Eastern Sierra Snowpack Summary - Dec 11 2016

The backcountry is more accessible than it has been in years. Please help!  ... Click Here to Find Out How

THIS ADVISORY EXPIRED ON December 13, 2016 @ 8:05 am
Avalanche Advisory published on December 11, 2016 @ 8:05 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
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.

Recent moderately strong westerly to southerly winds over the past 24 hours have formed shallow wind slabs on NW-N-NE-E-SE aspects near and above treeline. Isolated wind slabs prone to triggered releases 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. Though these tender pockets may not be big enough to result in burial, they could entrain a rider and carry them over and into hazardous terrain with potentially high consequences.

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.

Recent new snow rests on top a well-developed melt freeze crust above ~ 9,700’ on most aspects and has had little time to bond to the underlying melt-freeze crust. Small shallow triggered release are possible on slopes of 35° and steeper are possible. Though these tender pockets may not be big enough to result in burial, they could entrain a rider and carry them into over and into terrain with potentially high consequences. 

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.

A significant weak faceted layer, 10cm+/-, can be found above ~ 9,000’ on NW-N-NE-E facing slopes on top of a thick melt freeze crust directly above ground. Human triggered avalanche are possible on slopes >35° on NW-N-NE-E aspects, especially above 10,000’, investigate the snowpack for yourself and perform your own stability tests. 

Snowpack Discussion

The snow season is off to an interesting start with a complicated snow structure developing, especially with the most recent storm and snowfall. The season started off with two significant warm storms in mid and late October dropped close to 6” of water in the Mammoth Basin with rain reaching well over 11,000’.  This resulted in the formation of thick melt-freeze crust above ground that can be found between 9,000’ to 11,000’+. A couple weeks of dry weather followed these storms, which resulted in the development faceted snow above the thick ice crust above ground.  Below 9,000’ the snowpack disappeared in most places. On November 27th, a colder storm system dropping almost 1.5” of water and a foot of snow at the 9000’ level, with strong post-frontal winds. Avalanche control on Mammoth Mountain produced a number of significant avalanches, some propagating considerable distances. (1.5- 4ft crowns!).  These avalanches failed in the facet layer above the upper thick rain crust. The latest storm system to move through the region came in very warm with moderate Southerly to Southwesterly winds. Precipitation amounts range from .5 to 1.6” of water over the past 24 hours. Initially, the storm produced 2 to 6” of new snow above ~9,500’ before changing to light rain and freezing drizzle. Post-frontal passage saw temperatures cool resulting a the formation of a 1 to 2”melt-freeze crust capping the the new snow. Where the crust is well-developed and supportive (~9,700’) it will limit the stress applied to the facets below. However, where the crust is weak and unsupportive (uppermost elevations or below ~9,700’), the crust may not be as supportive and well-developed, which may allow riders to stress the slab overlying the facets and trigger a release.  The likelihood of human triggered releases is unlikely below ~9000’ with only 6 to 12” snowpack with minimal layering but as you ascend above 9,500’ triggered avalanches are possible. Keep an eye out for wind slabs, shallow unstable storm slabs, as well as the presence of persistent slab weaknesses in the upper elevations on E-N-NW facing slopes. If you are going into the backcountry, take the time to dig a hole and assess the facet layer under the Nov 17th storm snow.

South of Mammoth, snowfall amounts have been lower, making access to skiable and rideable terrain even more difficult and long approaches.

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

 

recent observations

Red Cone, Mammoth Lakes Basin (12/10/16): Ascended the standard route past Lake George. Consistent 6 to 12” of moist snow with minimal layering from ~8500’ to ~9,500’. Above ~9,500’, snowpack ~12” to 36” with ~2” of new snow over melt-freeze crust. The crust begins as unsupportive but gains strength with increased elevation. Moderate westerly to southwesterly winds loading NW-N-NE-E-SE aspects near and above treeline. Initially, due to the warm temperatures, snow accumulation was minimal. As the day progressed, temperatures began to cool with snow accumulating to ~three” by mid-day.

Snow stability tests produced moderate to hard failures on facets resting above the thick melt-freeze layer directly above ground. The new snow is still strengthening and developing bonds to the recently formed underling melt-freeze crust near the surface. One steeper terrain, ski tests easily peeled off the new snow and given the moderate Southerly to Southwesterly winds, anticipate sensitive shallow slabs or wind slabs forming near and above treeline on NW-N-NE-E-SE aspects.

 

weather

A break in the precipitation today and Monday before another round of storms moves into the region Tuesday through Thursday and brings the threat for more significant precipitation with travel possible impacting travel Tuesday morning and could persist into the weekend.

Sunday thru Monday - Water vapor satellite imagery currently showing a swath of dry air pushing across northern Nevada behind our departing atmospheric river system that provided copious rainfall to the Sierra. Areas of fog forming in Sierra valleys possible in the early AM hours. Generally seasonable temperatures and light breezes thru Monday.

Tuesday - The next system will move in early Tuesday with a prolonged period of precipitation continuing into the weekend.  An upper level low off the Pacific Northwest will send the next round moisture across the Sierra on Tuesday with colder temperatures than the recent AR system. An expansive cold continental low sitting across much of the US-Canadian border will slow progression and push the moisture plume farther south with the plume’s northern edge around the Tahoe Basin. Snowfall to become widespread across the Sierra snow levels possibly down to all valleys floors. Snow levels will rise through Tuesday as moisture and warmer air moves in aloft allowing for a change over to rainfall through Tuesday evening at the lower elevations.

Long-term

Models are coming into fairly good alignment with the second half of the next system will affect the region with a decidedly mixed bag of weather hazards. Late Tuesday the overall flow begins to buckle as a long wave trough digs offshore, which will raise mid/upper level temps and heights and push the snow levels above 7000 feet in many locations by late Wednesday afternoon with a decrease in precipitation and strong winds possibly developing as the precipitation lifts north late Wednesday afternoon into Wednesday evening. A deep plume of moisture moves onshore Thursday and spreads throughout northern California and northwest Nevada. A cold front pushes through much of the region by late Thursday with moderate to locally heavy rain possible with high snow levels initially...falling to the valley floors by late Thursday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Sunday Sunday Night Monday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW IN THE MORNING. MOSTLY CLOUDY. MOSTLY CLOUDY.
Temperatures: 37 TO 43 deg. F. 20 TO 26 deg. F. 37 TO 43 deg. F.
Wind direction: WEST WEST SOUTHWEST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 55 MPH 15 TO 20 MPH WITH GUSTS TO 55 MPH 10 TO 15 MPH WITH GUSTS TO 45 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW IN THE MORNING. MOSTLY CLOUDY MOSTLY CLOUDY
Temperatures: 31 TO 39 deg. F. 18 TO 24 deg. F. 29 TO 37 deg. F.
Wind direction: WEST WEST WEST
Wind speed: 20 TO 35 MPH. GUSTS UP TO 55 MPH INCREASING TO 65 MPH IN THE AFTERNOON. 20 TO 30 MPH WITH GUSTS TO 60 MPH. 20 TO 30 MPH WITH GUSTS TO 60 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 48 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the Eastern Sierra Avalanche Center who is solely responsible for its content.

ESAC receives support from ...