Eastern Sierra Snowpack Summary - 2016-01-16 06:17

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THIS ADVISORY EXPIRED ON January 18, 2016 @ 6:17 am
Avalanche Advisory published on January 16, 2016 @ 6:17 am
Issued by Josh Feinberg - 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.

Strong SW to W winds have been off and on since Wednesday and are forecasted to continue through the weekend into next week.  8-12” of new snow fell Thursday night, with another inch or two last night, and potentially a few more inches will fall today.  This is plenty of new loose snow for winds to transport.  These winds have and will continue to form wind slabs on NW-N-NE-E-SE facing slopes below ridgelines and across slopes at all elevations.  Widespread evidence of natural avalanche activity was seen throughout the Mammoth Lakes area, in the Sherwins and the Lakes Basin yesterday (Friday).  Danger for trigger is greatest on slopes >35 degrees, convex slopes, and sidewalls of gullies.  Be on the lookout for denser, hollow sounding snow.  Resulting avalanches could possibly be large enough to bury a person, or carry a person through trees or over cliffs. 

Avalanche Character 2: 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.

A weak basal snowpack still exists throughout much of the forecast area due to shallow early season snow conditions and cold temperatures.  While it is unlikely for a human to trigger an avalanche at this level, the resulting slide would likely be large with bad consequences.  This layer is becoming less and less of a concern as the snowpack deepens.  However, it is still something to be aware of, especially where shallower snowpack exists outside of the Mammoth area.  Regularly reevaluate the local snow stability by digging snow pits and performing stability tests to help you make informed decisions.  This layer will likely become a greater concern right after or during a significant new snow load.

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.

There are areas that have buried surface hoar 1-2ft deep in the snowpack.  It is uncertain how widespread this is, but the fact that it has been found recently (yesterday – Friday), is reason enough to be on the lookout for this potentially dangerous weak layer.  If consistent across a slope, an avalanche triggered at this level could be large with bad consequences.

Snowpack Discussion

Avalanche conditions will remain elevated today (Saturday) due to the 8-12” of new snow that fell Thursday night and high SW to W winds that have formed and will continue to form dense wind slabs below ridgelines and across slopes facing SE-E-NE-N-NW.  Exposed slopes >35 degrees are of greatest concern, especially at mid to upper elevations.  These high winds have also created dense, albeit softer, wind slabs in typically more protected areas such as widely spaced treed areas.  Widespread evidence of natural avalanche activity was seen throughout the Mammoth Lakes area, in the Sherwins and the Lakes Basin yesterday (Friday).  There will undoubtably be areas that are sensitive to human triggering today.  Natural avalanches will be possible, and human triggered avalanches very likely.  Some of these avalanches could likely be large enough to bury, injure or kill a person.   

The secondary avalanche concern is still the weak basal facets that are found throughout the region.  These are most concerning in areas of shallow snowpack where the weight of a skier or snowboarder (and even more likely the greater weight of a snowmobiler) could possibly trigger this deeper weak layer.  While a failure in these deep basal facets is unlikely at this point in time, it still remains a possibility, and the resulting avalanche would likely be large with severe consequences.  If and when a more powerful zonal flow impacts our area with heavy snow fall, this layer will be of much greater concern.  In addition, there are still areas where observers are finding buried surface hoar 1-2’ down in the snowpack.  This is a very weak layer, and if a human were to trigger it where it is consistent across a slope, a significant and potentially harmful avalanche could result.  These 2 avalanche problems are much trickier to detect than wind slabs.  Make your own observations by digging down in the snow and assessing localized conditions.

recent observations

Evidence of widespread natural avalanche activity was visible yesterday (Friday) in the Mammoth Lakes area in the Sherwins and in the Lakes Basin.  These were all relatively small avalanches (R1-R2, D1-D2).  These occurred mostly Friday night as close to a foot of snow fell, accompanied by high SW winds.  Crowns ranged in size from 1-2+ft and 30-200ft wide.  D2 means large enough to potentially bury, injure, or kill a person.  Mammoth Mountain Ski Patrol reported moderate avalanche activity due to control efforts (hand charges and ski cutting) on upper mountain and some mid mountain locations.  1-2' crowns, R3 (Medium relative to path), D2-3 (Could bury, injure, kill a person - to - Could destroy a car, damage a truck, destroy a wood frame house or break a few trees).

A knowledgeable party in the Sherwins yesterday (Friday) found buried surface hoar about 2/3-3/4 of the way up towards the ridge in a relatively open area below tree line.  ECT tests failed and propagated on this weak layer that was buried 1-1.5ft down.  This surface hoar was buried first with softer snow, which protected and preserved it from higher winds.  The winds then picked up and heavier snow fell laying a denser wind slab on top.  If widespread, this is a very dangerous combination.

Shooting cracks and settlement was observed in the Lakes Basin yesterday (Friday) at mid elevations in the trees on steeper slopes.  A pit on a ESE aspect at 9,600’ showed weakness just below a thin melt-freeze crust that formed during the sunny conditions on Thursday before close to a foot of new wind deposited snow accumulated on top.

weather

Another storm system is expected to move into our region today (Saturday).  Light snowfall is expected, with up to 3” of accumulation at higher elevations in the northern parts of the region, and less south of Mammoth.  High winds out of the West will continue in the 45-60mph range, with gusts reaching 80-90mph at high elevations.  High temperatures are expected in the low to mid 30s.  

Saturday night a short wave ridge will form, with weaker West winds and a dry period before another zonal flow effects our region Sunday night into Tuesday.  This will be the southern edge of the hyped “atmospheric river” that is bringing back to back storms and significant moisture from the pacific.  At the moment models show that this flow will be centered over Northern California and Southern Oregon, and that we will only be experience a grazing of it with warmer temps and a little wet and dense snowfall.  Later next week there looks to be another possible chance of significant snowfall. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. SCATTERED SNOW SHOWERS IN THE MORNING … THEN ISOLATED SNOW SHOWERS IN THE AFTERNOON MOSTLY CLOUDY MOSTLY CLOUDY
Temperatures: HIGHS IN THE UPPER 20S TO UPPER 30S deg. F. LOWS IN THE 20S deg. F. HIGHS IN THE UPPER 20S TO UPPER 30S deg. F.
Wind direction: WEST WEST BECOMING SOUTHWEST AFTER MIDNIGHT SOUTHWEST
Wind speed: 30 - 50 MPH WITH GUSTS TO 75MPH 30 TO 50 MPH WITH GUSTS TO 70 MPH DECREASING TO 20-30 MPH WITH GUSTS TO 45 AFTER MIDNIGHT 20 -30 MPH WITH GUSTS TO 45 MPH INCREASING TO 40-60 MPH WITH GUSTS TO 85 MPH IN THE AFTERNOON.
Expected snowfall: UP TO 3" in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. SNOW SHOWERS IN THE MORNING … THEN ISOLATED SNOW SHOWERS IN THE AFTERNOON MOSTLY CLOUDY MOSTLY CLOUDY
Temperatures: HIGHS IN THE MID TEENS TO MID 20S deg. F. LOWS IN THE MID TEENS TO LOW 20S deg. F. HIGHS IN THE 30S deg. F.
Wind direction: WEST WEST SOUTHWEST
Wind speed: 50 - 60 MPH WITH GUSTS TO 90 MPH 55 - 60 MPH WITH GUSTS TO 85 MPH DECREASING TO 30 - 35 MPH WITH GUSTS TO 50 MPH AFTER MIDNIGHT 25 - 30 MPH WITH GUSTS TO 45 MPH INCREASING TO 50 - 70 MPH WITH GUSTS TO 90 MPH IN THE AFTERNOON
Expected snowfall: UP TO 3" 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 24 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.

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