Snowpack Summary - Sat, Dec. 17, 2016

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THIS ADVISORY EXPIRED ON December 20, 2016 @ 7:38 am
Avalanche Advisory published on December 17, 2016 @ 7:38 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 72 hours are: Storm Slabs above ~8,500’. Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations. Additional concern, Wind Slabs or Storm Slabs avalanches stepping down and triggering a much larger Persistent Slab. Use extra caution in terrain 35 degrees and steeper, because of the widely variable new snow amounts and characteristics, assess the snowpack of the slope before you commit. 

 

Avalanche Character 1: 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 storm snow (above ~ 8,000’) is still gaining strength and bonding to adjacent layers. As you approach the Crest a density inversion within the new snow will likely remain sensitive to triggered release. It shows signs of bonding to the old/new snow interface but a density change within the new snow about 3”(~8 cm) from the snow surface shows slab tendencies with the potential to propagate more widely as it becomes more supportive. On terrain greater the 35 degrees or steeper it may be possible to trigger Slab Avalanches big enough to result in burial, capable of dragging riders into hazardous terrain, rocks or trees as well as possible triggering deeper instabilities with potentially serve consequences.

 

Avalanche Character 2: 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.

The recent storm arrived with moderate to strong Westerly to Southerly winds forming Wind Slabs on NW-N-NE-E-SE aspects near and above treeline. Wind Slabs prone to triggered release are possible on slopes of 35 degrees and steeper. Typically, Wind Slabs may be encountered below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around freshly formed drifts and hollow sounding slabs on steeper terrain. Due to the strength of these winds, even treed slopes that may normally be sheltered from the wind could have sensitive wind slabs. 

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.

Two layers deeper in the snowpack pose a Persistent Slab hazard. The storm snow is resting on top of a shallow layer of near surface facets over a melt-freeze crust, which is resting on top of another shallow facet layer. An additional persistent weak layer of facets is very deep in the snowpack, a ~ 4”(~10cm) thick faceted layer overlaying a basal melt freeze layer at ground.  This faceted layer can be found above ~ 9,000’ on NW-N-NE-E facing slopes. Though these weaknesses are deeply buried, a triggered release higher in the snowpack could easily step down into these deeper layers, possibly producing a large very destructive avalanche.

 

Snowpack Discussion

 

The season’s first significant winter has exited the region leaving in its wake 1 to 2 feet of new snow at the lower elevations and 2 to 4 feet of new snow in the mid to upper elevations while ushering in cold Canadian air into our region. The system came in warm with rain/snain up to 9,000’ and moderate SW to Westerly winds Thursday. Temperatures began to fall overnight Thursday with snow rapidly accumulating above 9,000’.  Snow totals this morning (Rock Crk 15”/38cm, June Lake Ski Area Study Plot 12”/30cm, Gem Pass 24”/60cm, Tioga Pass Entry Station 32”/81cm, Virginia Lakes Ridge 15”/38cm, Mammoth Pass 39”/99 cm.

The recent storm has created a complicated snow picture depending on elevation and location. Areas near the Crest exhibit an inverted snowpack with a denser slab over lower density snow with Storm Slab properties and a number of facet crust combinations  deper inthe snowpack while areas further from the Crest are reporting a “right side up” snowpack, no density inversion, little or no facet formation and adhering well to underlying snow. Elevation also played a big role. During the storm, below ~9,000 rain persisted longer, which resulted in lesser new snow amounts and a thinner density inversion layer of only ~4” to 6” (10 to 15 cm). Above 9,000’ new snow accumulation was significantly more and exhibits a well-defined density inversion layer, 12” to 15” (30 cm to 38 cm) thick layer just below 3” (~8cm) of new snow at the surface in shelter areas. This Storm Slab is relatively unsupportive but is expected to settle and strengthen significantly overnight (Friday) becoming much more supportive, increasing the potential for propagation. Deeper in the snowpack the old/new snow interface consists of a thin layer of facets over a thin melt freeze layer with more facets directly beneath. The new snow is still forming bonds to the adjacent layer, which is tenuous at best. Deeper still is a layer of facets resting on top of a basal melt freeze layer at ground. These faceted layers pose a significant Persistent Slab avalanche concern for deep releases. Additionally, as you approach the mid and upper elevations, moderate to strong SW to W storm winds produced extensive Wind Slabs, most likely be encountered on NW-N-NE-E-SE aspects. Use extra caution around freshly formed drifts and hollow sounding slabs on steeper terrain. Due to the strength of these winds, even treed slopes that may normally be sheltered from the wind could have sensitive wind slabs. Areas north and south of Mammoth Lakes received lesser amounts of snow. The concern in the regions are recently formed Wind Slabs in the mid to upper elevations and a possible Storm Slab hazard where the new snow hasn’t fully bonded to the underlying snowpack.

Other concerns: early season conditions exists, especially outside of the Mammoth Lakes Basin. There are plenty of rocks, stumps, down trees just under the snow surface, use caution while riding and playing in the backcountry. 

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

 

 

recent observations

Red Cone Approach (12/16/16) – Ascended standard route up from the Tamarack Trailhead along the X-C ski trails to the summer cabins at the base of Lake George and ascended the standard route.  Consistent 24 +/-” (60cm) new snow from the trailhead up to Lake George. Above Lake George new snow depth increases dramatically with 140 cm new at 9900’. New snow had a classic density inversion approx. 6” to 12”(15 cm to 30 cm) thick just below the surface (~ 2”/ 5cm). This density inversion was reactive to Compression Tests (CTV/Q2/30cm from surface, CT3/ Q2/30cm from surface, CT6 Q2/30cm from surface). Extend Column Test was negative (ECTN3, Q2, did not propagate, failed directly under the shovel). Snow pit was dug at 9,900’, NW aspect, 32 degree slope, site was just below treeline. Snowpack depth 83”, 215cm.

Lake George, saddle above Crystal Lake (12/16/16) - Observed several natural releases on the Lake George Headwall (D1-2). Shooting cracks up to 2.5m from skis on firmer wind affected snow, and kick-turn tests easily sheared 30-45 cm from surface as we skinned up. Quick test pit at 9700', NE aspect, 32 deg. slope. HS 180cm. Obvious storm snow layer at 157cm from bottom (CTE). Facet/crust layer was found at 60cm from bottom in this location, and two CTV results failed on this layer (one on isolation with saw, one on placing the shovel on the column). Ski conditions on the lower angle slope were not great.

Mammoth Mtn. (12/15/16) – ~1ft of dense new snow at 9,000' by 5pm.  Ski cut missions throughout day resulted in numerous avalanches with up to 1ft crowns especially in locations with wind deposition occurring across mid and lower mountain (9,000-10,000ft).  Many slopes were ski cut numerous times during the day. 

Mt Gibbs, N Facing Chute (12/14/16) - Dirt Rd clear to Gibbs Lake TH parking (locked gate).  20 minutes of walking on summer trail until enough coverage to skin.  6-12" of dense consolidated snow to Gibbs lake (very supportable with most obstacles visible), very variable with some areas of icy glaze (adventurous skiing back down following summer trail).  1-4ft+ of snow from Gibbs Lake up middle N. facing couloir.  Couloir quite well filled from windblown snow from the various storms.  (see attached pic of approach). Pit dug at 9900' just below mouth of couloir (see observation for profile and picture).  60cm deep snowpack, slightly moist throughout at noon (air temp was 0ndeg C).  2 CT tests produced no results, while one failed just above ground with hard force, very rough Q3 fracture. Multiple hand pits and one test pit dug in various locations in couloir testing upper windload / slab.  Consistent failures with hand shears and CT tests ~5" down from surface a 2-3cm layer of slightly less dense snow, however failures took moderate force, and ECT tests did not fail or propagate.  Some very small isolated slabs failed on edges of couloir from ski/board compression force.  Snow depth >1meter in couloir.  Potential deep buried facet layer may exist in this area. 

weather

Sat thru Sun - Northerly flow aloft combined with a cold Canadian airmass moving into the western Great Basin this weekend will keep valley temperatures well below average. The coldest day is expected to be Sunday as an upper disturbance passing well east of the region brings a reinforcing shot of cold air. The cold reinforcement Sunday is shown to be fairly shallow as an upper ridge builds overhead. This will strengthen inversions Sunday with afternoon temperatures fairly similar in valleys and at higher Sierra locations on Sunday with highs in the upper teens and twenties while lows will be range from the single digits to low teens.

Monday thru Tues – Beginning of the week begins cold and clear with a weak northerly tracking system brushing the area on Tuesday. The system will move through the Pacific Northwest into Idaho and Montana with the tail end of an associated cold front sweeping through northern NV with light precipitation limited to along the front, closer to the border will likely be snow, but if any moisture makes it farther south, a wintry mix is possible. 

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: SUNNY CLEAR SUNNY
Temperatures: 23 TO 28 deg. F. 3 TO 11 deg. F. 24 TO 29 deg. F.
Wind direction: NORTHWEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 45 MPH IN THE MORNING BECOMING LIGHT. LIGHT WINDS. GUSTS UP TO 25 MPH IN THE EVENING. LIGHT WINDS. GUSTS UP TO 30 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: SUNNY CLEAR SUNNY
Temperatures: 18 TO 24 deg. F. 6 TO 12 deg. F. 21 TO 26 deg. F.
Wind direction: NORTH SHIFTING TO THE NORTHWEST NORTHWEST NORTH
Wind speed: 20 TO 30 MPH WITH GUSTS TO 60 MPH DECREASING TO 15 TO 20 MPH WITH GUSTS TO 35 MPH IN THE AFTERNOON. 15 TO 20 MPH WITH GUSTS TO 35 MPH. 15 TO 20 MPH. GUSTS UP TO 25 MPH INCREASING TO 35 MPH IN THE AFTERNOON.
Expected snowfall: 0 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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