Avalanche Advisory - Sat, Jan. 28, 2017

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THIS ADVISORY EXPIRED ON January 31, 2017 @ 4:50 am
Avalanche Advisory published on January 28, 2017 @ 4:50 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 72 hours

Isolated newly formed wind slabs sensitive to human triggering in mid to upper elevations, primarily found under ridgelines and near and adjacent to exposed terrain features that promote drifting and loading. Friday, Northeast to Easterly winds formed fresh Wind Slabs above ~9,500’, especially along the Crest, primarily on N-NW-W-SW-S-SE facing slopes. North to Northeast winds of 15 to 25 mph are forecasted for Saturday over ~9,500’, which will continue to transport snow from windward aspects and forming isolated sensitive Wind Slabs on leeward slopes (NW-W-SW-S-SE-E). Sunday thru Monday winds will ease somewhat and swing toward the Northeast forming isolated Wind Slabs, from ~9,500’ and above, primarily on NW-W-SW-S-SE aspects.

Mid to upper elevations - Heightened avalanche conditions near ridgetops and in and around terrain features that promote drifting and loading. Natural avalanches unlikely; human- triggered avalanches possible.  Evaluate snow and terrain carefully and identify features of concern. Caution in steep sheltered terrain where the snow is loose and unconsolidated, sloughs are likely.

Low Elevations – Stability is improved over the course of the past few days. Overall generally safe avalanche conditions but watch for isolated or localized Wind Slabs in gullies, cut banks, and crossloaded terrain features. 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.

Winds at the lower and mid elevations have been relatively light over the past couple of days allowing Wind Slabs that formed during the recent Atmospheric River event storm time to stabilize.  However, above 10,000’, moderate North to Northeast winds continue to transport the low-density snow onto NW-W-SW-S-SE-E aspects, forming isolated Wind Slabs below ridgelines and along the sidewalls of couloirs, gullies, and depressions on slopes greater than 35 degrees on NW-N-NE-E-SE aspects.  Look for blowing snow, fresh cornices, denser snow/over low-density snow, and smooth hollow sounding slabs. Do your own stability assessments in safe terrain to see how recent and sensitive wind slabs are. Caution on wind loaded slopes steeper than 35 degrees, leeward convexities, slopes below ridgelines, and the side walls of gullies.

Mid to Upper Elevations - Natural avalanches are unlikely; human- triggered avalanches possible on leeward slopes, near and around terrain features that promote drifting. Use caution and avoid exposed terrain 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.  

Low Elevations (below 8,000’) - Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Natural and human- triggered avalanches unlikely. Small avalanches in isolated areas or extreme terrain.

 

Avalanche Character 2: Loose Dry
Loose Dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Loose Snow – Mid to lower elevations, there is the possibility of rider triggered loose snow releases in sheltered steep terrain where the snow is still loose and unconsolidated. Though not a danger for burial, can force a rider into hazardous terrain. 

Snowpack Discussion

Last week’s Atmospheric River (AR) event has left the region with substantial new snow amounts in its wake, some areas exceeding 70”. The storm ushered in one of the largest natural avalanche cycles for the season, to date. During the storm, an avalanche released on McGee Mountain and struck a house, which sustained significant damage. Storm slabs that formed during the storm have stabilized. As the system moved off to the east, high pressure moved into to dominate the regions with clear skies and somewhat unseasonably cold temperatures. Winds in the low and mid elevations have been relatively light during this time. However, moderate North to Northeast winds have continued to produce wind slabs in favored locations in the upper elevations because of continued wind transport. Sensitive Wind Slabs will most likely be encountered below ridgelines, adjacent to terrain features that promote drifting and loading in the upper elevations (above ~9,500’). As temperatures warm, especially Sunday/Monday, minor wet roller-ball on solar aspects around rocky terrain features in the low to mid elevations are possible with isolated Wet Loose possible at the lowest elevations.

 

recent observations

Sherwins, Mammoth Basin (1/27/2017) - Quick trip up the Sherwins from the end of Old Mammoth Rd., up the shoulder behind Mammoth Rock, walked along the Sherwin Ridge, dropped down over shoulder behind Mammoth Rock onto the benches below Mammoth Bowl, back down to car.

-Light NNE winds on the skin up. Snow surface showed signs of recent scouring from up slope N winds. 2mm thick wind skin at snow surface. Lots of wind affected snow in open areas. Past SW winds created small cornices and thick pillows on N aspects.

-Settlement cones around almost all the trees in The Hose area and decreased ski penetration from a few days ago show that snow is settling and getting thicker in the upper pack. Top 25cm are still F hardness over a 4F+ layer. Hand shears pulled off on top of the 4F+ layer with hard force, sometimes planar, sometimes rough.

-Debris from several past (>24 hrs old) avalanches still visible on looker's right of Mammoth Bowl and under the ridge above The Hose. All aspects: NE. All slides R1D1 and shallow. The three I saw appeared to have been skier triggered (intentional?) with either cornice drops or ski tracks above or near the debris.

-At the top of the ridge lots of show transport from moderate N winds. Snow banners pointing S from the Sherwin Ridge and the Mammoth Crest seen in the vicinity of Duck Pass. Cornice kick tests on N aspect above The Hose broke around my skis with some hard stomping. Resulting blocks created small shallow sluffing and didn't run more than 20 feet. It seems that there are still some small instabilities at the Sherwin Ridge line and on other features loaded by previous SW winds, but that they are becoming less reactive. Current northerly winds are stripping slopes near ridgeline and blowing the snow onto S facing aspects.

-Best skiing was on wind-sheltered slopes. Areas exposed to either past SW or current NE winds were a variety of wind board, breakable wind crust, or just thick powder.

Bishop Bowl, Bishop (1/27/17) - From parking at 8400 feet, toured to top of Bishop Bowl (~12,000 ft).  Light surface hoar seen at start of tour.  No cracking or whumping noted throughout tour. At start (8am) of tour we noted some banners from peaks to the north, with the wind out of the northwest.  By the time we reached the "summit" (11am) winds were calm, no snow banners noted

Snow stability and conditions were similar to those reported yesterday by observers.  We dug number hand shear pits on open, low angle northeast facing slopes.  In general, these showed 20 cm deep, fist hard snow over a firmer layer.  This firmer layer varied from 1 cm ice to 1 finger hardness and was generally well bonded to snow above.  Of note was a 5-10 cm thick wind crust (surface) on east facing slopes, near the ridge top at 11600 ft.  Under this slab is loose, poorly bonded snow.  Quick tests showed this surface layer to be easily reactive upon isolation. Temperature at 12,000 ft was -5 degree Celsius at 11am.

Variable, crusty skiing up high, followed by some very good turns back to the car.

Negatives, June Mountain (1/27/17) - Route:  Off of top of June up hourglass, across top of negatives ridge, down first main solar-bowl chute, up Hemlock, down to fern grotto and down treed slopes to Rd.

-Evidence of overnight and morning N to E winds up above 10,500'.  Many hand pits dug along route, areas without overlying windslab no layers of concern detected. 

-Areas with denser overlying newer windslab: typical thickness from 1-5cm, Pencil hard over Fist plus.  Hand shears failed with moderate force, with variable shear qualities.  Some thinner (2-3cm) areas failed with easy force.  None of the thicker spots tested failed with much energy. 

-Did an ECT in one of the more suspect areas we could find with a thicker (5cm) slab, 3 ECT tests did not propagate, but failed with 12-13 taps under that windslab layer.  (indicating low propagation propensity).  10,500', N facing, 36 degrees. 

-CT test at top of NW facing chute (11,200'), 40 degree slope, heavily windloaded from previous SW winds = CTX.  {Pencil to 1Fplus hardness at least 90cm down.)  Welded.

-Skied SE facing sunny line at 1:15pm, no noticeable moistening at surface, slight softening.  Creamy.

-More north facing aspects seemed more wind effected. 

-Western ridgeline of hemlock ridge had obvious wind-board.  Treed slopes had smooth consolidated powder all the way to Rd level.

-Light to moderate north winds at ridgetop (11,000') at 1pm.  Not much wind lower down below ridge.  Visible snow banner off the top of Mt. Ritter and a couple other 12,000' ridges nearby. 

 

 

weather

Sat thru Sun – High pressure dominates the region with moderate temperatures and Easterly winds thru Sunday morning with the strongest winds along the immediate Sierra crest. The gradient should remain intact through at least Sunday afternoon with modest easterly winds continuing at and west of the Crest.

Mon thru Tuesday - The ridge of high pressure will remain intact over northeast CA through Monday. Mountain temperatures increasing due to the strong subsidence (descending motion of air in the atmosphere occurring over a rather broad area.) warming under the ridge. Ridge will continue to push east on Tuesday with a deep Pacific trough developing off the West Coast with increasing ridgtop winds on Tuesday as the trough moves closer to the coast. 

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 THEN BECOMING PARTLY CLOUDY. PARTLY CLOUDY THEN BECOMING CLEAR. SUNNY.
Temperatures: 36 TO 41. deg. F. 13 TO 23. deg. F. 40 TO 45. deg. F.
Wind direction:
Wind speed: LIGHT WINDS. LIGHT WINDS. LIGHT WINDS.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: SUNNY. PARTLY CLOUDY THEN BECOMING CLEAR. SUNNY.
Temperatures: 32 TO 38. deg. F. 20 TO 26. deg. F. 38 TO 43. deg. F.
Wind direction: NORTHEAST LIGHT WINDS BECOMING NORTHEAST NORTH
Wind speed: 15 TO 25 MPH. GUSTS UP TO 40 MPH DECREASING TO 30 MPH IN THE AFTERNOON. 10 TO 15 MPH WITH GUSTS TO 25 MPH AFTER MIDNIGHT. 10 TO 15 MPH WITH GUSTS TO 30 MPH IN THE MORNING BECOMING LIGHT.
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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