Avalanche Advisory - Tue, Jan. 24, 2017

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THIS ADVISORY EXPIRED ON January 26, 2017 @ 6:57 am
Avalanche Advisory published on January 24, 2017 @ 6:57 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 48 hours

Wind Slab is the primary concern for the forecast period – exposed slopes and open trees (primarily mid to upper elevations), where Southwesterly storm winds loaded 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 be expected. As these Wind Slabs begin to heal and become less sensitive, winds are forecasted to veer to North/Northwest with much lower winds speeds of 10 to 20 mph, which may form new potentially sensitive Wind Slabs on NE-E-SE-SW-W aspects. Be on the lookout for blowing snow and signs of recent dense wind-deposited snow. Avoid freshly formed drifts and hollow sounding slabs and steep slopes that are being wind-loaded, and do your own stability assessments on areas where dense wind-deposited surface snow is found.

The threat of natural Storm Slab releases is diminishing as the snowpack adjusts to the recent rapid loading with the passage of time. Though unlikely, it may be possible for a Wind Slab release to trigger a weakness deeper in the snowpack, this would be an isolated event. 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.

 

 

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 latest AR moved through the region with moderate to strong Southwesterly winds and depositing copious amounts of low-density snow, which  formed large and extensive Wind Slabs on NW-N-NE-E-SE aspects on all exposed slopes and in open trees. Winds are forecasted to be light below 10,000’ during the forecast period limiting new snow transport, which will help prevent new Wind Slab formation and allow previously formed Wind Slabs to strengthen. However, caution is still advised as the older Wind Slabs heal and strengthen. They may still be sensitive to triggered failure and due to the combination of low-density snow and strong storm winds extend further downhill than normally expected. Above 10,000’, the combination of forecasted North to Northwest winds of 10 to 20 mph (optimal wind transport 15 to 25 mph) and the extensive amounts of new low-density snow available for transport will likely form new sensitive Wind Slabs on NE-E-SE-S-SW-W aspects. These will primarily be found below ridgelines and near terrain features that promote drifting and crossloading. Avoid hollow dense slabs, especially on convex rolls. 

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 latest AR event has moved out of the region with snowfall amounts of 39” to 70” of new snow. Ahead of the system, relatively warm air and light rain moved into the area, which helped breakdown the Surface Hoar and facets at the snow surface, which aided bonding at the new/old snow interface but the extreme snowfall amounts formed thick Storm Slabs with several large avalanches observed and reported during the cycle. 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 Big Pine.  The threat of natural Storm Slab releases is diminishing as the snowpack adjusts to the recent rapid loading with the passage of time. Though unlikely, it may be possible for a Wind Slab release to trigger a weakness deeper in the snowpack, this would be an isolated event. 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.

 

Avalanche Character 3: 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.

Mid to lower elevations, there is the possibility of rider triggered loose snow releases. Though not a danger for burial, can force a rider into hazardous terrain. 

Snowpack Discussion

The latest Atmospheric River (AR) event has begun to move out of the region with historic amounts of snow in its wake. As the systems moved into California off the Pacific, they pounded the region with heavy snow / winds and were responsible for a number of highway closures, avalanche evacuation orders for threatened locations, and one avalanche striking and damaging a building. The system came in a series of three waves over the course of a week with the last impulse (Sun thru Monday) being the biggest, wettest, and windiest. Storm totals ranged from 29 inches at Virginia Lakes to a whopping 70 inches at Mammoth Pass. The storms were cooler than the previous AR events, bringing a bounty of low-density snow and on occasions strong Southwesterly winds. The immense amount of snow has brought welcomed drought relief and good riding condition down to elevations that haven’t seen heavy snows in years. The snowpack is showing signs of adjusting to the recent rapid loading with the risk of Storm Slabs subsiding with stability tests indicating minor weakness within the recent new snow and no recent Storm Slab avalanches reported or observed. However, as the last of the AR events moved through the region (Sunday thru Monday) it was accompanied by moderate to strong Southwesterly winds and with relatively low-density snow, which formed extensive Wind Slabs in exposed terrain and open trees (primarily mid to upper elevations), primarily on NW-N-NE-E-SW aspects. Winds are forecasted to be light below 10,000’ limiting new snow transport, which will help prevent new Wind Slab formation and allow previously formed Wind Slabs to strengthen. However, caution is still advised as the older Wind Slabs heal and strengthen. They may still be sensitive to triggered failure and due to the combination of low-density snow and strong storm winds extend further downhill than normally expected. Above 10,000’, the combination of forecasted North to Northwest winds of 10 to 20 mph (optimal wind transport 15 to 25 mph) and the extensive amounts of new low-density snow available for transport will likely form new sensitive Wind Slabs on NE-E-SE-S-SW-W aspects. These will primarily be found below ridgelines and near terrain features that promote drifting and crossloading. Avoid hollow dense slabs, especially on convex rolls. Dangerous avalanche conditions exists. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. Natural avalanches possible; human- triggered avalanches likely. Wind Slab failures could possible trigger a deeper release within the upper snowpack.

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. 

Storm Snow Fall Totals

Virginia Lks (9445’): New Snow 29”, SWC 4.4”

Gem Pass (10750’): New Snow 44”, SWC 5.3”

June Wx Plot (9148’): New Snow 38”, SWC 3”

Mammoth Pass (9500’): New Snow 70”

Rock Crk (9600’): New Snow 39”

Big Pine Creek (10,000’): 40”

  

 

 

recent observations

Trees Below June Lake Ski Area, June Lake (1/23/17) - Ski cuts by ski patrol on face of J1 produced no results (viewed from Rd), toured in NW facing trees at approx 8600 feet, no whumphing or cracking...observed older crowns (probably mid storm) in Carson bowl area at approx 10,000 feet in NE facing rocky terrain that had been blown back in by the wind. Calm winds at lower elevation but saw evidence of wind transport above 9500 feet (mainly west winds).

Mammoth Rock Bowl, Sherwins, Mammoth Basin (1/23/17) - -Toured up from the end of Old Mammoth Rd past Mammoth Rock assessing snowpack, being cautious to avoid steeper wide open areas, up to ridgeline. Mammoth Rock Chute slid with stomping at starting zone.  1.5-2ft crown dense soft slab.  42 degrees is upper slope steepness, windloaded. Multiple hand pits failed with moderate force with Q2 - Q1.5 shear quality, at density change in new snow ... most notably failures occurred above a very thick 1Finger hard layer in a 4finger layer (which was overtopped by light fist hard snow).  Right-side-up snowpack. Quick test pit dug a couple hundred feet below ridge on 30 degree slope. ECTP21 at same density change as above hand pits, 50cm down. Some small shooting cracks at steep convexities. Calm winds, except at ridgetop where moderate south winds were blowing (very cold!).  These winds were forming small cornices which were failing frequently and not getting too large. Stayed in terrain less than 35deg .... terrain over 40 degrees (especially if windloaded!) could produce avalanches, as seen above in poop chute. 

Mammoth Mtn., Mammoth Basin - Main Lodge (1/23/2017):  Good action in storm layers.  8-10" crowns with hadn chargers, 18-24" with ski cuts following hand charges.  Ski cuts resulting in 2' crowns, slides down to previous bedsurface from day before. Soft slab avalanches occuring with hand charges and ski cuts mid mountain Interesting phenomenon, usually hand charges achieve bigger results.  33" of new snow in last 24 hours. Upper Mtn socked in. Main lodge side (Bear etc.): 8-10" crowns failing at old/new snow interface.  Soft slabs running 300-350ft. Lincoln Mtn: 4-6" crowns failing within new snow, some small pockets lower down mid-slope in steep areas 1-1.5ft crowns. 

Mammoth Rock Bowl, Sherwins, Mammoth Basin (1/23/2017) - Route:  End of Old Mammoth Rd up past Mammoth Rock and up along ridge to ~9,700', back down.

Mammoth Rock chute slid with stomp at upper convexity start zone ~7" crown (failed in new snow layer not far above old melt/freeze crust). Natural slide happened last night from cross-loading ~9,700' just on leeward (eastern) side of ridgeline, ~1' crown, 100ft across, followed steep section of slope just below rock outcrops / cliffs which normal skin track up goes right thru.  R1-D1. Multiple hand-pits showed easy to moderate failure of new snow, which is forming a cohesive storm slab (much denser snow than previous few storms). Quick test pit ~9,700', 35deg slope, N, CT tests failing upon isolation ~16" down just above 3cm wind crust that formed yesterday afternoon during most intense period of winds. Lots of small shooting cracks on slopes about 35deg and steeper. Thick dense snow.  ~8" at 9,000' by 7am. Light SW winds this morning so far below 9,500'. Impression: Much more cohesive storm slab overall in sheltered areas than the past few storms (soft slab releases more likely now than simple sloughs on slopes >35deg).  Wind-deposit areas more susceptible to releases.  Sluggish to trigger, but higher density makes potential slides more concerning. Skiing quality ok, staying on surface, very hard to get up if you fall!  Deep snow immersion a real concern, watch out for tree wells and stay with a buddy even in low angle terrain.     

 

weather

Tues. thru Wednesday - Dry and cold weather through Wednesday. Low level clouds will thin and clear Tuesday allowing for a cold night with temperatures easily plummet to single digits or even below zero. Low temperatures are not expected to drop as much for Wednesday night.

Thurs - increase in cloud cover Thursday as a weak disturbance crosses northern Nevada. A few light upslope showers will be possible across the eastern Sierra slopes Thursdays afternoon as an east surface gradient develops with an expansive surface high pressure area developing across southern Idaho. Overall, chilly weather with only low chances for light flurries or light snow showers are expected.

Friday - High pressure will build over the Great Basin Friday leading to strengthening inversions. On Friday, a strong easterly gradient develops across the Sierra. This will lead to gusty E/NE winds with gusts of 70-100 mph possible across the higher terrain of the Sierra. With conditions remaining stable below 700 mb, unlikely to see strong winds surfacing much below ridgelines, though periodic gusty winds are possible.

 

 

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: MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS. PARTLY CLOUDY THEN BECOMING CLEAR. PARTLY CLOUDY.
Temperatures: 17 TO 22. deg. F. 2 BELOW TO 8 ABOVE ZERO. deg. F. 22 TO 27. deg. F.
Wind direction: LIGHT WINDS. LIGHT WINDS. LIGHT WINDS.
Wind speed:
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS THROUGH THE DAY. PARTLY CLOUDY THEN BECOMING CLEAR. PARTLY CLOUDY.
Temperatures: 10 TO 16. deg. F. 6 TO 12. deg. F. 17 TO 23. deg. F.
Wind direction: NORTH NORTH NORTHWEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 35 MPH. 10 TO 15 MPH WITH GUSTS TO 30 MPH. 15 TO 20 MPH WITH GUSTS TO 35 MPH.
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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