Eastern Sierra Snowpack Summary - 2016-01-23 06:50

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THIS ADVISORY EXPIRED ON January 25, 2016 @ 6:50 am
Avalanche Advisory published on January 23, 2016 @ 6:50 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.

New snow and high winds out of the southwest during this storm will create wind slabs on NW-N-NE-E-SE slopes, below ridgelines and across exposed slopes.  Some of these wind slabs will likely avalanche naturally during the storm, and others will very likely be sensitive to human triggering.  Pay attention to firm snow.  These wind slab avalanches could likely be large enough to cause injury or death.  These slabs will stabilize and become less sensitive with time, but how much time this will require isn’t know. Do your own quick hand shear tests in safe locations to assess how sensitive these new wind slabs are, while being aware that sensitivity can vary greatly across a slope.   

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.

Avalanche danger will be heightened today (Saturday) and into tomorrow, even in wind-protected areas on slopes steeper than 35 deg. at all elevations and aspects due to the 7-13" of new dense snow that has fallen since yesterday morning, and the 3-6" more that is expected today (Saturday).  Although the “right-side-up” nature of this storm (warmer denser snow down low with lighter colder snow on top) tends to result in better bonding within the new snow and between the old snow and new snow than vice versa (cold/light on bottom and warm/dense on top), this significant amount of new snow will mean that human triggered avalanches remain a likely possibility.  Use good assessment skills to determine for yourself if a slope is safe enough for you to travel on.            

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.

The relatively rapid added load to the snowpack during this storm could possibly be enough stress to cause failures deep in the snowpack at the basal facets in isolated areas.  Should this occur, resulting avalanches will likely be large and destructive.  While unlikely, this concern will be greater if snowfall totals end toward the higher side of projections (>2ft), and this concern will be greater for areas with previously shallower snow packs (<1.5m, areas outside of Mammoth).

*This added weight of new snow also adds concern in terms of very isolated pockets where buried surface hoar may still exist 1.5-2.5ft down in the snow pack.  This can be a potentially very weak layer.  While this buried surface hoar problem is nowhere near as wide a concern as it has been north of our region in Tahoe, it is still something to consider.  Sheltered open areas below tree line are where these isolated pockets may still exist.    

Snowpack Discussion

Another winter storm is upon us!  7-13” of new dense snow has fallen since the storm began yesterday (Friday) morning, and another 3-7” is expected throughout the day today.  This snowfall has been and will continue to be accompanied by high winds out of the southwest.  This storm started out warm with a high snowline above 7000’, laying down a solid layer of Sierra Cement, before cooler air moved in last night and snowline dropped below 6000’.  Snowline will likely drop below 5000' today.  

Heightened avalanche conditions will exist today (Saturday), and into tomorrow.  Greatest concern will be new wind slabs sensitive to human triggering on mid to upper elevations slopes >35 deg.  These will most likely be found on NW-N-NE-E-SE slopes, below ridgelines and across exposed slopes.  Storm slabs will also be of concern on slopes >35 deg that are protected from the wind at all aspects and elevations.  Natural avalanches are possible, and human triggered avalanches are likely.

The relatively rapid added load to the snowpack during this storm could possibly be enough stress to cause failures deep in the snowpack at the basal facets in isolated areas.  Should this occur, resulting avalanches will likely be large and destructive.  While unlikely, this concern will be greater if snowfall totals end toward the higher side of projections ( close to 2ft), and this concern will be greater for areas with previously shallower snow packs (<1.5m).  This added weight of new snow also adds concern in regards to very isolated pockets where buried surface hoar may still exist 1.5-2.5ft down in the snow pack.  While this buried surface hoar problem is nowhere near as wide a concern as it has been north of our region in Tahoe, it is still something to consider.      

Sunday’s clear skies, cool temperatures and calmer winds out of the NW will result in decreasing avalanche conditions throughout the day.  The “right-side-up” storm snow (heavy dense snow at the bottom, lighter on top) will settle and stabilize quicker than storm snow that started cold and then warmed up.  Natural avalanches on Sunday will be unlikely, except on steep southerly facing slopes where sun radiation could weaken the upper snowpack.  Human triggered avalanches will still be possible.  

recent observations

Observations in the Sherwins yesterday (Friday) revealed a settling and densifying upper snowpack as a result of the warm moist air mass and warm snowfall.  The varying high winds were creating some new shallow wind deposits mid-slope in more exposed areas, some of which broke out into chunks in between switchbacks. 

Large snow plumes were observed over high peaks just south of Mammoth due to the high SW winds.

weather

Snow will continue today (Saturday) with snow levels falling below 5000 feet.  An additional 2-6” of snow is expected, bringing storm totals up to 1-2’ above 7000 feet, and possibly 4 to 8” along highway 395 near Mammoth.  Highest snowfall amounts are expected for Mammoth and north, with significantly less south.  High Temperatures are expected in the mid 20s at 10,000.  High southwest winds in the 50-60mph range with gusts in the 90s are expected to decrease slightly in the afternoon. 

For Sunday expect clearing skies, continued colder temperatures in the mid 20s at 10,000’, and much calmer winds out of the NW in the 10-15mph range with gusts up to 25mph.  High pressure is expected to remain through the beginning of next week.   

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. OCCASIONAL SNOW SHOWERS MOSTLY CLOUDY. SNOW LIKELY IN THE EVENING...THEN SLIGHT CHANCE OF SNOW AFTER MIDNIGHT PARTLY CLOUDY TO SUNNY
Temperatures: UPPER 20S TO MID 30S deg. F. LOW TO UPPER TEENS deg. F. UPPER 20S TO MID 30S deg. F.
Wind direction: SOUTHWEST NORTHWEST NORTHWEST
Wind speed: 30 - 35 MPH, GUSTS TO 55 MPH 15 - 20 MPH, GUSTS TO 30 MPH 10 - 15 MPH
Expected snowfall: 3-6 in. UP TO 2 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. OCCASIONAL SNOW SHOWERS MOSTLY CLOUDY. SNOW LIKELY IN THE EVENING...THEN CHANCE OF SNOW AFTER MIDNIGHT PARTLY CLOUDY TO SUNNY
Temperatures: MID TO UPPER 20S deg. F. SINGLE DIGITS TO MID TEENS deg. F. UPPER TEENS TO MID 20S deg. F.
Wind direction: SOUTHWEST WEST BECOMING NORTHWEST AFTER MIDNIGHT NORTHWEST
Wind speed: 60 - 65 MPH WITH GUSTS TO 100 MPH, DECREASING TO 40 - 45 MPH WITH GUSTS TO 75 MPH IN THE AFTERNOON 35 - 40 MPH WITH GUSTS TO 60 MPH DECREASING TO 15 - 20 MPH WITH GUSTS TO 35 AFTER MIDNIGHT 10 - 20 MPH WITH GUSTS TO 30 MPH
Expected snowfall: 3-6 in. UP TO 2 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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