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 January 5, 2007:


January 5, 2007 at 1:00 am

Forecast Discussion:


This advisory was posted on Friday, January 5, 2007 at 7:02 am

Early this morning the winds shifted to the north and east after the storm ended. These easterly winds will continue at 30-40 mph along the ridgetops today. Storm snow totals above 8000' range from 13-17 inches providing ample snow for transport by these east winds. The high pressure following this storm will settle in for the next few days and temperatures will start to warm up at the upper elevations.

The new snow can can be divided up into two parts: the initial dense, warm and wet 4-6 inches that fell at the start of the storm on Wednesday night and the colder, less dense powder that fell during the day yesterday and overnight last night. That initial dense 4-6" bonded quite well to the old snow surface as the temperatures cooled off yesterday and provided a great base layer to support the rest of the storm snow. This layer also added to the series of strong layers with strong bonds between them that make up the middle of the current snowpack. Even though there are still some weak layers below this series of well consolidated layers, they are not prone to failure right now due to the difficulty in breaking though that series of strong layers. This series will serve as a "bridge" to protect us from those lower weak layers until enough weight is added to the snowpack to overwhelm the strength of these layers. In the colder, less dense powder, subtle differences during the storm changed the density and crystal shapes in the storm snow and resulted in weaknesses forming in the new snow. This colder, less dense snow was (and still is) easier for the wind to transport and thus allowed more widespread and thicker windslabs to form.

Even though we never got quite enough snow to see widespread natural activity yesterday, several human triggered avalanches occurred in the windslabs formed with the colder, less dense snow on NE windloaded slopes. As the windslabs grew over the course of yesterday the avalanches also increased in size. Today the winds will be stripping the new snow from these NE aspects and moving it to the NW aspects. As this snow transport takes place the avalanche danger will also shift. Again, today will be tricky because the conditions will be changing throughout the day. Early in the day there will still be some reactive windslabs on the N-NE-E aspects since the wind will not have had time to move much snow from them. By midmorning and into the afternoon the winds will have had ample time to move snow to the NW-W-SW aspects and those will become the more suspect slopes. Most of the avalanches that occur today will likely be recently formed windslabs failing on weaknesses within the new snow. They could be large as we now have enough snow cover to create more uninterrupted starting zones.

Keep your eyes open today and watch out for any signs of windloading: drifting, cornices, ripples, blowing snow, etc. These windloaded slopes will be the most dangerous. Find some small, steep test slopes that would not harm you if they did slide and jump on them to see how they react. These miniature pieces of terrain can be great tools to indicate what could happen on similar larger terrain. Watch out for other obvious signs of instability like recent avalanche activity, cracks shooting out from your skis, and whumphing noises. On days like this when the snow is rapidly changing, you have to always be watching for indicators like these and be ready to move off of steeper terrain into safer areas like ridgelines and heavily treed slopes.

Above treeline the avalanche danger is CONSIDERABLE on any wind loaded aspects, 35 degrees and steeper. This morning the N-NE aspects will be most suspect and by midmorning and into the afternoon the NW-W-SW aspects will become the most likely slopes for avalanche activity. Below treeline the avalanche danger is MODERATE with isolated pockets of CONSIDERABLE on steep, open, wind loaded slopes.

Andy Anderson , Avalanche Forecaster

Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet):
4 deg. F
Max. temperature at Sierra Crest past 24 hours:
26 deg. F
Average wind direction at Sierra Crest past 24 hours:
East northeast
Average wind speed at Sierra Crest past 24 hours:
30-40 mph
Maximum wind gust at Sierra Crest past 24 hours:
112 mph
New snow fall at 8,200 feet past 24 hours:
8-10 during the day yesterday and another 2 inches overnight.
Total snow depth at 8,200 feet:
43 inches
Mountain Weather Forecast For Today:
Partly cloudy this morning and clearing as the day progresses.
Temperature forecast for 8,000 to 9,000 feet:
7-17 degrees F.
Ridgetop winds forecast for the Sierra Crest:
East 35 - 45 mph, G 80 mph
Snowfall expected in the next 24 hours:
0 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet
For today, partly cloudy this morning and then becoming clear in the afternoon. Highs 10 to 20 degrees F. East winds 20 to 30 mph. Overnight clear with lows 10-20 degrees F, and temperatures increasing through the night. East winds at 15 to 25 mph. Saturday will be partly cloudy and a little warmer. Daytime highs 25-35 degrees F. Northwest winds at 15-25 mph.

Above 8000 Feet
For today, partly cloudy this morning and then becoming clear in the afternoon. Highs 7 to 17 degrees F. East winds 35 to 45 mph with gusts to 80 mph. Overnight clear with lows 7 to 17 degrees F, and temperatures increasing through the night. East winds at 25 to 35 mph with gusts to 50 mph. Saturday will be partly cloudy and a little warmer. Daytime highs 23-33 degrees F. Northwest winds at 25-35 mph.


The bottom line:


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: deg. F.
Max. temperature in the last 24 hours: deg. F.
Average wind direction during the last 24 hours:
Average wind speed during the last 24 hours: mph
Maximum wind gust in the last 24 hours: mph
New snowfall in the last 24 hours: O inches
Total snow depth: inches

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

For 7000-8000 ft:

  Friday: Friday Night: Saturday:
Weather:
Temperatures: deg. F. deg. F. deg. F.
Wind direction:
Wind speed:
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Friday: Friday Night: Saturday:
Weather:
Temperatures: deg. F. deg. F. deg. F.
Wind direction:
Wind speed:
Expected snowfall: O in. O in. O in.