Eastern Sierra Avalanche Advisory

Avalanche Advisory published on December 19, 2014 @ 7:00am: Issued by Sue Burak - Inyo National Forest
This advisory is valid for 24 hours.
weather

A couple of weak storms bring the chance of snow showers over the Sierra Crest today and Saturday before high pressure builds over California. A warming and drying trend will bring mild temperatures to the mountains with highs reaching the low 40's by Sunday.

Long term, a powerful storm will spill over the ridge centered off the coast of California next Wednesday, bring snow to Idaho, Utah and Colorado. Our area could see anywhere from heavy to light snow Christmas eve and Christmas Day. While snowfall amounts are uncertain, cold temperatures are certain for Christmas Day.

recent observations

Blowing snow transported by southeast winds created pockets of sensitive wind slabs yesterday on northwest facing slopes above treeline below the Mammoth Crest yesterday. In less than an hour, gusty southeast winds loaded low angle northwest facing slopes with enough snow to create sensitive wind slabs and propagating cracks. An hour earlier, extended column tests done on north, northwest and northeast aspects in the same area did not propagate (ECTN).  It doesn't take strong storm winds to create wind slabs and it doesn't take very long for unstable conditions to develop.

Two days ago, a skier remotely triggered a small slab avalanche in a wind loaded pocket on a northwest facing slope above treeline in the Red Cone Bowl area. This morning, there is another report of a skier triggering a small wind slab in a isolated pocket on a larger slope came from the San Joaquin Ridge area.

Depth hoar at the base of the snowpack is not reacting in extended column tests at this point. The layer bears careful monitoring If an unwelcome prolonged period of cold and dry weather occurs after Christmas.

 

Avalanche Problem 1
type aspect/elevation characteristics
likelihood size trend
likely
unlikely
large
small
description

Unstable pockets of wind slab have been observed on northwest slopes in alpine terrain. Wind transport from southeast winds has created pockets of unstable wind loaded snow within areas of seemingly stable snow. Very little evidence of instability is expected to exist but don't let the lakc of instabiity clues lead to lazy and inattentive backcountry travel. 

A limited number of extended column tests suggest snowpack stability is generally good but reports of small isolated areas of unstable wind slabs are a heads up reminder that small avalanches are possible. A shallow snowpack does not eliminate the risk of avalanches but it does increase the chance of impact iwth rocks.

advisory discussion

It's been a slow start to the winter. Last year at this time in December, the Mammoth Pass snow pillow was recording 22" of snow depth and 2 inches of water. Today, the snow depth reading is 25" of snow with over 3.5 inches of water. Even though total snow depth is similar to last December, the difference is there is more water content and a supportable . The first December storm was mild with high snowlines followed by a colder storm that ended with a period of light winds. Cold temperatures after the storms have allowed snow to remain on steep rock faces, giving the appearance there is more snow than there really is. Rocks, stumps and logs are covered with a thin layer of snow and trusting skiers and riders are hitting obstacles. Snow depths range from about 18 to 24" in the Mammoth Lakes Basin with around 18" above treeline.

A shallow snowpack increases the likelihood of impact with rocks and shallowly buried obstacles. A fall or hard turn could have big consequences without ever triggering an avalanche.

The early season snowpack has a depth hoar layer found in higher elevation terrain and on Mammoth Mountain. The layer is not reacting in tests done on Tuesday and Thursday.  The ever present wind above treeline poses the current wind slab avalanche problem. It's tricky because wind slabs are forming in isolated areas under small convex features or under rocks on some, but not all slopes. Pay attention to areas where snow texture changes or rounded pillows have formed.


ESAC Weather Page

Weather Station Links:
June Mountain Summit
Mammoth Summit
Rock Creek
CURRENT CONDITIONS  Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 19 deg. F.
Max. temperature in the last 24 hours: 28 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 10 mph
Maximum wind gust in the last 24 hours: 35 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 24 inches
Two-Day Mountain Weather Forecast  Produced in partnership with the Reno NWS
For 8,000 to 10,000 ft.
  Friday Friday Night Saturday
Weather: partly cloudy partly cloudy mostly cloudy
Temperatures: 27-36 deg. F. 25-31 deg. F. 27-35 deg. F.
Wind direction: SW SW SW
Wind speed: 15-25 15-25 10-15
Expected snowfall: t in. 0 in. 0 in.
For above 10,000 ft.
  Friday Friday Night Saturday
Weather: Mostly cloudy mostly cloudy clouds
Temperatures: 22-30 deg. F. 15 deg. F. 25-30 deg. F.
Wind direction: W SW SW
Wind speed: 20-30 20-30 20-30
Expected snowfall: 1-3 in. 0 in. 0 in.
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