Avalanche Advisory - Thu, Jan. 19, 2017

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THIS ADVISORY EXPIRED ON January 21, 2017 @ 6:57 am
Avalanche Advisory published on January 19, 2017 @ 6:57 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 48 hours

Wind Slab and Storm Slab potential has risen, primarily in the mid to upper elevations, due to the first in a series of storms transits the area with moderate to strong winds, which has increased the threat of avalanches in the forecast area. Southwesterly winds will continue to load NW-N-NE-E-SE aspects producing extensive Wind Slabs along ridgetops, extreme terrain, and exposed terrain that promotes drifting and crossloading, some extending further down slope than would normally be expected. Avoid freshly formed drifts and hollow sounding slabs. Storm Slabs are possible in sheltered areas, especially from Mammoth Lakes Basin south to Rock Creek where up to 20” of new snow reported. Also, there is the potential for isolated pockets of Surface Hoar above ~9,500’ in sheltered terrain and possible reactive. Natural Avalanches are possible, Human Triggered Avalanches likely. Dangerous avalanche conditions exist. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Be on the lookout for blowing snow and signs of recent dense wind-deposited snow. Avoid steep slopes that are being wind-loaded, and do your own stability assessments on areas where dense wind-deposited surface snow is found.

 

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.

Forecasted moderate to strong Southwesterly winds at upper and mid elevations (9,000' and above) Thursday thru Friday will form Wind Slabs on NW-N-NE-E-SE aspects, along ridgetops, leeward sides of ridgelines, and in and around terrain features that promote drifting and crossloading. Natural avalanches are possible; human- triggered avalanches likely on leeward slopes, near and around terrain features that promote drifting. Use caution and avoid exposed terrain steeper than 30 degrees.

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.

 

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 6” to 20” new snow being reported as of 0400, Thursday. The storm system moved into the region Wednesday and came in relatively warm which will aid bonding at the new/old snow interface but in areas with higher snowfall amounts, the bonds may not be strong enough hold the snow in place, if stressed. Areas reporting greater amounts of new snow also have greater potential for slabs and weakness within the new snow, which may be tender and prone to release. Areas with the greatest potential for Storm Slab failures is greatest in the Mammoth Lakes Basin South to Rock Creek but may be encountered elsewhere were the new snow hasn’t fully bonded to the underlying snowpack. Natural avalanches are possible; human- triggered avalanches likely on terrain steeper than 30 degrees slopes. 

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.   

 

Snowpack Discussion

After several days of mild weather, winter returns to the region with the first in a series of storms moving through the region with 6” to 20” new snow reported in its wake. Prior to the storm’s arrival, high pressure dominated the region briefly with cold clear nights driving strong facet and Surface Hoar growth at and near the snow surface while the mid and deeper snowpack strengthened overall. A few wet loose releases were reported on solar aspects from 1/16 – 1/17. Surface Hoar formed throughout the region, up to 10,000’. Fortunately a strong warm-up Tuesday night combined with light rain (up to 9,300’) and gusty winds on Wednesday has helped breakdown the Surface Hoar at the surface. The rain and moisture, as it refreezes, will form stronger bonds between faceted grains near the old snow surface. The Surface Hoar may still linger in isolated sheltered pockets above 9,500’ and possibly reactive. Mid and Deep faceted layers, where present, are currently not reactive but isolated step down potential remains possible in shallow areas where these weaknesses are closer to the surface and more easily triggered.       

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.   

 

recent observations

Crown Profile, V-Bowl Trees, Dana Plateau (1/18/17) – Toured to the top of V-bowl to assess how the Pre-Frontal weather was affecting the Surface Hoar that has been widely observed throughout the forecast region. Temperatures in the AM were already above freezing and the snow surface already quite wet at 10:30AM with obvious signs of a strong warm-up over night given the saturation of the snow surface. Moderate to strong Southwest winds with strong gusts ahead of approaching storm. Light and wet snow by midday. Surface snow progressively got wetter through the day with fairly large rollerballs where skier disturbed on steeper terrain. The combination of wind, rain, and warm pre-frontal temperatures have taken a toll on the Surface Hoar from 7,000’ to 9,300’. The Surface Hoar has been drastically reduced because of the Pre-Frontal weather has broken up the feathery crystals, laid over, or metamorphosing and sintering to adjacent crystals. Remote possibility areas of Surface Hoar survived the Pre-Frontal weather in isolated sheltered locations, above 9,000’ where it escaped the Pre-Frontal warming, winds, and light rain. Encountered a four large crowns in and around the top of V-Bowl that occurred last week mid storm cycle. Crown height varied from 3’ to 6’, all on NE aspects, at ~9,300’. Crowns were irregular and debris showed signs of being quite wet with one slide running 2/3 down V-Bowl.

Lundy Canyon (1/17/17) - Observer: d lewis, Eastern Sierra Avalanche Center

Lat/Lon:  38.028204°, -119.176746°

Elevation: 7300’, Aspect: N Angle: 25°

Sky Cover: -BKN VF 6400-7500

Precipitation: No Precipitation

Wind Speed: Light, subjective

Wind Direction: W, Blowing Snow: None

Snow Depth: 95cm, Boot Penetration: 20cm

Comments: Surface Hoar 3-6mm in exposed area, 10-15mm in sheltered areas.1-3cm thick with rime from 7100 to ~ 8000'. Facets below Surface Hoar, loose, 10 -20 cm @ the bottom 15-30cm @ 9000'. Compression Tests- CTH 21, 28, 29, Q3 @ 10cm above ground. Surface Hoar & Facets at the surface possible weak layer with a well formed bed surface of either melt/freeze crust or dense wind slab for approaching storm . 3 Stability Tests: CT28 Q3 85cm CT29 Q3 85cm CT21 Q3 85cm Failed in facets above ground.

Snowpack consistent 2 to 3 feet (meter +/-) at the Trailhead. Coverage is good. Snow surface impressive display of Surface Hoar, rimed at the lower elevations (~7,000' - ~8,700'). Well-established inversion with basin fog since the last storm system exited the region has been coating the lower elevation Surface Hoar with a thin rime coating. Looks like 70's ceiling popcorn. Surface Hoar was consistent 2 to 3cm thick from 7,000' to 9,300'. Size varied from 3-6mm in more exposed areas, ribs, ridges, crests while sheltered areas had 10 to 15mm in more sheltered locations. Below the Surface Hoar, 2-3 mm loose facets with sloughing on test slopes (~7,500', N-NE aspects). The surface snow skis well but may become a weak layer if buried. Next storm system worth watching closely and checking the advisory. No recent avalanche activity noted on southerly aspects of lower Lundy Canyon. NW-N-E aspects loose facets, anything beyond East noticeable light Melt/Freeze Crust under 4-8mm Surface Hoar.

Virginia Lakes Rd is closed ~ 1/4 mile from 395. Lundy Canyon open to first gate above homes off 395 (~2 miles). Parking is limited.

Red Cone, Mammoth Basin (1/17/17) - Toured up to Red Cone via the usual route from Tamarack, down north facing trees to Horseshoe Lake, back out on the cross country trails. 1050am: 3 R1D1 recent wet point releases visible on S aspect of the Sherwins. Started under some rocks. 1120am: Recent winds had stripped down to the rain crust/ice layer below Crystal Crag, N aspect. 1145am: Stomped on a W facing rollover with an obvious wind drift, no results. Surface hoar was melting down or broken up by previous winds at and above 9400'. 1230pm: Dug a pit on W aspect of Red Cone Bowl around 10,000' See Observation. 145pm: Stomped on another wind lip on W aspect of Red Cone Bowl. No results. Skied the NW aspect of the bowl, wind affected and not great until hitting the trees down to Horseshoe where it became good but thickening powder. Skiing in sheltered trees would probably be the best right now.

weather

* Winter weather advisory now in effect until 10 am PST Thursday morning above 6000 feet.

* Winter weather advisory in effect from 4 am Friday to 10 am PST Saturday...

* timing: moderate snow will continue through early this morning before tapering to showers by early afternoon. The next wave of heavy snowfall will begin early Friday morning into Saturday morning.

* snow accumulations Friday-Saturday: 6 to 12 inches west of highway 395 above 7000 feet with 15-20 inches on the crest. 2 to 5 inches possible east of highway 395 below 7000 feet.

Thurs thru Friday - The first in a series of winter weather systems pushing across the Sierra. Snow will thin out and become more light and showery by late morning and early afternoon. After a brief lull this afternoon and evening, the next winter system will begin to impact the Sierra early Friday morning. Winter Weather Advisories have been posted for this next system during this period across the Sierra with snow levels early Friday morning start generally around 4,000` and rise to nearly 4,500` during the afternoon Friday. Snow totals from Friday morning through Saturday morning in the Sierra look to be in the 12-18" above 7,000`.

Sat thru Sunday - Model simulations are still on track with a dynamic cold system pushing into the region late Saturday night and persisting over the area through early Monday. Ahead of this system some lingering snow showers will continue in the Sierra through Saturday. Gusty winds are likely ahead of the system Saturday afternoon and evening for much of the area with the strongest winds in the Sierra and just east of the Sierra. The system for Sunday comes with a very strong jet aloft...150+ kt. Early on this system will tap a fairly deep moisture plume in the eastern Pacific. The tapping of this moisture plume in the model simulations has resulted in increased QPF for the past few model runs. The moisture push comes in two distinct segments. The first is during the day Sunday when the Sierra will see the brunt of the precipitation. Then another burst develops Sunday evening associated with a secondary vorticity lobe and surface cold front. It is highly likely that if the models continue this trend we will see up to 3 feet in the higher elevations of the Sierra and near 2 feet around Mammoth Lakes.

 

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. SNOW IN THE MORNING, THEN CHANCE OF SNOW SHOWERS IN THE AFTERNOON. CLOUDY. CHANCE OF SNOW SHOWERS IN THE EVENING, THEN SNOW SHOWERS LIKELY AFTER MIDNIGHT. CLOUDY. SNOW.
Temperatures: 23 TO 28. deg. F. 13 TO 18. deg. F. 24 TO 29. deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTH
Wind speed: 20 TO 30 MPH WITH GUSTS TO 60 MPH DECREASING TO 10 TO 15 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON. 10 TO 15 MPH. GUSTS UP TO 25 MPH INCREASING TO 35 MPH AFTER MIDNIGHT. 15 TO 25 MPH WITH GUSTS TO 60 MPH.
Expected snowfall: 2 TO 4" in. UP TO 2" in. 5 TO 10" in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SNOW IN THE MORNING, THEN CHANCE OF SNOW SHOWERS IN THE AFTERNOON. CLOUDY. CHANCE OF SNOW SHOWERS IN THE EVENING, THEN SNOW SHOWERS LIKELY AFTER MIDNIGHT. CLOUDY. SNOW.
Temperatures: 15 TO 23. deg. F. 8 TO 14. deg. F. 16 TO 24. deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 30 TO 45 MPH WITH GUSTS TO 85 MPH BECOMING WEST 15 TO 25 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON. 15 TO 25 MPH. GUSTS UP TO 35 MPH INCREASING TO 45 MPH AFTER MIDNIGHT. 20 TO 35 MPH WITH GUSTS TO 70 MPH.
Expected snowfall: 3 TO 6" in. UP TO 3" in. 5 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.

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