Eastern Sierra Snowpack Summary - 2015-12-26 06:38

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THIS ADVISORY EXPIRED ON December 28, 2015 @ 6:38 am
Avalanche Advisory published on December 26, 2015 @ 6:38 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.

The winds have shifted direction and increased in intensity again last night.  They are out of the NE now, blowing moderately at mid elevations and gale force over ridgetops.  Snow is being re-distributed in different places than it has been in the two previous storms, when the winds were consistently out of the W-SW.  Look out for new sensitive windslabs mostly on W-SW-S facing terrain, but also pay attention to older windslabs on E-NE-N facing terrain that may still have isolated areas where they are sensitive enough to be human triggered.  

Avalanche Character 2: 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.

A weak faceted basal snowpack exists over much of the forecast area.  This is an atypical snowpack for the Sierra.  It is much more tlike the more dangerous continental snowpack of the rockies at the moment.  It is possible that a skier or rider could add enough force to the snowpack if they hit just the right "sweet spot" to trigger this layer to cause it to fail.  It this happens, the resulting avalanche will likely be large and very dangerous.  Thinner areas of the snowpack is where these sesitive trigger spots may exists, near rock outcrops, and the edges of slopes.     

Snowpack Discussion

Yesterday’s clear and calm weather gave the snowpack a break from new loading from snowfall and wind.  The light low-density snow from Dec 23rd did not add too much new stress to the snowpack either.  However the moderate W-SW winds associated with that event did likely create some new windslabs on the lee-side (E-NE-N) of ridges and terrain features at upper elevations where human triggering may still be possible.  The dreaded northeast winds are back and have been blowing strong most of the night with consistent gusts in the 120mph range over Mammoth Mtn.  These winds will continue throughout the day, stripping away the beautiful soft snow in more exposed terrain, and forming new windslabs on different aspects than previously (W-SW-S).  So be on the look-out for a variety of windslabs on all aspects of exposed mid-high elevation terrain, and do your own stability tests in safe locations to see how sensitive they are. 

The dense Sierra cement that fell with high winds during the two days prior increased the avalanche danger considerably.  It created dangerous dense windslabs, as can be seen by the 4-6ft crown that ran ~300 across the top of Mammoth Rock Bowl that failed naturally during the event.  The very dense snow also put the added stress to the lower weak faceted snowpack that made it susceptible to failure, as can be seen by the explosive work on Mammoth Mountain that resulted in large avalanches running to the ground at mid to upper elevations.   It also created storm slabs that were susceptible to human triggering in more protected steep mid-high elevation terrain.  The avalanche danger has now decreased for these potential problems.  However, it still may be possible to find isolated areas of this older windslab that may still be sensitive to human triggering (particularly in thin areas, such as sides of gullies).

The persistent weakness deep in the snowpack still exists in many areas.  This layer actually seems more sensitive at lower to mid elevations in our stability tests.  At higher elevations, the new snow has created a thick tough bridge above this weak layer in most areas, however there may be isolated areas where the snowpack is thin enough where a hard turn may be able to trigger it.  Observers in VA Lakes report a shallower snowpack there than in Mammoth, and that this weak layer is even more prominent and sensitive there.  While this layer is unlikely to trigger, the resulting avalanche would be large and dangerous, and probably rip out above the skier or rider.  Don’t be the unlucky one!  This problem is much less obvious and harder to “see” then windslabs.  Evaluate your slopes carefully.

Also of note, the snowfall amounts during these events varied greatly throughout the forecast area.  Mammoth area got the heaviest hit with records showing 4-5” of new water, while June area received much less than half that.  South of Mammoth, snow coverage is dramatically thinner and there are few reports of backcountry travel.  If you know of anyone getting out south of Mammoth please let us know! 

recent observations

Recent observations in the Mammoth Lakes area (Lakes Basin, Punta Bardini, Sherwins) and in Virginia Lakes are all showing the existence of a weak basal snowpack.  Stability tests range from very easy failure on this layer, to no failure.  Stability tests also have shown very variable failures in the new snow, without propensity to propogate.  

weather

Clear and sunny skies are expected for today and Sunday, with highs in the upper teens to mid 20s today and a bit warmer tomorrow with highs in the upper 20s to mid 30s.  Strong Northeast winds will blow this morning in the 60-70mph range with gusts over ridgetops over 100mph.  These will gradually die down through the day and be light out of the SW by tomorrow.  A weak low pressure system is expected to move into the area Sunday night, bringing light snow fall and colder temperatures through Monday night.  Accumulation of 1-2" is possible.      

Long Term

Mostly clear conditions with blustery winds are expected through the end of the week, except for the possiblity of another very weak low pressure system on Wednesday that could bring some snow flurries to the area.  By the end of the week a high pressure ridge will be set in over the area, keeping conditions dry likely through New Years weekend.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Patly cloudy, then becoming sunny Clear Sunny then becoming partle cloudy
Temperatures: High around 25 deg. F. Lows in the teens deg. F. Highs in the low to mid 30s deg. F.
Wind direction: NE NE changing to SW
Wind speed: 55 TO 60 MPH WITH GUSTS TO 85 MPH DECREASING TO 45 TO 50 MPH WITH GUSTS TO 70 MPH IN THE AFTERNOON 25 TO 30 MPH WITH GUSTS TO 40 MPH DECREASING TO 10 TO 15 MPH AFTER MIDNIGHT LIGHT WINDS 10 TO 15 MPH WITH GUSTS TO 25 MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Partly cloudy then becoming sunny Clear Sunny
Temperatures: Highs in the upper teens to low 20s deg. F. Lows in the low to mid teens deg. F. Highs in the upper 20s to low 30s deg. F.
Wind direction: NE NE changing to SW
Wind speed: 70 TO 75 MPH WITH GUSTS TO 105 MPH DECREASING TO 55 TO 60 MPH WITH GUSTS TO 80 MPH IN THE AFTERNOON 0 TO 45 MPH WITH GUSTS TO 65 MPH DECREASING TO 15 TO 20 MPH AFTER MIDNIGHT 15 TO 20 MPH WITH GUSTS TO 30 MPH
Expected snowfall: 0 in. 0 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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