Avalanche Advisory - Sat, Feb. 11, 2017

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THIS ADVISORY EXPIRED ON February 13, 2017 @ 6:54 am
Avalanche Advisory published on February 11, 2017 @ 6:54 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 48 hours

Wind Slab – The combination of 10” to 30 + inches of new transportable snow with moderate North to Northeast winds will form sensitive Wind Slabs will on NW-W-SW-S-SE-E aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. Anticipate fragile cornices along ridgelines. Natural avalanches are possible; human- triggered avalanches likely in the mid to upper elevations.

Low Elevations – potential for Wind Slabs exists in and around expose isolated features that promote drifting and loading (Gullies, creek beds, shallow depressions, etc.) Natural avalanches unlikely; human- triggered avalanches possible.

Storm Slab – the snowpack is adjusting to 1 to 3 feet of new snow in the past 24 hours and will require additional time to strengthen. The potential for Storm Slab exists primarily in the mid to upper elevations on all aspects in protected areas at least thru Saturday. Saturday - Natural avalanches are possible; human- triggered avalanches likely, especially in steep protected terrain. Sunday - Natural avalanches unlikely; human- triggered avalanches possible. Avoid terrain traps that will result in deep burial, even with relatively small releases.

Low Elevations (~ below 8,000’) considerable less snow has fallen on a rain soaked snowpack in the lower elevations. The bonding is fairly good and continues to strengthen. Saturday - Natural avalanches unlikely; human- triggered avalanches possible. Sunday - Natural and human- triggered avalanches unlikely.

 

 

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.

Wind Slab – The combination of 10” to 30 + inches of new transportable snow with moderate North to Northeast winds will form sensitive Wind Slabs will on NW-W-SW-S-SE-E aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. Anticipate fragile cornices along ridgelines. Natural avalanches are possible; human- triggered avalanches likely in the mid to upper elevations. Avoid wind loaded terrain and runout zones below wind-loaded terrain. Human-triggered wind slab avalanches remain likely today due to continued wind loading of leeward slopes across the forecast area in the mid to upper elevations. Dangerous avalanche conditions exist. Carefully evaluate the terrain and snowpack before committing to any slope and use this information plan conservative travel routes that avoid the avalanche problems.  

Low Elevations – potential for Wind Slabs exists in and around expose isolated features that promote drifting and loading (Gullies, creek beds, shallow depressions, etc.) Natural avalanches unlikely; human- triggered avalanches possible. Evaluate snow and terrain carefully; identify features of concern. 

 

 

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 Slab – the snowpack is adjusting to 1 to 3 feet of new snow that accumulated in the past 24 hours and will require additional time to strengthen. The potential for Storm Slab exists primarily in the mid to upper elevations on all aspects in protected areas at least thru Saturday. Snow surface cracking are possible clues for Storm Slabs.  Natural avalanches are possible; human- triggered avalanches likely, especially in steep protected terrain. By Sunday, the new snow will have had additional time to settle and strengthen reducing the Storm Slab potential. Sunday - Natural avalanches unlikely; human- triggered avalanches possible, especially in sheltered extreme terrain. Avoid terrain traps that will result in deep burial, even with relatively small releases.

Low Elevations (~ below 8,000’) considerable less snow has fallen on a rain soaked snowpack in the lower elevations. The bonding is fairly good and continues to strengthen. Saturday - Natural avalanches unlikely; human- triggered avalanches possible. Possible areas where shallow Storm Slabs may exist in localized areas, especially unusually deep deposits. Sunday - Natural and human- triggered avalanches unlikely. 

Snowpack Discussion

The latest atmospheric river (AR) cycle rolled into the region late Monday with warming temperatures and rain in the lower and mid elevations and heavy snowfall in the upper elevations (above ~10,000’) through much of the week, saturating the lower elevation snowpack and penetrating deeply into the upper portion of the snowpack in the mid elevations. This was accompanied by very strong Southwest winds aloft forming wind slabs throughout the higher elevations. The combination resulted in significant avalanche activity throughout the forecast region by Thursday. Avalanche activity (Storm Slab, Wind Slab, Loose Wet) was observed throughout the mid and upper elevations, where visibility permitted. Late Thursday, precipitation continued as cooler air began filtering into the region with the snowline falling to below ~8000. Friday snow continued to accumulate above ~7500’ with amounts ranging from 10 to 30 plus inches above 9,000’.  The new snow is bonding relatively well to the old snow interface but density changes within the new snow are a concern for ongoing Storm Slabs through Saturday. By Sunday, the new snow will have had time to settle and strengthen reducing the threat of Storm Slabs. However, Wind Slabs will continue to be a concern throughout the forecast period as previously formed storm Wind Slabs on NW-N-NE-E-SE-S aspects begin to strengthen, new Wind Slabs will begin to form on NW-W-SW-S-SE-E as North to Northeast winds begin to  late Saturday into Sunday. Anticipate Wind Slabs throughout the upper elevations, possibly in uncommon locations due to the exceptionally strong storm winds.

As temperatures begin to climb Sunday, Loose Wet may be possible on solar aspects in the Low to Mid Elevations as the new snow warms under the mid-February sun.

Caution – Snow creep cracks have been observed throughout the forecast area with some large enough to swallow a person. They have been found at all elevations on steep slopes, base of cliffs, couloirs, and below buried rock bands. This latest round of wet snow/rain may cause these to become more pronounced.

Wx Stats (0500, 2/11/17)

Virginia Lks (9445’): Temp 17.8, HN 18 ”/ 4.7” SWE

Gem Pass (10750’): Temp 28, HN N/A / 3” SWE

June Wx Plot (9148’): Temp 18.8, HN 13” / 1.5” SWE

Mammoth Pass (9500’): Temp 20, HN 31”  

Rock Crk (9600’): Temp 23, HN 14.5”

Big Pine Creek (10,000’): HN 10.8”

 

recent observations

June Mountain BC (2/10/17) - A great day in the BC near June Mountain Ski area on the south and east facing slopes on the opposite side of the ridge near chair J7.  The terrain we were on today was 20-25 degree angle range and widely spaced with trees.  Ski pen was about 30-50cm on average and deeper at times.  New snow depth on top of the old rain soaked snow was averaging about 135 cm.  This layer of snow was very stable and bonding well to the wet snow below.  The best skiing was sheltered slopes.  Ridgeline winds were averaging about 30mph (best guess) but only when gusting.  Off the ridge the wind was not a factor.  Lightly snowing all day.  I can't speak to the windslabs, but the snow in this area seemed to represent a nice, stabilizing post storm snowpack.

Mammoth Rock, Sherwins (2/10/17) - Toured from the end of Old Mammoth Road. Parking area adequately plowed. 6-8" new snow approaching Mammoth Rock. Light winds NW with some visible snow transport in exposed areas. Light snow falling (S-1) throughout tour with periods of heavier precip (S1-S2). Hand pits revealed good bonding between new snow and old wet snow surface. Some separation in new snow. Probing revealed a right side up snowpack. Pit dug on NE aspect NTL (approx. 9500') in an area with previous wind transport. Moderate force in column tests produced Q3 shears and non-propagating results within the new snow, and within the old rain soaked snow. See attached profile. We observed a slightly stiffer wind effected snow surface near the ridgetop, but did not experience any shooting cracks or whumphing. These newly formed windslabs were isolated to just beneath ridgetop in open areas and were still quite soft. At ridgetop the snow surface was more supportable with ski penetration reducing to just a few centimeters. Visibility was poor at ridgetop with light winds. Newly formed cornices were observed along the ridge from Mammoth Rock Bowl to the top of the Hose. Small crowns in the vicinity of The Hose were visible from a short distance but difficult to discern in the flat light. Another skier reported skiing bed surface in the Hose and finding large blocky debris in the Hose itself and the trees nearby, presumably from a recent interstorm avalanche. (Possibly cornice collapse?)

The Perch, Sherwins (2/10/17) - Natural avalanche of the new storm snow across the majority of the small upper headwall on the ridge at the top of the Perch in the Sherwins. I spotted the crown from afar as I skinned up the ridge solo. Once there, I was able to safely approach on the bed surface, which was the harder old snow surface from before Thursday Night/Friday's storm with ~5 cm of new snow from daytime Friday on top. I did a few handpit tests on analogous unslid snow on the ridgeline and got a very easy shear about halfway (~25 cm) down in the ~50cm of new storm snow where there is an density interface - the first pit slid on isolation, the second with almost no force. This mid-slab interface matches with what we saw in a pit profile on Punta Bardini earlier in the day (see Josh's pit observation). The slide itself released all the way to old snow (both layers of new snow), but looking at the point in the crown, it looks like a point release directly by the ridgline at the cornice that likely propagated across the slope to either side (my guess is that it started in the more sensitive top layer and stepped down to entrain all of the new snow). Based on the amount of new snow on the bed surface, I'm guessing that this went naturally sometime in the late night hours or early morning Friday. I saw one similar-looking slide in the bowl further southeast, but it was too far away and in out of the clouds, so I wasn't able to get much detail on it.

Punta Bardini, Sherwins (2/10/17) - Route: From parking lot at propane tanks, up standard route, down Old Growth and skier R Tele Bowl.  Parking lot at propane tanks (7,800'): 8:30am and 1:15pm: Super wet slush, pools of water, see attached photo.  1:15pm temp = 34deg F. 7,800' to 8,100': Heavy dense wet snow. 8,100' to 8,500': 5-8" of new Dense snow, but becoming drier quickly with elevation, skiing actually not bad. 8,500' to 10,000':  Light dry surface snow, up to 1-1.5ft of new on top of wet saturated snow from yesterday and before.  See attached profile done at 9,400'.  Stable profile in this sheltered location.  Very right-side-up upper snow, and wet moist snow below fairly solid.  CTs resulted in mostly Q3 results, with one Q2 result 85cm down with 30 taps.  Air temp 11am: -2deg C. No shooting cracks anywhere, no fracturing even on steep rolls stomping on snow above skin-track. Hand pits throughout sheltered area resulted in no fractures, just crumbling soft snow.  Skinned up with small switchbacks up rib between chutes and Old Growth where there is usually wind-effected snow.  No wind effected snow today here!  The snow was very unconsolidated. At ridgetop ~9,900', where light SW wind was present with occasional moderate gusts, isolated areas of slightly denser (Fist +) wind deposited snow was present.  Hand pits failed with moderately easy force cleanly ~15 - 25cm down.  Gave the clue that more widely wind-effected areas would be much more concerning.  Reports from another experienced party that we encountered who skied the Bardini Chutes:  Surprisingly un-wind-effected snow.  Extensive heavy sloughing, and some isolated crowns, 4-6" of very soft slab, one of which propagated ~50ft.  Snow from these sloughs/soft slabs ran entire length of chutes, with moderate amounts pilling up on the uphill side of trees, and they were surprised how much debris filled the lower parts of chutes.  They believed that main chute may have slid naturally (likely from steep head-wall area) within the previous few hours due to the amount of debris in the lower parts of chute, although they didn't see crown.  Sounded like if someone had gotten caught in their slough and fallen above a tree, they could have gotten buried.  Surprisingly great skiing thru Old Growth, light fluff.  Became denser in the Tele-bowls below 8,500'.   

Table Mountain, Bishop Creek (2/9/17) - Short tour in the Bishop Creek drainage to look at snow and weather conditions. Evidence of many wet slides on E-NE aspects of Table Mountain (see photos).  Most of these slides appeared to have happened within the last 24 hours with start zones (those that were visible) fairly low 9000-9500 ft. 1015am: 8400 ft- South Lake Rd parking. Strong SW winds.  Light rain.  Air temp=10C. Snow on road cuts at parking lots ~1m deep, moist throughout profile. 1120am: Skinned up road.  Cloud ceiling ~10,500 ft.  Could not visualized cornice or snow transport over ridgelines. Snow surface moist. Moist to about 80cm depth.  Ski penetration=5-10cm. Air=6C. 1230pm: ~9500 ft. Continued strong wind.  Snow surface moist to 40cm depth.  Ski penetration 10-15 cm. Dense but generally dry below that. Rain has turned to light snow, usually melting on contact. No snow accumulation noted. 1pm: 10,300ft. Strong, gusty winds transporting snow. Snow surface pencil firm (2cm thick), moist crystals noted in this layer. Dense, dry snow below that. Ski penetration 0-2 cm.  Air= -1C. 130p: No snow accumulation at parking lot since leaving for tour. Air temp=5C.

weather

Saturday- North to northeast moist upslope flow Saturday will allow residual showers to continue across Mono and Alpine counties thru the day, which will likely contribute a few more inches of snow to those areas. Cold air advection will also allow the east-west gradient to set up and produce gusty easterly ridge winds across the Sierra Crest. Winds light Northerly winds then veering to the Northeast, with temperatures Highs in the upper teens to mid 20s, Lows single digits to mid teens.   

Sunday thru Tuesday - High pressure will keep the Sierra dry and also allow inversions to set up over the valleys. Inversions will moderate warming for the lower valleys early next week, while the ridges and mid-slopes will see a more noticeable warming trend. Moderate North to Northeast winds, temperatures will hover near normal with Highs in the mid 20 to low 30s, Lows single digits to mid teens.    

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. Scattered snow showers in the morning, then isolated snow showers in the afternoon. Partly cloudy then becoming clear. Sunny.
Temperatures: 25 to 30 deg. F. 10 to 18 deg. F. 32 to 37 deg. F.
Wind direction: Light winds becoming north. North Light winds.
Wind speed: 15 to 20 mph with gusts to 40 mph in the afternoon. 15 to 20 mph in the evening becoming light. Gusts up to 45 mph.
Expected snowfall: 2 to 3 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Mostly cloudy. Scattered snow showers. Partly cloudy then becoming clear. Sunny.
Temperatures: 18 to 24 deg. F. 9 to 14 deg. F. 28 to 33 deg. F.
Wind direction: North Northeast Northeast
Wind speed: 10 to 15 mph with gusts to 35 mph increasing to 20 to 30 mph with gusts to 65 mph in the afternoon. 20 to 35 mph with gusts to 60 mph. 20 to 35 mph with gusts to 65 mph.
Expected snowfall: 2 to 4 in. 0 in. 0 in.
Disclaimer

This Snowpack Summary is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. 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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