Avalanche Advisory - Sat, Jan. 14, 2017

THIS ADVISORY EXPIRED ON January 17, 2017 @ 7:06 am
Avalanche Advisory published on January 14, 2017 @ 7:06 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 72 hours

Low to mid elevations are showing signs of adjusting to the new load from the latest snow cycle with Storm Slab, Deep Slab, and Persistent slab concerns slowly improving but Wind Slabs remain a concern due to Northeasterly winds that picked-up during the day Friday and are forecasted again on Sunday, loading NW-W-SW-S-SE aspects. Wind slab releases will likely be relatively small but potential numerous along ridgetops and in and around terrain features that promote drifting and crossloading. Natural avalanches unlikely; human- triggered avalanches possible. Evaluate snow and terrain carefully; identify features of concern. Though Wind Slab avalanches may be relatively small, there is potential for triggering deeper releases. In the upper elevations, conditions remain more uncertain due to a complicated snowpack structure and limited data. An additional consideration for the Low and Mid elevations is the potential increasing instability on solar aspects this afternoon due to solar warming. With temperatures forecasted to climb into the low to mid thirties thru Monday, solar aspects could see rapid warming, decreasing stability. Low and mid elevations are also at elevated risk for loose sloughs on steep terrain. Not a burial hazard by itself but has the potential to carry a rider into hazardous terrain. As a result careful snowpack evaluation, cautious route-finding and conservative decision-making are essential. 


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.

Moderate Northeasterly winds at upper and mid elevations (9,000' and above) Friday have formed Wind Slabs on NW-W-S-SE aspects, along ridgetops, leeward sides of ridgelines, and in and around terrain features that promote drifting and crossloading. Friday’s Wind Slabs have had some time to strengthen but remain sensitive. Human triggered avalanches will likely be encountered on leeward terrain features where wind transported snow is deposited. Sunday, Northeasterly winds are forecasted to pick-up once again, which will form another round of sensitive Wind Slabs in the mid to upper elevations on NW-W-S-SE aspects. Use caution and avoid exposed terrain steeper than 30 degrees where Wind Slabs with varying levels of sensitivity exist until they have more time to stabilize.  

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.

The recent historical snowfall brought multiple feet of snow over the past week, mostly “right side up” (density increases with depth) with snowfall amounts in feet. Overall conditions are improving as bonding continues to build between the various layers and the old/new snow interface. However, areas where the storm slab is relatively shallow (~4 feet or less) may still be reactive to skier triggered release, especially areas north and south of the Mammoth Lakes Basin.  Natural avalanches are unlikely but human- triggered avalanches remain possible on sheltered slopes on all aspects and elevations through Monday on slopes 35 degrees and steeper.  Do your own stability assessments / test pits, and keep an eye out for signs such as shooting cracks and whoomphing before deciding to commit to steeper terrain. An additional consideration for the Low and Mid elevations is the potential increasing instability on solar aspects this afternoon due to solar warming. With temperatures forecasted to climb into the low to mid thirties thru Monday, solar aspects could see rapid warming, decreasing stability. Low and mid elevations are also at elevated risk for loose sloughs on steep terrain. Not a burial hazard by itself but has the potential to carry a rider into hazardous terrain.


Avalanche Character 3: Deep Slab
Deep Slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to Storm or Wind Slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

Early season storms In October and early December formed a combination of Melt/Freeze crusts and facets between 9,000 and 10,500', primarily found on NW thru N thru East aspects forming a Persistent Deep Weaknesses that has been found throughout the forecast region. The recent heavy snows have added significant stress to the snowpack. An avalanche failure in the upper snowpack could potentially trigger a step down failure deep in the snowpack triggering a potentially very large and destructive avalanche.  While this is unlikely, it is still a possibility, especially in areas with shallower snowpacks south of Mammoth and North of June. It is worth doing your own investigation by digging deep and checking out if these weak layers exist, and how sensitive they are with your own stability tests.

Because of the widely varying conditions, especially in the upper elevations, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.   


Snowpack Discussion

The latest in a series of storms moved out of the region Friday and in its wake historical amounts of snow. The extended wet weather has greatly improved drought conditions throughout the region with many sites reporting 195 to 205% of average. Over 16” of water has fallen in the mountains surrounding Mammoth in January, total snow depths reading well over 140” base at the Mammoth Ski Area while Mammoth Pass is reporting 147”.  It started precipitating January 4th, with only a couple of small breaks. As temperatures cooled, lower elevations (~6,000’) received significant snowfall with reports of good coverage. The recent Atmospheric River (AR) started very warm with the rain line at ~9,500’, which saturated the lower elevation snowpack to the ground. This resulted in a number of Wet Slab and Loose Wet avalanches along the Sierra Front. Wet Slab avalanches were observed Tele-Bowls in the Sherwins, Mammoth Basin. Cooler temperatures have returned and will begin to re-freeze the underlying snowpack, which will ultimately strengthen the lower snowpack between 8,000’-10,200’+.

Snowpack Details past 7 days

Mammoth Pass: Storm snow 62”, 11 water"; Virginia Lks: Storm snow 38", SWC 9.5”; Rock Crk: 29.9"; SWC - 6.05. Gem Pass: 55.5", SWC 9.6”; June Study Plot: Storm snow 39.4, SWC 9.1”.

Because of the widely varying conditions along the length of the Range, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.   


recent observations

Aspendell, Bishop (1/13/17) - Had a midday 2-hr window and wanted to see how much snow was up Bishop Creek. Dug a quick pit on a NE facing 25 degree slope above Aspendell in that little granite mini bowl below north lake. 170 cm deep; evidence of windloading, and east face of Bishop bowl had avalanches during the middle storms. Low angle skinning was 6-8" powder on top of nice dense snow - no sounds- solid base no collapsing. Dug a pit in wind loaded gentle gully near top. Overall behavior was three layers - recent powder and wind slab; middle dense storm layer on top of rain crust; basal layer of moist rounding former sugar crystals. Surface to 15 cm was fist new powder (Thurs pm storm). 15 cm to 45 cm slab layer (probably round 3 three of recent storm cycle) 4 finger hard slab. 45cm to 105 cm was dense fairly homogenous 1 finger hard slab - didn't geek out with mini layers as its appeared to be a solid unit - well bounded to rain crust below. 2cm hard rain crust above basal layer. From 110 to 170cm was large old sugar crystals that were beginning to round, had some moisture to them - 4 finger; base was rocks and dirt. No Isolate block in top 45 cm - no compression failures; sheer test required strong outward pull- just short of "prying" - clean block slid out (great igloo material); isolated from 45 to below rain crust- couldn't get anything to go with compression, sheer prying - bottom layer seems well bounded. Did Rutschblock test R4 failure - clean block at the 45 cm slab base- somewhat surprising first real jump and it slid right out - 25 degrees slope. So seems like that top slab is still bonding to middle storm layer; should have looked more closely at the interface but ran out of time; jumped all over the middle layer R7 nothing. Skied the short slope- no signs of instability. Hope this helps a little - Also someone put a skin track up the standard moraine gully for Bishop bowl but I didn't see any ski tracks; people have also skied Habaggers today on the open west face and the north facing trees above; no one hit Table Mtn as far as I could tell.

Convict Lake, Convict Creek (1/13/17) - Route:  From Convict Lake resort up east facing bowl to ridgeline, back down. A half dozen quick hand pits done in route, one quick test pit and ECT test.  In summary, some interesting layers, but after investigation, nothing proving concerning.  Details follow: 7,800:  ~7" of new low-density snow on top of rain crust. Some tiny pockets of thin wind crusts above 8,400' on edge of gully where northerly winds deposited snow just on other side of rib.  ~2 cm thick, very weak, easy to ski right through.  Just below ridgetop on SE facing slopes this windcrust was slightly thicker (~3cm) and denser, but still too thin to be of concern to propagate as a slab. 8,500', ENE facing, 36degree slope:  some interesting layers, but tests proved they weren't concerning. 18" (47cm) of snow on top of rain crust, top 14" (35cm) low density new snow, 4" below that more dense Fist + snow on top of crust.  Total snow depth = 40.5" (103cm).  Top rain crust is thin, ~2cm thick, underneath that a 3-4cm layer of soft decomposing precip. particles exists, underneath that is re-frozen rain saturated snow to the ground. Of note - hand shears did fail with hard force within that thin low density layer between crusts (when pushing on that upper crust, but CT and ECT tests did not fail in that layer.  ECT test did fail, but barely propagated width of shovel in subtle density change within Fist+ snow above rain crust. Calm winds this late afternoon on this slope (appears that winds did blow light to moderate earlier in the day toward in the upper reaches of this slope).  Snow banners were visible this evening over the tops of Laurel Mountain and Mt Morrison from NE winds, most likely resulting in potentially sensitive sizeable Wind Slabs on the SW side of the peaks.       19 deg F at 3:30pm just over 9,000' at ridgetop.       

Mammoth Mountain, Mammoth Lakes Basin (1/13/17) - Consistent moderate (20-35mph) NE winds at 10,000', transporting yesterday's new light snow over ridges and across slopes creating sensitive windslabs on leeward features, especially other side of ridges and sidewalls of gullies.

Punta Bardini, Mammoth Lakes Basin (1/12/17) -Route: Propane tanks up Old Growth to summit, back down thru skier R Tele Bowl. Light low density snowfall accumulating rapidly all day until mid afternoon.  Over 1 ft at 8,000' at 10:15am, over 2ft by 2pm at 9,500'.  No signs that it is consolidating into any kind of slab all day today.  Just loose sloughs on steeper pitches.  Occasional to rare moderate gust out of the South at the ridgetop (10,000'), but mostly calm.  No signs of slabby surface snow anywhere today on Punta. Ski cut top of main Bardini Chute resulted in some sloughing.  Skied Old Growth and Tele Bowl.  No signs of shooting crack, woomphing, although small pockets of settlement on steeper rolls, but nothing propagating. One pit dug in wind sheltered location just under 9,000', 225cm total snow depth.  Of note, bottom half of snowpack is isothermal, still wet, with 10cm refrozen rain crust at top.  Upper half of snowpack above this rain crust is right-side-up, with upper 50cm new very low density unconsolidated snow. Some CT results in denser snow between the crust and todays new fluff, failing with moderate to hard force, Q2 results. ECTs did not propagate. Another pit dug on exposed ridgeline 3/4 of the way up (~9,400', N facing).  On windward side of this ridge (W side), new fluff from today sat directly on top of rain crust.  The farther East from ridge (leeward side), the more dense snow was found between rain crust and todays fluff.  ECT test slightly to the lee-side with about 15cm old dense snow (from yesterday or the day before) on top of rain crust, ECT tests failed with hard force and propagated, but not too energetically.  Tests failed on thin facet layer under this upper rain crust. Overall little evidence was found of instabilities today on our tour. Of most concern would be extensive sloughing on steeper slopes.  As this new low-density snow settles however, it will most likely develop some slab tendencies, which may effect stability assessment in future days. Some of the better skiing of the year ... much snow in the face :-)


Sat thru Sunday - Easterly flow and orographic lifting continue to support a fairly widespread low level cloud deck along the Sierra Front into the Sierra itself. As a result, temperatures dropped more slowly overnight and will warm more a bit than forecast. Temperatures are colder than recent conditions with the potential for more cold temperatures Saturday night into Sunday morning. Models project a westerly, off-shore track for an approaching wave Sunday with increasing cloud cover.

Mon thru Tuesday – weak short wave ridge over the region. The ridge flattens and allows over running precipitation to move into the northern part of the forecast area by late Tuesday, spreading south overnight ahead of the main upper low.


Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Temperatures: 30 TO 35 deg. F. 9 TO 19 deg. F. 31 TO 36 deg. F.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Temperatures: 27 TO 33 deg. F. 11 TO 16. deg. F. 27 TO 33 deg. F.
Wind speed: 15 TO 20 MPH WITH GUSTS TO 30 MPH. 15 TO 25 MPH WITH GUSTS TO 40 MPH.
Expected snowfall: 0 in. 0 in. 0 in.

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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