Eastern Sierra Snowpack Summary - 2016-01-21 07:35

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THIS ADVISORY EXPIRED ON January 23, 2016 @ 7:35 am
Avalanche Advisory published on January 21, 2016 @ 7:35 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.

The recent warm temperatures and light winds have given the snowpack some time to adjust to the new snow load (bonding between layers and sintering between snow grains) as evidenced by 1” to 3” of settlement noted at many remote reporting stations (Mammoth Pass Snotel, 2”).

However, strong Southerly winds (20 to 25 mph, increasing to 55 to 60 mph, gust 90 mph after Midnight) are forecast begining Thursday afternoon with snow by Friday AM, which will produce new windslab concerns on leeward slopes (NW/N/NE/E/SE) 35 degrees and steeper, especially newly deposited drifts, cross-loaded gullies, below ridgelines. These wind slabs will be denser, possibly hollow sounding. Nearby cornice formation and rippled snow surface can be clues to where these wind slabs may exist. While exposed slopes will be of most obvious concern, high winds can form windslabs in open forested terrain. These deposits in more protected terrain may only be slightly denser than snow in un-wind affected areas.  Shooting cracks are an obvious warning sign that these softer wind deposits exist and may be unstable. These avalanches will likely relatively small, they could carrying a person into objects (rocks, trees, etc.), or undesirable terrain (cliffs, terrain traps, etc.).

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.

A growing concern for Friday into the weekend is the potential for storm slab instabilities with 1’ to 2’ forecasted throughout the Crest. As the snow accumulates Friday thru Saturday, avalanche danger will increase even in wind protected areas on slopes steeper than 35 deg. at all elevations and aspects.  It is unknown how well this new, relatively dense snow will bond to the underlying snow surface. The potential for natural avalanche will increase through the weekend and triggered avalanche will be possible on terrain 35 degrees and steeper. Reassess conditions regularly and be willing to adjust route selection, both ascending and descending.

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.

Basal facets still exists throughout much of the forecast area, which formed early season due to the shallow snow conditions and cold temperatures.  The likelihood of human triggered avalanches at this level is remote; this next storm has the potential to provide enough stress to reawaken deep instabilities and producing large natural avalanches.  This layer is especially a concern north of June Lake and south of Mammoth Lakes where the snowpack is inherently shallower. 

Added note: A layer of Surface Hoar and Dendrites formed early January and was subsequently buried under several inches of snow. This layer is variably distributed throughout the sheltered forested terrain surrounding Mammoth Lakes Basin and June Lake (especially the lower elevation trees), as well as other locally favored areas. Where present, it has shown consistent mod-hard failures with clean planar faces and propagation tendencies. This layer may become more reactive this weekend with further loading. Because of its varied distribution, this should be considered when riding steep sheltered lines. Stay close to your riding partners, ride one at time, have avalanche rescue equipment and know how to use it. 

Snowpack Discussion

Avalanche danger increased with the last two storm systems with a total of 17” or snow and 2.64” of Water Equivalent (Mammoth Pass Snotel, 9300’) over a 5-day period accompanied by strong Southwesterly winds. Evidence of natural avalanche activity was observed in Red Cone Bowl along the western flank as well as reports of natural activity skier’s left of the Hose along Sherwin Ridge. The latest storm came in cool and exited with strong winds, warm temperatures, and increased snow density producing the classic upside down snowpack (higher density snow over lower density snow), which produced widespread windslabs on leeward slopes (NW-N-NE-E-SE aspects), especially mid to upper elevations where there is an abundant supply of transportable snow. Wednesday dawned clear and cold but warmed quickly to above normal, which helped the snow to settle, easing the windslab brittleness a bit, while promoting bonding to the previous snow surface. Forecasted mild temperatures and light winds early Thursday will allow further strengthening to continue prior to the arrival of the next storm system (late Thursday). However, caution is still advised while traveling in avalanche terrain on NW-N-NE-E-SE facing slopes, 35 degrees and steeper, especially areas prone to windloading and drifting, or on slopes with strong warming (Southerly aspects). Likely locations: below ridgelines and cross-loaded slopes. Exposed slopes are the most obvious concern but high winds can form soft windslabs prone to human triggering in open forested terrain. While these avalanches may be relatively small in nature, they could easily carry a person into obstacles, into undesirable terrain, or trigger existing deeper weak layers and initiating larger avalanches. This is especially a concern in areas outside of the Mammoth Lakes Basin where the snowpack is shallow in nature and weaker overall.

With the arrival of the next system and the threat of additional windslabs, there’s potential of 1-2 feet of added load with these next storm systems and the potential for storm snow instabilities. Additionally, the added stress to the snowpack from this new load will increase the possibility that lingering persistent weak layers (Surface Hoar/Stellars) or the well documented deep persistent facets above ground to become more reactive and fail, increasing the possibility of natural avalanches as well as an increased threat of human triggered releases at all elevations  

recent observations

(1/20/16) Mammoth Lakes Basin received 12”-24” snow with 2.64” snow water equivalent over a 5-day period. Recent control efforts by the Mammoth Patrol produced moderate results on the upper mountain (R2-D2s) with 1-3' crowns confined to the new snow from the most recent storm systems. This somewhat mimics conditions in the backcountry. Avalanche debris was noted along the western basin floor of Red Cone Bowl as well as reports of natural activity skier’s left of The Hose along Sherwin Ridge.

Snow stability tests from the Red Cone: compression tests scores of mod to hard failures in the new snow (32cm/39cm from surface) with an irregular bed surface surface (Q3). Consistent hard compression test scores were observed (59cm/70cm from surface) on preserved Dendrites and Surface Hoar on East and West aspects, above 9800 feet (slope angle 33 degrees, 40 degrees) Q1 to Q2, Extended Column Test (East Slope) ECTP, 22, 59 from surface.  Test slopes in sheltered treed terrain were inconclusive.

 

weather

A pair of weather systems will begin to push into the region late tonight (Thursday) into Saturday with strong winds developing across ridges this afternoon, with increasing winds of 50 to 60 mph, gusts of 80-100 mph by midnight. The first frontal system will move into the Sierras by early Friday with plenty of moisture. The Jet Stream will briefly stall over northern California, keeping southerly wind flow over the Sierras and the majority of the precipitation along the Sierra Crest and western slopes and limiting snowfall east of the Sierras (aka, shadowing). Snow levels starting out around 6000-7000 feet on Friday.

The second shortwave (a disturbance in the mid or upper atmosphere which produces upward motion ahead of it) moves through late Friday night into Saturday with colder temperatures, lowering snow levels, and increasing precipitation with a better chance of moisture spilling over into the eastern Sierras. Snow will continue in the Sierras on Saturday with snow levels falling to around 4000-5000 feet behind the front with the unstable conditions and spillover into the lee of the Sierras with forecasted storm totals of up to 1-2’ in Sierras above 7000 feet, 4 to 8 inches expected along highway 395 in Mono county. Behind the cold front, breezy winds will make for a brisk day on Saturday. 

Sunday - Cool and mainly dry conditions will prevail as High Pressure builds along the west coast, with areas of mid-high level clouds spreading across the region.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY CLOUDY. MOSTLY CLOUDY. CLOUDY. CHANCE OF SNOW.
Temperatures: 35 TO 42 deg. F. 24 TO 30 deg. F. 30 TO 36 deg. F.
Wind direction: LIGHT WINDS BECOMING SOUTH. SOUTH SOUTHWEST
Wind speed: 10 TO 20 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON. 15 TO 25 MPH WITH GUSTS TO 35 MPH INCREASING TO 25 TO 45 MPH WITH GUSTS 50 TO 70 MPH AFTER MIDNIGHT. 30 TO 50 MPH WITH GUSTS TO 70 MPH.
Expected snowfall: 0 in. 0 in. UP TO 2 INCHES. in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY CLOUDY. MOSTLY CLOUDY. CLOUDY. CHANCE OF SNOW IN THE MORNING. SNOW LIKELY IN THE AFTERNOON.
Temperatures: 27 TO 35 deg. F. 21 TO 27 deg. F. 25 TO 30 deg. F.
Wind direction: LIGHT WINDS BECOMING SOUTH SOUTHWEST SOUTHWEST
Wind speed: 20 TO 30 MPH WITH GUSTS 35 TO 55 MPH IN THE AFTERNOON. 25 TO 45 MPH WITH GUSTS TO 70 MPH INCREASING TO 40 TO 60 MPH WITH GUSTS TO 90 MPH AFTER MIDNIGHT. 45 TO 65 MPH. GUSTS 80 TO 100 MPH.
Expected snowfall: 0 in. 0 in. UP TO 4 INCHES 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 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the Easter Sierra Avalanche Center who is solely responsible for its content.

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