Eastern Sierra - Fri Jan 15 2010
Eastern Sierra avalanche advisory
Posted January 15, 2010 by Sue Burak
The avalanche danger is MODERATE on slopes steeper then 35 degrees especially on northwest through east aspects with fresh deposits of wind transported snow. Human triggered avalanches are possible. Evaluate conditions on individual slopes before riding or skiing. Follow backcountry protocol by only putting one person in avalanche terrain at a time and clear the runnout zones.
Main avalanche concern: wind loading
Strong winds continued to blow yesterday from the north, blowing new snow around near ridgetops and redepositing in dense drifts on both north and southwest aspects. Significant drifting and wind transport was observed by several groups yesterday on the Mammoth Crest and Rock Creek areas. Any slope with drifting of wind blown snow should be avoided. In alpine and open treeline areas avoid north through east facing slopes near the top of ridge crests or terrain features. Ride slopes or cross avalanche paths one at a time, and regroup well away from avalanche slopes on high ground.
Observer reports and a handful of extended column tests gave mixed results. One group reported fracture propagation across the entire one meter column in 2 taps. In my tests, the fracture did not propagate across the entire column. In both cases the weak layer was the change in grain type and size between Tuesday’s snowfall and the Wednesday morning storm. Test results along with no reports of skier triggered slides suggest the storm snow has had enough time to settle but instabilities remain in specific terrain features such as deposits of wind drifted snow in steep terrain below ridgetops and in short steep convex rollovers in the trees.
The secondary avalanche concern is buried surface hoar and weak faceted snow. The large avalanche in the trees below June Mountain likely occurred on surface hoar above a rain layer. The problem with surface hoar is that it forms on all slopes but only survives on a few. This makes it very pockety- you find it in one place but not another. There is more danger from surface hoar at middle elevations than alpine terrain and is often observed in open areas in the steep trees below the Mid Chalet at June Mountain. It occurs anywhere where the snow is exposed to a clear sky
Enjoy the dry weather for the next two days because the weather is going to
change by Sunday. For today, a high pressure ridge over California will
keep us dry. Some valley inversions are possible, especially in the Mono
Lake area. North winds at the higher elevations will continue to move snow
around, with average winds speeds around 25 mph with gusts to 40 mph.
Expect high clouds today with temperatures in the upper 30’s to low 40’s
at the 8,000 to 9,000 ft elevations. Higher elevations will be warm with
highs in the mid 30’s to around 40. Temperatures will fall and southwest
winds will pick up in advance of the first system on Sunday/Monday.
The ridge breaks down Saturday with a chance of snow late Saturday
into Sunday. The real action begins on Monday when the first of a series of
4 storms impacts our area. The screaming 200 kt jetstream is so strong and
the longwave trough is so deep, that individual storms or waves will spin off
the jet. The strongest portions of the waves will favor Southern California
and the southern Sierra. The area of the trough covers the western United
States to the central Pacific.
The main event is forecasted for Wednesday into Thursday. High winds will create blizzard conditions.
Please note that the avalanche danger rating in this advisory expires in 24 hours. This advisory is our best interpretation of snow pack conditions and NWS forecasts issued today. Backcountry travelers should be aware that elevation and geographic distinctions are approximate and that a transition zone exists between upper and lower elevations. Avalanches do not happen by accident and most human involvement is a matter of choice not chance. Most avalanche accidents are caused by slab avalanches that are triggered by the victim of member of the victim's party. Even small slides can be dangerous. Always practice route finding skills and carry avalanche rescue gear. Remember that avalanche danger ratings are only general guidelines. Distinctions between geographic areas, elevations, slope aspects and slope angles should be made.
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