Eastern Sierra Snowpack Summary - 2016-03-05 07:07

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THIS ADVISORY EXPIRED ON March 7, 2016 @ 7:07 am
Avalanche Advisory published on March 5, 2016 @ 7:07 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.

For today (Saturday) heightened avalanche concern exists in the form of windslabs on steep exposed slopes >35deg above 9000’ that face W-NW-N-NE-E.  These dense windslabs will likely be found just below ridgelines and mid slope where cross-loading has occurred.  Areas closer to Mammoth where greater amounts of new snow fell overnight will be of greater concern than areas further south and north which received only 1-3”.  Increasing SW winds throughout the day will continue to transport this new snow and continue to develop fresh windslabs that will be sensitive to human triggering in isolated areas.  Small natural avalanches will be possible and small human triggered avalanches likely. 

For Sunday, much greater avalanche concern will exist throughout the region as the result of high intensity snow fall beginning Saturday night into Sunday morning that will be accompanied by strong winds out of the SW, gusting upwards of 90mph over ridgetops.  1-2’+ of new snow is expected.  Dangerous windslabs will form just below ridges and across exposed slopes facing W-NW-N-NE-E, especially at mid to upper elevations.  Natural wind slab avalanches will be likely and human triggered avalanches very likely that could result in burial and 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.

Some concern exists today on all aspects on steep slopes >35deg that are not exposed to the wind above 9000’ that received greater than 3-4” of new snow.  While the warm temperatures likely resulted in good bonding of this new snow to the old snow surface, small human triggered avalanches may be possible especially on steep convex roll-overs. 

For Sunday much greater avalanche concern will exist throughout the region as the result of high intensity snow fall beginning Saturday night into Sunday morning.  While the new snow will likely bond well to the old snow surface due to the initially warm temperatures, density changes within the new storm snow will likely exist that will increase avalanche concern for storm slabs in wind protected areas for slopes >32deg facing all aspects.  Steep convexities are areas of obvious concern, as are gully bottoms that act as terrain traps where a relatively small slide could bury someone deeply.  Natural storm slab avalanches will be possible, and human triggered avalanches likely. 

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. The significant new snowload expected Saturday night into Sunday morning could very well overburden these weak areas and result in larger avalanches to the ground.  Natural and human triggered avalanches are likely in these isolated areas.   

Snowpack Discussion

Our February thaw has come to an end, and the first signs of the start to a miracle March are upon us.  Rain and wet snow fell overnight, ranging from 1-6” of snow above 9000’, with highest amounts around Mammoth.  1-2” of snow fell in Old Mammoth, and 6” of snow with a water content of almost an inch fell at Main lodge.  This was accompanied with moderate SW winds.  Light precipitation and warmer temperatures are expected through the day today before the main event rolls in tonight dropping 1-2’+ of new snow by Sunday morning.    Snow levels are expected to start high around 8500’, but quickly drop in the early morning hours of Sunday down to 5000’.  A High Wind Advisory is in place starting this afternoon into tonight, and a Winter Storm Warning is in place beginning tonight into early Monday morning.   

Avalanche concerns have now shifted away from wet slides to wind slabs and storm slabs.  For today (Saturday) heightened avalanche concern exists in the form of windslabs on steep exposed slopes >35deg above 9000’ that face W-NW-N-NE-E.  These dense windslabs will likely be found just below ridgelines and mid slope where cross-loading has occurred.  Areas closer to Mammoth where greater amounts of new snow fell overnight will be of greater concern than areas further south and north which received only 1-3”.  Increasing SW winds throughout the day will continue to transport this new snow and continue to develop fresh windslabs that will be sensitive to human triggering in isolated areas.  Small natural avalanches will be possible and small human triggered avalanches likely.  Some concern exists as well today in the form of storm slabs on all aspects on steep slopes >35deg that are not exposed to the wind above 9000’ that received greater than 3-4” of new snow.  While the warm temperatures likely resulted in good bonding of this new snow to the old snow surface, small human triggered avalanches may be possible especially on steep convex roll-overs. 

For Sunday, much greater avalanche concern will exist throughout the region as the result of high intensity snow fall beginning Saturday night into Sunday morning that will be accompanied by strong winds out of the SW, gusting upwards of 90mph over ridgetops.  1-2’+ of new snow is expected.  Dangerous windslabs will form just below ridges and across exposed slopes facing W-NW-N-NE-E, especially at mid to upper elevations.  Natural wind slab avalanches will be likely and human triggered avalanches very likely that could result in burial and death.  While the new snow will likely bond well to the old snow surface due to the initially warm temperatures, density changes within the new storm snow will likely exist that will increase avalanche concern for storm slabs in wind protected areas for slopes >32deg facing all aspects.  Steep convexities are areas of obvious concern, as are gully bottoms that act as terrain traps where a relatively small slide could bury someone deeply.  Natural storm slab avalanches will be possible, and human triggered avalanches likely. 

In addition, In exposed, high elevation terrain (>11,500') that faces N to NE some areas of very faceted shallow snowpacks have been found to exist. The significant new snowload expected Saturday night into Sunday morning could very well overburden these weak areas and result in larger avalanches to the ground.  Natural and human triggered avalanches will be likely in these isolated areas.   

recent observations

Observations yesterday on Punta Bardini revealed moist surface snow on all aspects including north up to atleast 9200' due to the warm air and clouds, and a very stable underlying snowpack.  

Observations last week in several high elevation NE aspects (>11,800') where shallow snowpacks existed revelaed very faceted snowpacks that were unsupportable.  The significant snowload expected tonight into Sunday morning could overlaod these weak areas resulting in larger avalanches.       

weather

An Atmospheric River or Pineapple Express has set up that will bring heavy moisture from the tropics and multiple storm systems into the West Coast beginning tonight through Monday.  Rounds of strong SW wind and heavy snowfall are expected, that could result in 2-3’ of new snow by Monday morning.  A Wind Advisory will begin this afternoon and Winter Storm Warning will begin tonight through Monday Morning.

Today (Saturday), expect warm temperatures in the 40s at mid-mountain elevations, periods of light precipitation in the form of wet snow and rain below 9000’, and increasingly strong winds out of the SW, gusting up to 90mph over ridgetops.

Tonight, the most intense precipitation is expected, as the atmospheric river combined with a cold front will dump 1-2’+ of new snow by Sunday morning.  Snow level is expected to start high above 8500’, but drop quickly early Sunday morning down to 5000’.  Winds will remain high out of the SW, nearing 100mph over ridges. 

Snowfall will continue Sunday, but with much less intensity.  2-5” of snow is expected through the day.  Temperatures will be normal in the upper 20s and low 30s range, and strong SW winds will continue.

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: CLOUDY. CHANCE OF SNOW THROUGH THE DAY. CHANCE OF RAIN IN THE AFTERNOON CLOUDY. SNOW AND RAIN CLOUDY. SNOW
Temperatures: HIGHS IN THE MID 40S TO LOW 50S deg. F. LOWS IN THE 20S deg. F. HIGHS IN THE 30S deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 40 TO 45 MPH WITH GUSTS TO 60 MPH INCREASING TO 55 TO 60 MPH WITH GUSTS TO 75 MPH IN THE AFTERNOON 50 TO 60 MPH WITH GUSTS TO 80 MPH 35 TO 45 MPH WITH GUSTS TO 60 MPH
Expected snowfall: 1-3 in. 10-20 in. 2-5 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. SNOW IN THE MORNING...THEN CHANCE OF SNOW IN THE AFTERNOON CLOUDY. SNOW CLOUDY. SNOW
Temperatures: HIGHS IN THE UPER 30S TO MID 40S deg. F. LOWS IN THE TEENS deg. F. HIGHS IN TE MID 20S TO LOW 30S deg. F.
Wind direction: SOUTHWEST SOUTHWEST WEST
Wind speed: 45 TO 50 MPH WITH GUSTS TO 70 MPH INCREASING TO 65 TO 70 MPH WITH GUSTS TO 90 MPH IN THE AFTERNOON 60 TO 70 MPH WITH GUSTS TO 95 MPH 45 TO 55 MPH WITH GUSTS TO 85 MPH
Expected snowfall: 1-4 in. 12-22 in. 3-5 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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