Avalanche Advisory - Thu, Feb. 02, 2017

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

THIS ADVISORY EXPIRED ON February 4, 2017 @ 6:59 am
Avalanche Advisory published on February 2, 2017 @ 6:59 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 48 hours 

Wind Slab - Moderate to strong Southwesterly winds, above 10,000’, will form sensitive Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. Be wary of smooth hollow sounding snow. Caution on wind loaded slopes steeper than 30 degrees, leeward convexities, slopes below ridgelines, and the side-walls of gullies. 

Mid and Upper Elevations:
Thursday - Natural avalanches are unlikely; human- triggered avalanches possible on leeward slopes, near and around terrain features that promote drifting. 

Friday – Natural avalanches possible; human- triggered avalanches likely. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. 

Lower elevations (below 8,000'): 
Thursday - Natural and human- triggered avalanches unlikely. Generally safe avalanche conditions. 

Friday - Natural avalanches unlikely; human- triggered avalanches possible. Heightened avalanche conditions near terrain features with large adjacent fetches.

Storm Slab - The first in series of storms is moving through the region with 1 to 3 feet of snow forecasted by Saturday AM. Areas with the greatest potential for Storm Slab failures will be greatest in the Mammoth Lakes Basin along the Crest but may be encountered elsewhere were the new snow hasn’t fully bonded to the underlying snowpack. 

Mid to Upper Elevations:
Thursday  - Natural avalanches unlikely; shallow human- triggered avalanches possible as snow begins to accumulate.

Friday - Natural avalanches possible; human- triggered avalanches likely, especially near the Crest. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Low Elevations (below 8,000’) 
Thursday - generally safe avalanche conditions. Natural and human- triggered avalanches unlikely. 

Friday - Natural avalanches unlikely; shallow human- triggered avalanches possible. 

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 to strong Southwesterly winds, above 10,000’, will form sensitive Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. Be wary of smooth hollow sounding snow. Caution on wind loaded slopes steeper than 30 degrees, leeward convexities, slopes below ridgelines, and the side-walls of gullies.

Thursday - Natural avalanches are unlikely; human- triggered avalanches possible on leeward slopes, near and around terrain features that promote drifting.

Friday – Natural avalanches possible; human- triggered avalanches likely. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. 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.

Lower elevations (below 8,000"), the snow has settled significantly and has experienced appreciable warming and sun effect, which has helped strengthen to snow. As this storm progresses the lower elevations will see a mixture of rain and snow, with limited accumulation but isolated Wind Slabs may form in and around terrain features that promote drifting and crossloading, especially terrain features with large adjacent fetches.

Thursday - Natural and human- triggered avalanches unlikely. Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Natural and human- triggered avalanches unlikely.

Friday - Natural avalanches unlikely; human- triggered avalanches possible. Heightened avalanche conditions near terrain features with large adjacent fetches. Evaluate snow and terrain carefully; identify features of concern.

 

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.

The first in series of storms is moving through the region with 1 to 3 feet of snow forecasted by Saturday AM. The storm system came in relatively cool then warming as it progresses through the region before it cooling on exit. This has the potential to reduce initial bonding at the old/new snow interface. Areas receiving greater amounts of new snow will have greater potential for slabs and weakness within the new snow, which may be tender and prone to release. Storm Slab potential is greatest in the Mammoth Lakes Basin along the Crest but may be encountered elsewhere were the new snow hasn’t fully bonded to the underlying snowpack. This is primarily a concern for the mid to upper elevations, especially near the Crest.

Thursday (Upper and mid Elevations) - Natural avalanches unlikely; shallow human- triggered avalanches possible as snow begins to accumulate.

Friday - Natural avalanches possible; human- triggered avalanches likely, especially  near the Crest. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

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

Low Elevations (below 8,000’)

Thursday - 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.

Friday - Natural avalanches unlikely; shallow human- triggered avalanches possible.

 

 

Snowpack Discussion

The weather warmed dramatically toward the end of the fair weather period that followed the last Atmospheric River (AR) event, which exited the region on the Jan 23rd. Near record snow depths throughout the region have help bridge over deeper early season weaknesses. Over the course of the past 10 days the snowpack has settled and strengthened, adjusting to the recent heavy loading. As the weather cleared cold temperatures initially drove strong facet formation in the upper snowpack capped by a layer of Surface Hoar. Temperatures began to warm-up dramatically early during the week with many locations over 9,000’ recording temperatures well above freezing with some small wet instabilities observed on solar aspects. The warming temperatures helped to form a melt/freeze crust on most aspects below 9,500’ (N to NE, possible exceptoons) and while breaking down the Surface Hoar and form a melt/freeze crust capping the recently formed upper snowpack facets. Wednesday, strong pre-frontal Southwest winds were observed over the upper elevations forming fresh Wind Slabs in the upper elevations on NW-N-NE-E-SE aspects. Thursday, a moderate Atmospheric River is forecasted to impact the region with moderate to strong Southwest to South winds and moderate to heavy snowfall with 1 to 3 feet forecasted for the Crest by Saturday AM, lesser amounts as you move east. As a result, Storm Slabs will likely be encountered in the mid to upper elevations, especially near the Crest. The storm is forecasted to come in two distinct waves with a break Thursday afternoon to early Friday due to winds shifting to the south ahead of the second wave, which will favor snowfall along the Crest but limit amounts further East. As this system moves through the region, moderate to strong Southwest winds will form sensitive Wind Slabs on W-NW-N-NE-E-SE aspects in the mid and upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. Lower elevations, the snow has settled significantly and experienced appreciable warming and sun effect, which has helped strengthen to snow. As this storm progresses the lower elevations will see a mixture of rain and snow, with limited accumulation but isolated Wind Slabs may form in and around terrain features that promote drifting and crossloading, especially terrain features with adjacent fetches. 

Caution – Snow creep cracks have been observed throughout the forecast area with some large enough to swallow a person. They have been found at all elevations on steep slopes, base of cliffs, couloirs, and below buried rock bands.

Last Week's Blistering Hi Temps

Virginia Lks (9445’): Hi Temp 49 (1/31/17)

Gem Pass (10750’): Hi Temp 52 (1/30/17)

June Wx Plot (9148’): Hi Temp 52.6 (1/31/17)

Mammoth Pass (9500’): Hi Temp 58 (1/30/17)

Rock Crk (9600’): Hi Temp 53 (1/30/17)

Big Pine Creek (10,000’): Hi Temp 40 (1/31/17)

 

recent observations

Mammoth Basin (2/1/17) - Snow banners visible over Pyramid Peak and high ridgelines, SW winds.  50mph w 60mph gusts at 10,100'. 

Crest, Mammoth Basin (1/31/17) - Variable is the name of the game right now. Toured out of Tamarack and noticed lots of damp snow in all but sheltered NE - NW aspects below ~9500 climbing the Crystal/Red Cone ridge to the Crest. I didn't see significant rollerball activity, although I was mostly avoiding S. slopes for snow quality purposes. Still some nice recrystallized powder in the trees and on the N third of the compass. Above treeline a wide variety of old wind slabs and wind scouring dominate the snow surface. Some of the wind slabs are quite hard and up to 15 - 20 cm, but they don't seem reactive to me in hand pits and cuts on test slopes - I wasn't able to get any significant cracking anywhere. In places along the ridge, the wind has scoured down to old ice layers, so it's still worth being cautious of unexpected ice in steeper terrain, and it's possible that in isolated pockets that could provide a bed surface/dangerous interface after the next storm - it seems fairly isolated though, and I only found it right up on the ridgeline.

I skied a few lines in the Crystal Chutes and Jaws area and was surprised how much relatively good snow still existed - above ~9500 E and even SE aspects weren't totally destroyed by warming, especially where trees or rock features provided a bit of shading. There are few small old crownlines in Jaws that are almost certainly naturals from the wind loading late last week. When breaking trail up Jaws I was getting anywhere from 0 to ~15cm ski penetration depending on wind slab. I switched to booting for the last 100m to the ridgeline and found some pretty hard snow from the wind action requiring multiple kicks for each step for security. A whippet was nice for this section.

Earthquake Dome, Mammoth Basin (1/30/17) -

Lat/Lon:37.6674, -119.0017

Elevation: 9200 f

Aspect: NNW Angle: 32°

Signs of Instability: Pin Wheels

Sky Cover: -BKN

Precipitation: No Precipitation

Wind Speed: Calm subjective

Current Temp: 5°C Trend: →

Blowing Snow: None

Snow Depth: 335cm

Snow Temperature 20cm: -6.5° C

Boot Penetration: 35cm

Comments: Warm temperatures penetrating top 2-3 cm. 8000' wet, 9000' moist on N-E aspects. Surface Hoar over facets, decreasing in size with elevation, 6 mm at 8000', decreasing to  ~ 3mm @ 9000'. Surface Hoar (Laid over and rounding) and upper facets wet to moist, will likely form a melt/freeze crust once buried. Trees dropping their load forming roller balls on even sheltered slopes and shaded aspects. Stability Tests: CTN - Multiple tests yielded no results.  Lots of skier traffic in the area near skin track. Less as you move toward the east. Parking has been cleared in the traditional locations.

Negatives, June Mountain (1/30/17) - Toured in the Negatives, Hemlock Ridge. Saw widespread rollerball activity below about 10'200 ft on S- SE -E aspects. Steep terrain underneath rock bands. Looked like point releases, fan shaped but was mostly just the cumulative effect of all the roller balls from yesterday and today.

Snow on solar aspects today had MF crust on it. Still thin and it softened into good skiing. Top 3-4 cm of snow below 9500 ft seemed wet, balled up easily on solar aspects. The increasing cirrus cloud cover today made the snow grabby in some places below 9500 ft. Today, the clouds trapped a lot more heat and it really changed the snow below 10'000 ft even on N facing aspects. Still found widespread wind crusts between 1 cm to 5 cm thick on ENE aspects in wind affected areas on ascent through hourglass.  

 

 

weather

...Winter Storm Warning Remains In Effect until 4 Am PST Saturday Above 7000 Feet...

 * Timing: moderate to heavy snow this morning. Snow will decrease substantially in intensity or even stop this afternoon and evening before becoming moderate to heavy again late tonight through Friday.

 * Snow accumulations: 1 to 3 feet above 7500 feet...highest along the sierra crest...with 3 to 12 inches between 7000 and 7500 feet and up to 2 inches below 7000 feet.

 * Winds: south to southwest 15 to 25 with gusts up to 50 mph. Sierra ridge gusts up to 95 mph.

 * Snow levels: around 6000 feet this morning...rising to 7000 to 7500 feet this afternoon. Snow levels will then fall to around 6500 to 7000 feet Friday afternoon or evening.

Thursday thru Friday - winter storm impacting the area this morning (Thursday) through Saturday night. The first wave of the latest storm is bringing mountain snow across the Sierra this morning with snow levels near 6000 feet MSL rising to 6500-7500 feet this afternoon. Precipitation will decrease this afternoon into this evening before the next, stronger wave arrives late tonight. Moderate to heavy snow is forecasted for late tonight into Friday as a cold front crosses the region. 1 to 3 feet above 7500 feet...highest along the sierra crest...with 3 to 12 inches between 7000 and 7500 feet and up to 2 inches below 7000 feet. Snow levels lower to around 6000-6500 feet Friday and Friday night. Between about 6000-7000 feet elevation range in the Sierra, the snow level could be quite variable during this storm and periods of both rain and snow are likely. Locations in this elevation range could see snow accumulations anywhere from a couple of inches up to a foot. Southwesterly winds of 30 to 40 MPH Thursday, increasing to 40 to 60 Friday with gusts up to 100 mph.

Saturday - Moist, weakly forced flow continues to be expected through Saturday Wind and precipitation will be tapering off but lingering snow showers could continue over the Sierra through Saturday evening.

Sunday - Models favor another weak-to-moderate, multi-wave system to move into the Sierra late Sunday. The pattern is looking wetter, warmer, and windier leading into next week, as the latest models continue to favor the undercutting of the Bering Sea ridge. The first wave will move in late Sunday afternoon, with snow levels around 6000-6500 feet. The system is forecasted to deliver another 1 to 2 feet of snow for the high Sierra. Heaviest snow amounts look to be above 6500 feet, with lesser amounts below 6500 feet, as snow will likely mix with rain.

 

 

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: CLOUDY THEN BECOMING MOSTLY CLOUDY. SNOW IN THE MORNING, THEN SLIGHT CHANCE OF SNOW IN THE AFTERNOON. MOSTLY CLOUDY. CHANCE OF SNOW IN THE EVENING. SLIGHT CHANCE OF SNOW AFTER MIDNIGHT. CLOUDY. SNOW.
Temperatures: 33 TO 39 deg. F. 23 TO 28 deg. F. 31 TO 37 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 25 MPH WITH GUSTS TO 45 MPH. 15 TO 25 MPH WITH GUSTS TO 45 MPH. 20 TO 35 MPH WITH GUSTS TO 55 MPH.
Expected snowfall: 4 TO 7 in. UP TO 4 in. 4 TO 10 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SNOW THROUGH THE DAY. MOSTLY CLOUDY. SNOW LIKELY IN THE EVENING. SNOW AFTER MIDNIGHT. CLOUDY. SNOW.
Temperatures: 29 TO 35 deg. F. 18 TO 24 deg. F. 25 TO 31 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 30 TO 45 MPH. GUSTS UP TO 85 MPH DECREASING TO 75 MPH IN THE AFTERNOON. 30 TO 45 MPH WITH GUSTS TO 80 MPH. 40 TO 60 MPH WITH GUSTS TO 100 MPH.
Expected snowfall: 2 TO 7 in. UP TO 3" in. 4 TO 10 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.

ESAC receives support from ...