Eastern Sierra Snowpack Summary - 2016-02-20 06:42

Our goals are lofty, but our ask is simple... We need your help... Click Here to Find Out How

THIS ADVISORY EXPIRED ON February 22, 2016 @ 6:42 am
Avalanche Advisory published on February 20, 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.

There is two parts to this avalanche concern.  The first part deals with potentially lingering sensitivity of the large wide-spread windslabs that formed during the intense SW winds Wednesday night and Thursday.  While initially these windslabs were of great concern, they have now had time to settle and bond, and are much less likely to be sensitive enough to be triggered by a human.  The second part of this avalanche concern deals with the more recent smaller more isolated windslabs that formed yesterday (Friday) and that will continue to form today and Sunday as result of lighter more moderate SW winds.  These, depending on how recently they have formed, could be very sensitive to human triggering.  These sensitive windlabs are most likely to be found on E-NE-N facing slopes, below ridgelines and cross-loaded faces and gullies.  Be on the lookout for dense smooth potentially hollow sounding snow, and shooting cracks.  Do your own stability assessments to figure out of how sensitive these wind deposits are.  Quick hand pits can tell you a lot by isolating a small column of snow and seeing how cleanly and with how much force it takes to push the column to failure.  Do these in safe locations before exposing yourself to steeper terrain, and recognize that great variability can exist across a slope.  While these avalanches are likely to be small and isolated, they could have severe consequences if they result in a fall in steep technical terrain, above a terrain trap, or if they produce enough force to trigger a deeper failure resulting in a larger avalanche.  Natural windslab avalanches over the weekend remain unlikely, but human triggered avalanches possible.

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.

8”-18” of new snow fell Wednesday night into Thursday.  Temperatures started warm at the beginning of this storm, with snow levels above 7500’, and cooled relatively quickly, dropping snow levels down to 4500’.  This resulted in a very right-side-up snowpack, with dense snow at the bottom and lighter snow on top, which bodes well for stability.  In addition, this snow has now had almost 2 days to settle and for bonds to strengthen both within the new snow and between the new snow and old snow.  While rare and unlikely, it is still possible that some isolated areas may exist where a human could trigger a small storm slab avalanche.  This would most likely occur at a steep convex roll on a NE-N-NW facing sheltered slope at mid to upper elevations where the new snow still has not bonded well to underlying faceted sugary snow.

Avalanche Character 3: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Warmer temperatures in the upper 30s to low 40s and clear sunny skies today and Saturday may be enough to warm the snow on some solar aspects (SE-S-SW) to the point where some loose wet point release avalanches are possible.  The breezy SW winds that are predicted to continue through this weekend will help keep some of these exposed slopes cool and lessen this risk.  Use normal caution on solar aspects.  Be on the lookout for rollerball activity or slopes becoming unsupportable in the afternoon as indicators of slopes becoming unstable.  Small loose wet slides could be possible on steep SE-S-SW aspects at low to mid elevations in the afternoon this weekend, especially near rock bands in areas where winds are calm.  This concern is likely to rise after this weekend through next week as warmer temperatures are predicted with calmer winds, and will shift to encompass E facing slopes as well.     

Snowpack Discussion

The main avalanche concern for the weekend is windslabs in mid to upper elevation steep exposed terrain.  These will most likely be found on leeward slopes that face mostly E - NE – N, below ridgelines, and cross-loaded faces and gullies.  Some isolated lingering storm slab sensitivity may exist in sheltered locations on steep slopes at mid to upper elevations.  If encountered, these soft storm slabs could most likely be human triggered at steep convex points of a slope.  Some wet avalanche concern will develop on solar facing aspects (SE-S-SW) as temperatures rise and solar exposure increases throughout the upcoming days.

8”-18” of new snow fell at mid mountain elevations throughout the forecast area between Bishop and VA lakes during Wednesday Night’s storm.  Areas near Mammoth received the most snow, with areas north receiving somewhat less and areas south toward Bishop receiving much less toward the 8” side of the range.  Temperatures started warm at the beginning of the storm, with snow levels above 7500’, and cooled relatively quickly, dropping snow levels down to 4500’.  This resulted in a very right-side-up snowpack, with dense snow at the bottom and lighter snow on top, which bodes well for stability.  Very strong SW winds, well in excess of 100mph over ridgetops, blew during the night on Wednesday and through much of the day on Thursday.  These winds resulted in heavy snow transport in exposed areas at all elevations, and especially at mid to upper elevations.  Many natural avalanches, albeit mostly small, occurred during this intense wind period.  Winds continued yesterday (Friday) out of the southwest in the moderate range, resulting in continued snow transport, but in much more isolated areas than the day before.  Little to no evidence of natural avalanche activity was witnessed as a result of these winds yesterday (Friday),

recent observations

Observers in the Sherwins yesterday (Friday) reported evidence of old crowns/slides in the Sherwins and the Mammoth Crest (small to medium D1-D2) that occurred Wednesday night or Thursday.  No recent avalanche activity today.  No shooting cracks or whoophing, albeit that skier stayed in more sheltered lower angle terrain.  Moderate to strong W to SW winds. 

Test pit yesterday (Friday) on White Wing near June mountain on NW slope, 9300’, showed consistent CT and ECT failures at new snow / old faceted snow interface.  Q2 shear quality.  Some visible snow transport occurring today over ridge tops, including the negatives, (although much less than the previous day).   

Observer outside of Bishop yesterday near South Lake reported evidence of a natural avalanche from the previous day south of table mountain.  No whoophing/cracking today.  Lots of wind transport occurring from strong SW winds.  6”-8” of new snow above 10,000’.

Observer on earthquake dome yesterday (Friday) morning reported evidence of previous wind transported snow, but none during their tour yesterday.  8”-10” of new snow ontop of firm surface.  No signs of whoomphing or cracking. 

Mammoth Mountain Ski patrol reported significant avalanche results from control efforts (explosives/artillery/ski cutting) on Thursday, but very little results from the same efforts yesterday (Friday). 

Observers in Lundy Canyon yesterday (Friday) reported strong SW winds over ridgetops transporting snow in localized areas.  Some evidence of small natural avalanches that likely occurred Thursday. 

weather

A high-pressure ridge is gradually building over our region this weekend, and will become more pronounced by Monday.  Expect clear skies, breezy winds out of the southwest and average temperatures with highs in the upper 30s to low 40s at mid mountain elevations for Saturday and Sunday.  By Monday, above average temperatures will be back, with highs into the upper 40s at mid mountain elevations.  This ridge looks to remain in place through the majority of next week keeping conditions dry, temperatures above average, and skies clear at least until next 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: MOSTLY SUNNY WITH SOME CLOUDS CLEAR SUNNY
Temperatures: UPPER 30S TO UPPER 40S deg. F. MID 20S TO LOW 30S deg. F. LOW 40S TO 50 deg. F.
Wind direction: WEST SHIFTING TO SOUTHWEST BY AFTERNOON SOUTHWEST WEST
Wind speed: 15 TO 25 MPH WITH GUSTS TO 35, DECREASING TO 10 TO 15 MPH IN THE AFTERNOON 10 TO 20 MPH WITH GUSTS TO 30 MPH 10 TO 20 MPH IWTH GUSTS TO 30 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: MOSTLY SUNNY WITH SOME CLOUDS CLEAR SUNNY
Temperatures: UPPER 20S TO UPPER 30S deg. F. LOW TO HIGH 20S deg. F. LOW 30S TO LOW 40S deg. F.
Wind direction: WEST SHIFTING TO SOUTHWEST IN THE AFTERNOON SOUTHWEST NORTHWEST
Wind speed: 35 TO 50 MPH WITH GUSTS TO 65 MPH, DECREASING TO 10 TO 20 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON 15 TO 25 MPH WITH GUSTS TO 45 MPH 15 TO 25 MPH WITH GUSTS TO 35 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.

ESAC receives support from ...