This Avalanche Advisory was published on December 29, 2010:
December 29, 2010 at 8:04 am | |
Near and above treeline, CONSIDERABLE avalanche danger exists on N-NE-E aspects with pockets of CONSIDERABLE danger on NW and SE aspects on slopes steeper than 35 degrees. Below treeline, pockets of CONSIDERABLE danger exist on N-NE-E aspects on slopes steeper than 35 degrees. On other aspects, MODERATE avalanche danger exists on slopes 35 degrees and steeper. |
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Forecast Discussion:
Snowfall and winds increased last night ahead of two cold fronts approaching the area. 15-20 inches of snow has fallen across the forecast area since yesterday afternoon. The southwest winds have averaged between 45 and 50 mph with gusts over 100 mph for most of the night. As the first of these fronts moves through the region this morning, expect temperatures and snow to continue falling. The winds and snowfall should decrease during the day. The second, colder front should arrive over the forecast area this afternoon bringing even colder temperatures. It will leave behind a mass of cold air that should push lows into the low single digits tonight. The winds should also start to shift to the north tonight as this second front exits. Cold temperatures and north winds should continue through tomorrow.
Snowpit data, layer bonding tests, and general observations yesterday on Carson Pass and Lincoln Ridge (near Yuba Pass) showed a mostly stable snowpack in good shape to handle new loading. On Carson Pass a thin, wet, melt-freeze crust existed up to 8900 ft on the snow surface. Wet surface snow existed up to 7500' on Lincoln Ridge as well. On Mt. Judah in areas where more intense wind-loading continued to occur, 4-8 inch wind slabs that failed in response to ski cuts and small cornice drops had already formed on steep N-NE-E facing test slopes (photos, videos). Rain fell up to at least 7000 ft in this area yesterday. Expect to see evidence of natural avalanches that occurred last night in wind-loaded areas today.
Primary Avalanche Concern: Wind Slabs
Over the last 24 hours, new wind slabs have formed due to the strong winds and 15-20 inches of new snow. These wind slabs have formed faster than the bonds holding them to the snowpack. As more wind and snow impact the forecast area today, these wind slabs will continue to grow larger and more widespread. Natural avalanches due to these wind slabs overloading the weak bonds holding them to the snowpack have already occurred during the night. These kind of natural avalanches will remain possible today on steep slopes in the most heavily wind-loaded areas. On any slopes where new wind slabs exist that have not already avalanched, adding a person to the snowpack could easily overload those weak bonds and cause the wind slabs to fail. Human triggered avalanches will be likely on wind-loaded slopes today. Due to the strong and consistent winds expect these fragile wind slabs to extend into traditionally protected areas like trees, farther down-slope than usual, and in open areas below treeline. Large, deep, dangerous avalanches could result from failure of these wind slabs. The wind-loaded N-NE-E and cross-loaded NW and SE aspects will hold the largest and most fragile wind slabs.
Secondary Avalanche Concern: Loose Snow
Human triggered loose snow sluffs will be likely on any steep slopes today. The colder NW-N-NE-E aspect at higher elevations will hold the best potential for larger loose snow sluffs. Even though these slides entrain less snow than slab avalanches, they can still push someone into an area that could have serious consequences like over a cliff, into a gully or creek, or into trees or other obstacles (terrain traps).
The bottom line:
Near and above treeline, CONSIDERABLE avalanche danger exists on N-NE-E aspects with pockets of CONSIDERABLE danger on NW and SE aspects on slopes steeper than 35 degrees. Below treeline, pockets of CONSIDERABLE danger exist on N-NE-E aspects on slopes steeper than 35 degrees. On other aspects, MODERATE avalanche danger exists on slopes 35 degrees and steeper.
Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:
0600 temperature: | 21-24 deg. F. |
Max. temperature in the last 24 hours: | 27-29 deg. F. |
Average wind direction during the last 24 hours: | Southwest |
Average wind speed during the last 24 hours: | 45-50 mph |
Maximum wind gust in the last 24 hours: | 109 mph |
New snowfall in the last 24 hours: | 15-20 inches |
Total snow depth: | 77-108 inches |
Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS
For 7000-8000 ft: |
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Wednesday: | Wednesday Night: | Thursday: | |
Weather: | Snow this morning tapering off to snow showers in the afternoon | Mostly cloudy with a slight chance of snow showers | Mostly cloudy with a slight chance of snow showers |
Temperatures: | 20-27 deg. F. | 2-8 deg. F. | 8-15 deg. F. |
Wind direction: | Southwest | West shifting to the North | North |
Wind speed: | 20-30 mph with gusts to 45 mph | 15-20 mph decreasing to 10-15 mph with gusts to 35 mph after midnight | 10-15 mph with gusts to 25 mph in the afternoon |
Expected snowfall: | 5-9 in. | trace in. | trace in. |
For 8000-9000 ft: |
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Wednesday: | Wednesday Night: | Thursday: | |
Weather: | Snow this morning tapering off to snow showers in the afternoon | Mostly cloudy with a slight chance of snow showers | Mostly cloudy with a slight chance of snow showers |
Temperatures: | 16-23 deg. F. | 0-6 deg. F. | 8-14 deg. F. |
Wind direction: | Southwest | Northwest | North |
Wind speed: | 45-60 mph with gusts to 95 mph decreasing to 30-40 mph with gusts to 75 mph in the afternoon | 20-30 mph with gusts to 45 mph | 10-15 mph with gusts to 25 mph increasing to 35 mph in the afternoon |
Expected snowfall: | 5-10 in. | trace in. | trace in. |