the forecast area. Precipitation began yesterday afternoon with snow level near 7,800'. Snow level has fallen to near 6,000' this morning and is forecast to lower to around 5,000' during the day today. In the northern portion of the forecast area, over 1 inch of rain water equivalent has fallen as 5 to 7 inches of new snow in areas above 8,000'. In the southern portion of the forecast area, only around 0.3 inches of rain water equivalent has fallen as 2 to 3 inches of snow above 8,000'. An additional 11 to 23 inches of new snow is expected to accumulate across the forecast area over the next 24 hours. Ridgetop winds over the Sierra Crest remain very strong out of the southwest this morning with hourly average wind speed 90 to 100 mph and maximum gusts over 130 mph. Air temperatures are on a cooling trend with most remote sensors above 8,000' reporting air temperatures in the upper 20s this morning.

Observations:

Observations made yesterday on Powerhouse Peak (Luther Pass area) revealed that the existing snowpack is in good condition to handle new snow loading (pit profile). Snow surface conditions consisted of widespread melt freeze crust on northerly aspects below 8,200'. Above 8,200', a mix of melt freeze crust in sun exposed areas and up to 4 inches of unconsolidated snow unaffected by melt freeze existed up to the summit near 9,300'. Recent observations from around the forecast area have shown a layer of graupel crystals within the recent storm snow from this past week (more info). This layer has shown reactivity as recently as Sunday on Castle Peak (Donner Summit area) in near treeline terrain at 8,300' on a N aspect 36 degree slope (pit profile). In areas where this graupel layer remains active in layer bonding tests, the snowpack is not in as good of a condition to handle new snow loading.

Avalanche concern #1: Wind slabs

As new snow continues to accumulate today, wind slabs will become larger and more widespread. With very strong winds affecting the forecast area today, wind slabs will form in many areas both above and below treeline. Slabs will be most prevalent in open areas on NW-N-NE-E-SE aspects. Falling air temperatures will continue to deposit increasingly lower density new snow on top of higher density new snow. Weakness within the new snow will be focused on specific crystal types and overburden from wind loading.

Avalanche concern #2: Deep slab instability

New snow loading may add enough additional stress to the snowpack to cause failure on the recent graupel layer. This layer lingers on many, but not all NW-N-NE aspects and typically resides 6 to 24 inches below the old/new snow interface. This deep slab instability is not expected to be widespread, but may occur in isolated areas in response to human triggers.


The bottom line:

Near and above treeline, avalanche danger will rise to CONSIDERABLE as the day progresses on NW-N-NE-E-SE aspects, 35 degrees and steeper. Below treeline, avalanche danger will rise to MODERATE with pockets of CONSIDERABLE danger in open wind affected areas on NW-N-NE-E-SE aspects, 35 degrees and steeper.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature: 24 to 29 deg. F.
Max. temperature in the last 24 hours: 35 to 44 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 80 mph
Maximum wind gust in the last 24 hours: 133 mph
New snowfall in the last 24 hours: 2 to 7 inches
Total snow depth: 85 to 129 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Cloudy skies with snow showers in the morning. Periods of high intensity snowfall in the afternoon. Cloudy skies with snow. Cloudy skies with periods of snow.
Temperatures: 30 to 36 deg. F. 10 to 17 deg. F. 20 to 27 deg. F.
Wind direction: W to SW SW W
Wind speed: 25 to 35 mph with gusts to 75 mph. Gusts decreasing to 65 mph in the afternoon. 20 to 30 mph with gusts to 50 mph. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: 6 to 10 in. 5 to 10 in. 1 to 4 in.

For 8000-9000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Cloudy skies with snow showers in the morning. Periods of high intensity snowfall in the afternoon. Cloudy skies with snow. Cloudy skies with periods of snow.
Temperatures: 25 to 31 deg. F. 6 to 13 deg. F. 16 to 23 deg. F.
Wind direction: W to SW W shifting to SW SW shifting to W
Wind speed: 60 to 75 mph with gusts to 120 mph. Winds decreasing to 50 to 60 mph with gusts to 100 mph in the afternoon. 30 to 45 mph with gusts to 75 mph. Winds decreasing to 25 to 35 mph with gusts to 60 mph after midnight. 20 to 30 mph with gusts to 45 mph. Winds decreasing to 10 to 15 mph with gusts to 30 mph in the afternoon.
Expected snowfall: 8 to 12 in. 6 to 11 in. 2 to 5 in.

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