Eastern Sierra Snowpack Summary - 2015-12-22 07:11

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THIS ADVISORY EXPIRED ON December 24, 2015 @ 7:11 am
Avalanche Advisory published on December 22, 2015 @ 7:11 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

See Snowpack Summary for more details

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

Snowfall amounts varied greatly across the forecast area.  Mammoth Mountain received the most with 16" of new snow, while June Mtn received significantly less.  Greater danger will exist in the areas that received more new snow.  The snow surface prior to the start of this storm was loose and faceted, creating a potential weak layer below this new denser sierra cement snow.  Stability tests yesterday in a more sheltered area in the lakes basin showed easy failure of the 3-4” of new snow that had fallen.  Slightly more dense snow over less dense snow is a dangerous combination, and areas are likely to exist where human triggered avalanches are possible, especially at mid to upper elevations.  Steeper roll-overs and convexities are areas where triggering such an avalanche is more likely.  Perform your own stability tests in safe representative locations to see how well this new snow has bonded, and be aware of the variability that can exist across slopes.  Again, when in doubt, play it safe. 

Avalanche Character 3: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

Additional avalanche concern exists lower down in the snowpack, in the form of a persistent weak layer that formed under the old snow surface prior to the Dec 11-14 storms.  This layer has still been found to exist in recent pits in the Mammoth and Virginia lakes areas.  Stability tests are still showing failures in this layer, albeit not as clean as they were last week.  With this additional new snow and wind, the possibility exists that an avalanche in this new snow, or a skier or rider hitting just the right sweet spot, could create enough force to cause a failure in this lower persistent layer and result in a much larger avalanche.  This danger may still lurk in shaded and sheltered slopes in mid to upper elevations where early season snow accumulated and sat before the December 11-14 storms.  It is good to recall the Dragon's Back avalanche that occured on mountain a week and a half ago, where failure occured lower down the slope where this thin weak snow sat, as opposed to higher up the slope where winds had blown this snow away (check recent observation under the homepage for more details on this).  This weak underlying snowpack is of greater concern in areas that have received less snow, such as VA lakes, which was recently orbserved to had particularly large and well-formed DH compared to Mammoth-June areas.  

Snowpack Discussion

12-22 7pm update:  Mammoth Mtn Ski Patrol reported significant avalanches as a result of their control work this morning.  At least one avalanche failed just above the ground in a weak basal facet layer with a 3ft crown.  This slope had been skied hard prior to this storm.  Multiple stability test between 8900-9400' on the west edge of the Sherwins near Mammoth Rock failed easily and cleanly just above the ground in this basal facet layer.  This is evidence that the dense new snowload has re-awakened this deep persistent weak layer to potential human triggering.  Extremely strong winds gusted throughout the day today, over 100mph over the summit of Mammoth Mtn, and gusts up 160mph were recorded at the top of chair 1 near 9900'.  June Mtn remained closed all day due to the high winds.  Natural avalanches are likely, and human triggered avalanches are very likely especially at mid to high elevation slopes on wind loaded E-NE-N facing slopes.  Cautious route finding, careful snowpack evaluation, and conservative decision making is essential.     

We are in the midst of another Sierra storm that began yesterday morning and will begin to taper off late this morning.  5-16” of new Sierra cement fell so far with another 2-7” expected by early afternoon.  Mammoth Mtn SKi Patrol reported 16" of new snow this morning with almost 3" of water content (heavy wet snow!) near main lodge.  This new snow combined with the continuous very strong southwest to west winds has increased avalanche danger significantly.  Fresh wind slabs will be the greatest concern, while storm slabs on more protected slopes >32 degrees will also be of concern.  Look for these denser, hollow sounding wind-slabs on SE-E-NE-N facing slopes at all elevations, but especially mid to upper elevations.  At upper elevations, these slabs may be found lower down on slopes than they would normally due to the very high winds.  Be aware of slight terrain variations, convexities and rolls, both up and down slopes and across slopes, where one spot can be stable and just 10 feet away a sensitive and dangerous wind-slab can exist just waiting for a trigger.

While snowfall amounts have varied greatly over the forecast area.  (June Mtn reported no new snow yesterday, snowfall was continuous throughout the day in Mammoth with 3-4” of new snow, VA lakes reported 2”), the high winds have been consistent throughout.  Most lifts were closed yesterday on Mammoth Mtn, and June Mtn had to evacuate guests due to the high winds.  Winds in excesses of 100mph over ridgetops have been reported (up to 130 over Mammoth Mtn), and low elevation winds have been in the 30-50mph range.  These winds are forecasted to continue out of the west through the day today (although just about 10mph less intensely).  This means that sensitive wind-slabs will continue to form throughout the day today.  These slabs will strengthen with time, especially with the calm winds expected for Wednesday, but just how long they will take to strengthen is unknown.  It’s important to keep your eyes open and senses keen, and do your own quick stability tests to figure this out for yourself.  With storm and wind slabs the best information can be gained from observing the upper layers of the snowpack during backcountry travel and by checking very small test slopes in safe places with little to no consequence if snow starts to slide. If you observe cracking or whumpfing, or actual avalanches, it suggests that nearby slopes are likely unstable. When in doubt, play it safe.    

recent observations

Strong winds were observed throughout the forecast area over the last 2 days, transporting snow and creating new wind-slabs even before the new snowfall began yesterday.  Yesterday in the Mammoth Lakes Basin in low-mid elevation terrain, freshly deposited thin (3-4”) wind-slabs in exposed areas just below the Lake Mary Ridge top on NE facing slopes failed easily in hand shear tests.  Skiers near duck pass in the Mammoth Lakes Basin on Sunday reported strong west winds and visible snow transport, with wind-slabs developing on N, NE, E aspects.  In Hammil Bowl on the same day, great variability was observed across mid-elevation slopes just above tree line, where isolated wind-slabs failed easily with little force and some upon isolation.  ECT tests showed easy failure and propagation.  Whumphing was also observed.  A test-pit was dug on the same slope in a non-wind deposited area, and showed a very stable structure, with no significant failures.  This just goes to show the point of the great variability that exists across slopes.  Strong winds and fresh wind-slabs were also reported by skiers in the negatives behind June Mtn on Sunday. 

weather

Another major winter storm is working its way through the region with heavy Sierra snow, moderate to heavy valley rain, and strong winds.  Heavy snowfall amounts will remain at elevations 7000 feet and above, where storm total snow amounts may reach 6 to 18 inches west of highway 395, especially around Mammoth Lakes/June Lake and Deadman Summit. Strong Westerly to Southwest winds will continue to load Northwest thru Southeast aspects, especially near and above treeline. Wind gusts up to 55 mph will continue this evening, with ridge winds in the Sierra gusting over 100 mph tonight. Wind prone areas, especially along highway 395, which may see wind gusts up to 80 mph.

 

Long-term: main story will be a cold winter storm expected to impact the Sierra and western Nevada Christmas eve thru Christmas day. This system isn`t expected to be as wet as the current system but will be much colder. Temperatures on Christmas day will be very cold and may drop below zero overnight in colder Sierra valleys. Winds are forecast to increase along the ridges as the system moves through the region with an associated 140 kt jet. The heaviest period of snow in the Sierra will be on Thursday. Total QPF amounts are expected to be around 1 inch of water along the Sierra. Skies are forecast to clear with winds shifting to the east and remain quite cold. This will result in strong surface inversions and possible fog, depending on the extent of snow accumulation. East winds may become very strong along the Sierra Crest on Saturday with a possible downslope wind event on the west slope of the Sierra.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Snow. Chance of precipitation is 90%. Total daytime snow accumulation of 3 to 7 inches possible. Snow likely before 10pm, then a chance of snow showers, mainly between 10pm and 4am. Mostly cloudy. Chance of precipitation is 70%. Partly sunny.
Temperatures: High near 38 deg. F. deg. F. Low around 23 deg. F. deg. F. High near 36 deg. F. deg. F.
Wind direction: West West Southwest
Wind speed: 45 to 55 mph, with gusts as high as 80 mph 40 to 45 mph, with gusts as high as 70 mph 5-10 mph
Expected snowfall: 3-7 in. <0.5 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Snow. Chance of precipitation is 90%. Total daytime snow accumulation of 3 to 7 inches possible. Snow likely before 10pm, then a chance of snow showers, mainly between 10pm and 4am. Partly sunny.
Temperatures: high near 35 deg. F. deg. F. low around 18 deg. F. deg. F. high near 31 deg. F. deg. F.
Wind direction: West West West
Wind speed: 50 to 55 mph, with gusts as high as 100 mph over ridges. 35-45 mph, gusts to 70 mph. 15 to 20 mph, with gusts as high as 30 mph
Expected snowfall: 3-7 in. <1 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the Eastern Sierra Avalanche Center who is solely responsible for its content.

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