Eastern Sierra Snowpack Summary - 2016-03-12 06:56

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THIS ADVISORY EXPIRED ON March 14, 2016 @ 7:56 am
Avalanche Advisory published on March 12, 2016 @ 6:56 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.

Fresh new windslabs have formed as a result of new snowfall late yesterday afternoon and evening accompanied by very high winds out of the southwest.  2” - 10” of new snow has fallen since yesterday afternoon, once again with the greatest amounts being recorded around the Mammoth area, and significantly less both north and south.  High winds at mid to upper elevations, blowing near 80mph over ridgetops, have formed fresh sensitive windslabs that are most likely to be found just below ridgetops, on the sidewalls of gullies, and across cross-loaded slopes that face NW-N-NE-E-SE.  These windslabs will continue to form and be sensitive throughout the day today as moderate winds out of the southwest continue to transport snow.  Natural avalanches could be possible, and human triggered avalanches likely on exposed slopes greater than 35 degrees, especially at mid to upper elevations.  As snowfall begins once again and winds increase during the day on Sunday, this avalanche concern will become even greater.  The size and consequence of these avalanches will be greater for areas in the region that received greater amounts of new snow, but realize that even small avalanches in steep terrain where obstacles or terrain traps are present can results in injury or death. 

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.

Storm slabs that could be sensitive to human triggering may be found on all aspects in wind-protected areas, especially where snowfall amounts were greater than 6”.  Prior to this last snowfall, many mid and low elevation slopes, along with upper elevation slopes that have had sun-exposure (non-northerly aspects) have developed smooth melt-freeze crusts.  On more northerly facing slopes at mid to upper elevations, smooth surfaces exist in many areas in the form of windcrusts.  These various smooth snow surfaces that existed prior to this recent snow fall will make for good sliding surfaces for this new snow to fail and avalanche on.  The concern over storm-slab instability will begin to diminish as they are given time to settle towards the end of the day today and into tomorrow morning, but will again become heightened as new snowfall begins during the day on Sunday, 

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.

In exposed, high elevation terrain (>11,500') that faces N to NE some areas of very faceted shallow snowpacks have been found to exist prior to last weekend's storm.  It is likely that some of these weak areas avalanched naturally during this past storm, and it is also likely that some still exist that are now covered with windslab.  This underlying weak faceted layer wont stabilize quickly.  While a stiff thick overlying windslab may bridge over this weak layer effectively at many points, it is very possible that a human could trigger a failure where the slab is thinner.  It is possilbe that such a failure could propagate above the trigger and result in a dangerous avalanche that a human would have a hard time getting out of.  Be aware and on the lookout for this potential problem if you find yourself in this kind of high elevation terrain.  

Snowpack Discussion

Heightened avalanche concern exists today in the form of fresh new windslabs that have formed as a result of new snowfall late yesterday afternoon and evening accompanied by very high winds out of the southwest.  2” - 10” of new snow has fallen since yesterday afternoon, once again with the greatest amounts being recorded around the Mammoth area, and significantly less both north and south.  High winds at mid to upper elevations, blowing near 80mph over ridgetops, have formed fresh sensitive windslabs that are most likely to be found just below ridgetops, on the sidewalls of gullies, and across cross-loaded slopes that face NW-N-NE-E-SE.  These windslabs will continue to form and be sensitive throughout the day today as moderate winds out of the southwest continue to transport snow.  Natural avalanches could be possible, and human triggered avalanches likely on exposed slopes greater than 35 degrees, especially at mid to upper elevations.  As snowfall begins once again and winds increase during the day on Sunday, this avalanche concern will become even greater.  The size and consequence of these avalanches will be greater for areas in the region that received greater amounts of new snow, but realize that even small avalanches in steep terrain where obstacles or terrain traps are present can results in injury or death. 

There is also some avalanche concern over storm slabs that could be sensitive to human triggering on all aspects in wind-protected areas, especially where snowfall amounts were greater than 6”.  Prior to this last snowfall, many mid and low elevation slopes, along with upper elevation slopes that have had sun-exposure (non-northerly aspects) have developed smooth melt-freeze crusts.  On more northerly facing slopes at mid to upper elevations, smooth surfaces exist in many areas in the form of windcrusts.  These various smooth snow surfaces that existed prior to this recent snow fall will make for good sliding surfaces for this new snow to fail and avalanche on.  The concern over storm-slab instability will begin to diminish as they are given time to settle towards the end of the day today and into tomorrow morning, but will again become heightened as new snowfall begins during the day on Sunday, 

recent observations

Overnight snowfall from remote weather stations:

VA Lakes Ridge(9443’): 4” new snow / 0.2”H2O

GEM Pass(10750’): 4.5” new snow / 0.3” H2O

June Mtn(9148’): 4” new snow / 0.4” H2O

Mammoth Mtn(9014’): 10” new snow / 1.2” H2O

Mammoth Pass(9500’): 10” new snow / 1.2” H2O

Rock Creek(9800'):  2" new snow

Big Pine Creek(10000'): 2" new snow

Observations made in the Mammoth area, June area and Rock creek over the past several days are showing a mostly stable underlying snowpack with very variable surface conditions including many firm surfaces that are either melt-freeze crusts or wind crusts that could make for good sliding layers for new snowfall to fail on. 

weather

We are in the middle of another series of winter storms that are bringing high winds and periods of heavy snowfall to our region.  The bulk of the storms will be centered north of our area toward Tahoe and north, however we will still see impacts.  Today (Saturday) we are in a break between last night’s storm and the one that is expected to move in during the day on Sunday.  Expect mostly cloudy skies, temperatures in the low to upper 30s, and moderate winds out of the southwest at 25-40mph, with 60mph gusts over ridgetops.  Tomorrow (Sunday), precipitation will begin as the moisture plume dips south during the day.  Higher snow amounts are expected towards the north parts of our region, with potentially 1-1.5ft along the crest.  Strong high winds out of the southwest will persist through much of the day in the 40-60mph range, with gusts in the 90s over ridgetops.  Temperatures should be several degrees cooler than Saturday.

Snow showers should taper off by early Monday morning, strong winds are expected to continue through the day, and temperatures should be a few degrees warmer.

For Tuesday through the rest of next week, a high-pressure ridge near the coast will keep conditions dry, winds light, and temperatures near or slightly above average.             

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: MOSTLY CLOUDY CLOUDY. SLIGHT CHANCE OF SNOW CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING. SNOW SHOWERS IN THE AFTERNOON
Temperatures: HIGHS IN THE MID 30S TO MID 40S deg. F. LOWS IN THE MID 20S TO LOW 30S deg. F. HIGHS IN THE MID 30S TO LOW 40S deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 45 MPH 30 TO 35 MPH WITH GUSTS TO 50 MPH INCREASING TO 40 TO 45 MPH WITH GUSTS TO 65 MPH AFTER MIDNIGHT 35 TO 45 MPH WITH GUSTS TO 65 MPH
Expected snowfall: 0 in. UP TO 1 in. 2 - 6 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: MOSTLY CLOUDY CLOUDY. SLIGHT CHANCE OF SNOW IN THE EVENING. CHANCE OF SNOW AFTER MIDNIGHT CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING. SNOW SHOWERS IN THE AFTERNOON
Temperatures: HIGHS IN THE LOW TO MID 30S deg. F. LOWS IN THE MID TEENS TO LOW 20S deg. F. HIGHS IN THE UPPER 20S TO UPPER 30S deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 35 TO 45 MPH WITH GUSTS TO 65 MPH 50 TO 60 MPH. GUSTS UP TO 80 MPH INCREASING TO 90 MPH AFTER MIDNIGHT 50 TO 60 MPH WITH GUSTS TO 90 MPH
Expected snowfall: 0 in. UP TO 1 in. 2 - 6 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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