Eastern Sierra Snowpack Summary - 2016-01-05 06:42

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THIS ADVISORY EXPIRED ON January 7, 2016 @ 6:42 am
Avalanche Advisory published on January 5, 2016 @ 6:42 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.

Continuous new snowfall throughout the day today combined with strong SW winds will create dangerous windslabs especially on mid to high elevation leeward slopes facing N-NE-E.  Areas just below ridgelines, and cross loaded slopes are of greatest concern.  Slight variation in aspect and exposure on slopes can mean the change from soft unconsolidated snow to dangerous denser windslab in a matter of feet.  Just because the entrance to a pitch seems protected and un-wind affected doesn’t mean that it will be safe all the way down.  Be on the lookout for these subtle changes.

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.

More than a foot of snow is expected to fall today.  Avalanche danger at all aspects and elevations on slopes 35 degrees and steeper will increase as the day progresses.  An underlying faceted snow surface exists in many low to mid elevation areas, which could make the bond of this new snow to the old snow sensitive to failure.  In addition, widespread surface hoar developed last week in protected areas at low to mid elevations.  This is an especially weak layer if it gets buried before being destroyed.  While it is likely that most of this surface hoar did get destroyed before the 3” of new years night snow fell, atleast one observer reported finding it preserved in sheltered areas below tree line near TJ Bowl in the Lakes Basin.  It is likely there are isolated areas where it may have been preserved in the June Lakes area and VA lakes as well.  If so, this could be very sensitive to human triggering during today’s additional snow loading.  

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.

Basal facets still persist at the bottom of the snowpack through most of the forecast area.  Stability tests have been highly variable, some showing column failure and propagation upon isolation, and others not failing at all.  This is more of a concern where the snow pack is shallower, such as June and VA Lakes, however recent tests in the Sherwins in shallower areas have also been producing failures at this level as well.  While unlikely to find the sweet spot, with the added stress of new snowfall, it is more possible that a human could trigger a failure at this level, and the resulting avalanche could be quite large and catastrophic.  Areas where the snowpack is shallower, such as around rock outcrops, sides of gullies, and parts of slopes where wind has stripped some snow from, are areas where these sweet spots may exist.

 

Snowpack Discussion

We are in the midst of another winter storm that will continue throughout the day today and will likely drop over a foot of new snow at mid to upper elevations.  Avalanche danger will increase throughout the day on all aspects and elevations as more snow accumulates.  High winds from the SW are expected in the 30-50mph range, gusting up to 80mph over ridges, and will deposit snow rapidly and create dangerous wind slabs on leeward slopes facing E-NE-N, especially at mid to upper elevations.  Even steep protected slopes (>35degrees) at lower elevations could become dangerous, as the recent cold temperatures has faceted and weakened the underlying snow surface, which will likely result in poor bonding of the new snow to the old.  This near surface faceting is also a concern at and above tree line, where it’s presence will likely result in poor bonding between the old snow and the new softer storm slabs and denser windslabs that form today. 

Additionally, there is a concern over the widespread surface hoar that formed before new years in protected areas at low to mid elevations.  This large (4-8mm) feathery structure was found widespread behind June Mtn, in the Lakes Basin and the Sherwins last week.  While most of this fragile surface hoar likely was destroyed before the 3” of new snow that fell new years night, there are isolated protected areas where it was preserved.  One observer found it buried in areas below tree line near TJ Bowl in the lakes basin.    Other areas it could be found where it could be hazardous include small open slopes in forested terrain, protected gullies and in steep creek beds (especially if there is any open water).  Even small slopes can be dangerous if they end in a terrain trap where even a small amount of snow can end up pilling deeply.

recent observations

Mammoth Lakes Basin:  Yesterday, very small isolated pockets of recently deposited windslab just below the ridgeline on the northerly facing slopes of Red Cone Bowl were triggered by ski cuts.  Faceted surface snow was found at lower to mid elevations.  No buried surface hoar was found, but that does not mean it doesn’t exist.  On January 2, observers did find surface hoar under the ~3” of new snow below tree line near TJ Bowl. 

Sherwins:  Yesterday, observers witnessed recent windslab and cornice development in the vicinity of Rock Bowl.  Stability tests at mid-elevation were very variable, some failing easily on the basal facets, and others not. 

June Area:  Widespread surface hoar was found behind June Mtn from low elevations to tree-line (9700'!) last week before New Years.  It grows in areas protected from the wind.  While not a problem then, this 4-8mm feathery structure, if preserved and buried by new snow could act as a very sensitive weak layer that an overlying snow slab could fail on.  These areas of preserved surface hoar will likely be rare, but possible.     

weather

The first of several winter storms will affect our area today, with snowfall throughout the day and high winds from the south to southwest blowing 40-60mph, with potential ridge top gusts in the 90s.  Temperatures are expected in the teens to twenties.  Total snowfall today is expected to be between 8-14".  There will be a break tonight with light snowfall and calmer winds, until the second storm moves in early Wednesday morning with increased snowfall intensity and high SW winds picking up again thru Thursday morning.  Total snowfall tomorrow is expected to be between 4-8".

Long Term:  Unsettled weather through the end of the week/weekend, with lingering snow showers, but little substantial accumulation expected.  Looks to be another dry period Sunday into Monday, with cold temperature inversions in the valleys.

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 Snow before 10pm, then a chance of snow showers between 10pm and 4am, then snow likely after 4am Snow
Temperatures: 23-30 deg. F. 12-18 deg. F. 18-25 deg. F.
Wind direction: SW SW S
Wind speed: 35-40mph, gusts to 65mph 15mph, gusts to 25mph 30-40mph, gusts to 55mph
Expected snowfall: 8-13 in. 1-3 in. 4-8 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Snow Snow before 10pm, then a chance of snow showers between 10pm and 4am, then snow likely after 4am Snow
Temperatures: 18-25 deg. F. 5-12 deg. F. 13-20 deg. F.
Wind direction: S - SW SW SW
Wind speed: 55-65mph, gusts to 90mph 35-40mph, gusts to 60mph 35-40mph, gusts to 60mph
Expected snowfall: 8-14 in. 1-4 in. 4-8 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 ESAC who is solely responsible for its content.

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