Eastern Sierra Snowpack Summary - 2016-03-14 06:49

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THIS ADVISORY EXPIRED ON March 16, 2016 @ 6:49 am
Avalanche Advisory published on March 14, 2016 @ 6:49 am
Issued by Doug Lewis - 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.

Additional fresh new windslabs have formed as a result of new snowfall which began falling Sunday accompanied by very high winds out of the southwest, veering west Monday.  10” - 20” of new snow has fallen since Thursday afternoon with the greatest amounts being recorded near Mammoth/June Lakes area, significantly less amounts recorded north and south. The rather rapid succession of storms, cool temperatures, and strong mid to upper elevation winds of near 80mph over ridgetops, have produced tender windslabs on NW-N-NE-E-SE-S aspects throughout the range, which have yet to fully bond to adjacent layers. Extensive snow transport due the strong westerly winds will continue through Monday and continue to form additional fresh sensitive windslabs throughout the range. Windslabs typically form below ridgetops (extending downslope further than anticipated due to the strong winds), gully sides, depressions, and cross-loaded terrain features and slopes. Natural avalanches are possible, and human triggered avalanches are likely on exposed slopes greater than 35 degrees, especially at mid to upper elevations. Small releases may trigger deeper releases resulting in larger more destructive and dangerous avalanches with potentially greater consequence. Remember, small avalanches, especially in complex terrain, can results in injury or death. 

 

 

Avalanche Character 2: 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.

As high pressure builds over the region and temperatures begin to climb (high’s of 40’s to 50’s) the recent new snow is quickly cycling into a melt-freeze snowpack on E-SE-S-SW-W aspects. The melt-freeze cycle is just beginning with the surface melt-freeze crust varying in strength and supportiveness depending on elevation, aspect and nightly temperatures. As the sun advances around the compass rose, the melt-freeze crust will thaw and becoming less supportive during the day. Wet-Loose avalanches are possible, which has the potential to trigger larger Wet Slabs on steep slopes (>35 degrees) as southerly aspects heat up through the day. Plan to be off of these slopes early before they thaw and become unsupportive. Take note of Rollerball (Pin Wheels) activity, which is an indicator of weakening snow grain bonds and a rapidly warming snow surface. As the week progresses, natural and human triggered wet loose avalanches are possible, on southeast slopes in the morning, southerly aspects by mid morning to mid-day, and southwesterly by afternoon.  Use extra caution on large mountain faces with complex terrain and multiple aspects. Lower elevations (below 8000’), the snowpack is disappearing rapidly but the remaining snow will warm rapidly on all aspects with risk of wet loose activity increasing on slopes >35 degrees by the afternoon. Be especially cautious in complex terrain with multiple aspects, which can result in unequal heating and thawing.

 

Avalanche Character 3: 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.

While the potential for storm slab avalanches in wind-protected areas was an initial concern as the recent storm exited the region, the threat is declining as the new snow shows signs of bonding and overall strengthening. However, caution is still advised on southerly aspects, which have formed a well-developed melt-freeze crust prior to the recent storm cycle. , On northerly exposed mid and upper elevations aspects, smooth windslabs have formed with the potential for poor bonding with the recent new snow. The old smooth snow surfaces make for a good bed surface for avalanches. Storm slab instability is beginning to ease as the snow continues to settle and strengthen. 

Additional concern in the upper elevations where well-developed faceted snowpack continues to be found on isolated high elevation terrain (>11,500') with N to NE aspects where the snowpack remains relatively thin and shallow. Thick windslabs may bridge over this weak layer. However, it is possible a rider could trigger a slab failure where thinner, which could propagate and release a deeper and larger avalanche. Factor this in your decision making process when traveling in higher elevations and in complex terrain.

 

Snowpack Discussion

Primary avalanche hazard for this Snow Summary (valid 3/14 thru 3/16) period will be widespread windslabs formed during the recent two storms systems, which began on Thursday. Warming temperatures combined with new snow will increase to potential for Loose –Wet avalanches as the threat of storm slab avalanches begins to decline as the new snow settles and strengthens.

The recent series of storms and the associated strong South to West winds have produced multiple windslab events with lower density snow sandwiched between. Snowfall totals for the two systems range from 10” - 20” of new snow has fallen since Thursday afternoon with the greatest amounts being recorded around the Mammoth/June Lakes area, significantly less reported north and south. The rather rapid succession of storms, cool temperatures, and strong mid to upper elevation winds of near 80mph over ridgetops, has produced tender windslabs on NW-N-NE-E-SE-S aspects, which have yet to fully bond to adjacent layers. Extensive snow transport due the strong westerly winds will continue through Monday and form fresh sensitive windslabs throughout the Sierra. Windslabs typically form below ridgetops (extending downslope further than anticipated due to the strong winds), gully sides, depressions, and cross-loaded terrain features and slopes. Natural avalanches are possible, and human triggered avalanches are likely on exposed slopes greater than 35 degrees, especially at mid to upper elevations. Small releases may trigger deeper releases resulting in larger more destructive and dangerous avalanches with potentially greater consequence. Remember, small avalanches, especially in complex terrain, can results in injury or death. 

Additionally, as high pressure builds over the region and temperatures begin to climb (high’s of 40’s to 50’s) the recent new snow is quickly cycling into a melt-freeze snowpack on E-SE-S-SW-W aspects with the potential for Loose-Wet releases. The melt-freeze cycle is just beginning with the surface melt-freeze crust varying in strength and supportiveness depending on elevation, aspect and nightly temperatures. As the sun advances around the compass rose, the melt-freeze crust will thaw and becoming less supportive during the day with Wet-Loose avalanches possible, which has the potential to trigger larger Wet Slabs on steep slopes (>35 degrees) as southerly aspects heat up through the day. Plan to be off of these slopes early before they thaw and become unsupportive. Take note of Rollerball (Pin Wheels) activity, which is an indicator of weakening snow grain bonds and a rapidly warming snow surface. As the week progresses, natural and human triggered wet loose avalanches are possible, on southeast slopes in the morning, southerly aspects by mid morning to mid-day, and southwesterly by afternoon. Use extra caution on large mountain faces with complex terrain and multiple aspects. Lower elevations (below 8000’), the snowpack is disappearing rapidly but the remaining snow will warm rapidly on all aspects with risk of wet loose activity increasing on slopes >35 degrees by the afternoon. Be especially cautious in complex terrain with multiple aspects, which can result in unequal heating and thawing.

While the potential for storm slab avalanches in wind-protected areas was an initial concern as the recent storm exited the region, the threat is declining as the new snow shows signs of bonding and overall strengthening. However, caution is still advised on southerly aspects, which have formed a well-developed melt-freeze crust prior to the recent storm cycle. , On northerly exposed mid and upper elevations aspects, smooth windslabs have formed with the potential for poor bonding with the recent new snow. The old smooth snow surfaces make for a good bed surface for avalanches. Storm slab instability is beginning to ease as the snow continues to settle and strengthen.

 

recent observations

South Peak, Virginia Lakes (3/13/16) - Very wind affected with hand pits revealing 1 to 2 layers of wind-slab from 10cm to 25cm thick, failing with moderate to strong force. CTs failed fairly easily, ECTs failed but did not propagate.  No shooting cracks, whoomphing, signs of recent avalanche activity (main paths off summit did show evidence of avalanche activity likely from last snow/wind combo event on Friday.

Red Cone Bowl, Mammoth Lakes Basin (3/12/16) - Southwest winds picked up in the AM with large visible plume off the peaks south of Mammoth and along the Crest. 20-50 cm of new snow noted in the trees and bowl. The Bowl had a mixture of windslab and wind skin from the middle of the bowl and extending toward the western edge (North to Northeast aspects) with scoured and light scalloping along the eastern edge (west aspect). Much of the top of the bowl had a 10 cm to ~20 cm windslab from the summit ridge to well down into the bowl. The windslab was semi-supportable and not well bonded to the underlying snow. The windslabs easily failed during isolation and while weighting during Compression Tests but Extended Column Tests failing directly under the shovel. Test slopes yielded only small isolated sluffs. Southerly aspects developing light melt freeze crust. Snow conditions in northerly protected trees remains soft and very skiable.

Mammoth Rock, Sherwins (3/12/16) - Tests on switchback corners propagated and slid into trail below. Hand shear tests pulled easily and planar in new snow or at new/old snow interface 15-20cm down; one hand shear block popped out on isolation without pulling, sudden planar. Shovel Tilt Tests showed a little instability in the newer snow with a thin layer of wind transported snow above storm snow even down low on the slope. R3-D2.5 under the cornice in Mammoth Rock Bowl proper, saw recent wind slab debris below the Poop Chute, just skier's right of Mammoth Rock. Winds were still blowing Moderate out of the SW with snow transport occurring down to near the old Log Cabin Mine.

Mammoth Mtn. (3/12/16) - Control work produced widespread avalanches.  Mostly 12-24" crowns mid-mountain, mostly running full width of slide paths, 600'-1200' long. (Dragon's Wazoo 1,2,3, Avy 1/2 bowl, Shaft, Grizzly, Bear, Waterfall)  Widespread results across upper mountain as well, with 3-4' crowns in some places.  Sesame weather plot recorded 10" of new snow just above 9000', however strong SW winds created much deeper deposits.    

*Disclaimer:  Avalanche summary from explosive control work and ski cutting done by professionals. Control work in-bounds areas, with the majority of areas receiving a high amount of skier traffic.

Old Growth, Punta Bardini (3/12/16) - Winds moderate to strong out of the SW all day with significant snow transport all the way down in to the Tele Bowls (looker's left of Baby Tele was stripped to the old snow surface). New snow amounts ranged from 5 to 30 cm.  Reactive new wind slabs on the leeward side of the ridge between the Old Growth and Bardnini Chutes with shooting cracks, plating and sloughing on the old melt freeze crust in favored locations. Pillows with 5 to 10 cm thick harder wind slab above fist hard new snow. Surface snow at summit in the sun was already moist at 11:45. Hand shear above the main Tele Bowl failed easily and planar on the March 4 melt freeze interface roughly 40 cm down. A large slab had released all the way across Mammoth Rock Bowl.

Sherwins, Mammoth Basin (3/12/16) - Moderate winds predominantly out of the SW, but swirling from various directions across slopes. Wind effects were evident from low to high elevation in many areas (firm fresh windslab, wind stripped old snow surface, slightly more dense wind crusts of varying thicknesses).  Up to 6" of new snow in wind protected areas.  Numerous hand pits from low elevations to just below ridgetop revealed failures upon isolation of Pencil Minus/1 Finger plus windslab from 1"-8" thick on top of 4finger unconsolidated snow. Very variable in distribution across relatively small areas across slopes.  No shooting cracks or whoomphing. Crown visible just below ridgetop in Mammoth Rock Bowl, likely failing last night during wind loading after new snowfall.  Appeared to be ~100ft across, 1-2+" deep.  Debris not visible.

 

weather

Tues- Thurs: A much quieter weather pattern will return to eastern CA through Thursday as high pressure begins to build off the west coast. Tuesday - moderate northwest flow aloft will bring some late afternoon breezes with gusts 20-25 mph mainly near and east of Highway 395. Lighter east winds are expected Wednesday and Thursday as the ridge axis amplifies then moves slowly inland.

Temperatures will gradually rise a few degrees each day, with highs climbing to near seasonal normal. By Thursday, most lower elevations should push well into the 60s with a few warmer valleys possibly reaching 70 degrees. The next couple of nights will be cool. Areas of cirrus will spread across most areas Tuesday and Wednesday with little to no cloud cover expected Thursday.

 

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY CLEAR SUNNY
Temperatures: 45 TO 53 deg. F. 18 TO 25 deg. F. 49 TO 55 deg. F.
Wind direction: WEST NORTHWEST VARIABLE
Wind speed: 15 TO 25 MPH WITH GUSTS TO 35 MPH. 10 TO 15 MPH WITH GUSTS TO 25 MPH SHIFTING TO THE NORTHEAST AFTER MIDNIGHT. LIGHT WINDS.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY CLEAR SUNNY
Temperatures: 38 TO 46 deg. F. 16 TO 23 deg. F. 41 TO 49 deg. F.
Wind direction: WEST NORTHWEST SOUTHWEST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 45 MPH 15 TO 20 MPH. GUSTS UP TO 35 MPH DECREASING TO 25 MPH AFTER MIDNIGHT 10 TO 15 MPH. GUSTS UP TO 25 MPH IN THE MORNING.
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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