Snowpack Summary - Tue, Dec. 27, 2016

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THIS ADVISORY EXPIRED ON December 29, 2016 @ 6:54 am
Avalanche Advisory published on December 27, 2016 @ 6:54 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Isolated Wind Slabs along ridgetops and in and around exposed terrain features that promote drifting in the mid to upper elevations. These may be  encountered primarily on N-E-S aspects. Natural avalanches are unlikely, triggered releases are possible where wind slabs have formed. Usual caution in terrain that 35 degrees and steeper. As the upper snowpack continues to facet, Loose Dry releases may be possible in steep terrain. Generally not a burial concern unless combined with a terrain trap. Though small in nature, they can travel fast and entrain a rider quickly and possible dragging a rider into hazardous terrain. Early season variability exists throughout the Range, assess the snowpack before you commit and beware of thin snow cover in the lower elevations and in wind exposed locations.  

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 Westerly winds thru Wednesday will likely form isolated Wind Slabs on N-E-S aspects in the mid to upper elevations.  Wind Slabs will primarily be encountered along ridgetops and in and around exposed terrain features that promote drifting. Wind Slabs will likely be relatively small in size but they could easily capture an unsuspecting rider and drag them into hazardous terrain or if combined with a terrain trap, possibly result in burial. Avoid hollow sounding slabs or freshly formed drifts.

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.

As the upper snowpack continues to facet, Loose Dry releases may be possible in steeper terrain. Generally not a burial concern unless combined with a terrain trap. Though small in nature, they can travel fast and entrain a rider quickly and possible dragging a rider into hazardous terrain.  

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

Snowpack Discussion

A strong winter storm moved through the region Christmas Eve leaving 1 to 2 feet of low-density snow along the Crest, lesser amounts elsewhere. Snow fall amounts – Virginia Lakes 7”, Tioga Pass Entry Station 11”, Gem Pass 13”, Mammoth Pass 14”, Rock Creek 10”, and Big Pine Creek 15”. Winds during the storm were moderate forming isolated wind slabs along ridge tops and in and around exposed terrain features. Otherwise, minimal slab formation within the new snow with no major density changes within the new snow or adjacent layers. Cold temperatures continue to drive strong facet formation in the upper 1/3 of the snowpack, which makes for good skiing but could become a problem once buried and is worth noting and monitoring as it becomes buried. The weather through most of this week is relatively benign but moderate westerly winds may form isolated wind slabs on N-E-S aspects in mid to upper elevations along ridgetops and in and around exposed terrain features where drifting occurs. As the upper snowpack continues to facet, loose dry sluffs may be possible on steep slopes. These will be small in nature but if entrained, a rider could be dragged into hazardous terrain.

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.   

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

recent observations

Mammoth Rock Bowl, Sherwins (12/26/16) -

Lower elevations below Mammoth Rock (Trailhead up to ~ 9000) - snow depth varies considerably with elevation, lowest elevations plenty of objective hazards lurking just below the surface, especially where tree cover is thicker or slopes exposed to prevailing winds. Above ~9300, obvious avalanche activity, possibly from the Mid December cycle, now partially obscured by the latest storm  (12/23-24/2016). Mid to upper elevations coverage is good, skiing is overall good. Stability test - CTH21,23,25, Q3, @ 105 cm. No recent avalanches visible, small isolated wind slabs possible along exposed ridgetops and in and around terrain features, and vertical crests. Current faceting in the upper snowpack is making for good skiing now but is worth noting and watching as it gets buried.

Observer: doug lewis, Eastern Sierra Avalanche Center, Dec 26, 2016, 7:41 PM

Loc. - Mammoth Rock Bowl

Lat/Lon:37.6136, -118.9941

Elevation: 9150’

Aspect: NE Angle: 38°

Signs of Instability: Wind Loading

Sky Cover: CLR

Precipitation: No Precipitation

Wind Speed: Light subjective

Wind Direction: SW

Current Temp:-5.5°C Trend:➘

Blowing Snow: None

Snow Depth: 144cm

Snow Temperature 20cm:-10.5° C

Boot Penetration: 40cm Comments: Ground cover- talus. Crown visible top of bowl, Recent avalanche debris covering top 1/3 of bowl, partially covered by snow. Shallow isolated wind slabs near & around exposed terrain features.

Punta Bardini, Sherwins (12/24/16) - Snowline dropped to valley levels. Moderate morning winds, not transporting snow on Punta, but visible over ridgeline beyond Solitude.  Fairly calm with no more transport by noon. 35-40cm new snow at 9,000' and at most elevations (8,000' - 10,200') where wind did not transport. Pit dug at ~9,200' NNE aspect revealed overall stable snow structure.  Facet / Ice layers seem to be bonding better, facets are rounding. ECT test done just below ridgetop/summit in more dense area of wind deposit failed very easily, but had break. On Punta this dense of a wind slab was only found in limited locations just along ridgetop.  But it does indicate that in other more exposed areas, sensitive wind slabs are likely to exist. Main Bardini Chute viewed from the side, evidence that it slid during the storm, couldn't make out crown, could have been some heavy sloughing.  Evidence of small wind slab avalanches along ridgeline. Some small shooting cracks in exposed areas near ridgeline where soft wind slabs formed (1-4" for most part). We stayed almost entirely on slopes <32deg, and assessed small steeper pitches closely.  Silly good powder snow in Old Growth trees and Tele Bowls, and surprisingly my partner and myself didn't hit a single rock or stump from top to bottom of Tele Bowl despite the fairly low amount of snow.  Pick line wisely, be cautious, and it can be quite good.  The approach slopes were still quite thin and bushy.   

 

weather

Tues thru Wednesday - Strong jet stream winds in the Pacific Northwest are pushing upper level moisture through northern California and Nevada. Mostly this is bringing mid and high level clouds to the region along with an increase in ridge level winds.

Thursday thru Friday - Strong inversions are expected to continue through Thursday with light valley winds and limited mixing. A gradual warming trend with daytime high temperatures rising to the mid to upper 40`s for most areas by Thursday. Increasing clouds are expected for the eastern Sierra by Thursday as a cutoff low-pressure system begins to move inland. A Rex Block (a high pressure system located immediately north of a low pressure system) will develop over the Eastern Pacific as we go into the weekend, which will strongly influence the amplified long wave pattern over North America. This pattern will favor trough formation over the western U.S., allowing shortwave energy to drop down out of Canada and into the Sierra and Great Basin, bringing increasing chances for showers and colder temperatures.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: PARTLY CLOUDY THEN BECOMING SUNNY. CLEAR SUNNY
Temperatures: 36 TO 41 deg. F. 12 TO 18 deg. F. 40 TO 45 deg. F.
Wind direction: WEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 50 MPH IN THE MORNING BECOMING LIGHT. LIGHT WINDS, GUSTS UP TO 55 MPH DECREASING TO 35 MPH AFTER MIDNIGHT. LIGHT WINDS, GUSTS UP TO 30 MPH IN THE MORNING.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: PARTLY CLOUDY THEN BECOMING SUNNY. CLEAR SUNNY
Temperatures: 29 TO 37 deg. F. 11 TO 17 deg. F. 33 TO 41 deg. F.
Wind direction: WEST WEST WEST
Wind speed: 20 TO 30 MPH. GUSTS UP TO 55 MPH DECREASING TO 45 MPH IN THE AFTERNOON. 25 TO 35 MPH WITH GUSTS TO 55 MPH DECREASING TO 15 TO 25 MPH WITH GUSTS TO 40 MPH AFTER MIDNIGHT. 15 TO 20 MPH WITH GUSTS TO 35 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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