Eastern Sierra Snowpack Summary - 2015-12-19 08:52

THIS ADVISORY EXPIRED ON December 21, 2015 @ 8:52 am
Avalanche Advisory published on December 19, 2015 @ 8:52 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.

As another round of winter weather moves into the region today, the new snow may be slow to bond to the decomposing and faceted grains at the surface.  Of greater concern is the southwest winds associated with this storm that are expected to load N-NE-E aspects.  Windslabs sensitive to human triggering will likely be found on N-NE-E exposed slopes below ridgelines and the sides of gullies.   

Avalanche Character 2: 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.

The snowpack throughout the region remains relatively shallow with facets dominating the lower snowpack.  Two layers of concern exist:  

1)  A persistent weak layer of well-preserved facets mid-snowpack under the melt/freeze crust which was the old snow surface prior to Dec 10th (when last week’s heavy snows began).  

2)  Facets near the ground. 

Currently these layers are not highly reactive but may have the potential to be triggered by an event with substantial force such as a cornice fall or small slide.  However, with more snow on the way, these layers will likely become more reactive as additional snow/weight increase the stress within the snowpack . 

Snowpack Discussion

Avalanche danger will likely increase as the day progresses, and remain elevated tomorrow as snowfall begins later this morning and continues throughout the day.  While snowfall amounts are expected to be modest (2-8”), the fresh windslabs that will develop as a result of the moderate (30-55mph) SW winds associated with this storm will be the greater concern.  These slabs, which will likely be sensitive to skier triggering, will likely be found just below ridges and on the sides of gullies on exposed E-NE-N facing slopes at mid to upper elevations.  While the size of these avalanches may be small, the consequences of hitting early season obstacles could be severe.     

While the latest snowpack observations and reports from the Mammoth Lakes area north thru June Lake to Virginia Lakes continue to show a shallow snowpack structure with mid-pack and ground level weak, sugary faceted layers, the likelihood of triggering a slope failure within these layers is currently low, but not impossible.  However, with the added weight of new snowfall, and the potential force of a smaller slab avalanche as a result of the new snow and wind deposition, these deeper layers could be triggered resulting in a much larger avalanche.    

The cold temperatures early last week and the clear skies caused the upper surface layers of the snowpack to become faceted and sugary.  While this has been great for skiing conditions, this layer can become a weak point once buried under new snow.  

Ridgetop winds increased significantly yesterday with SW winds in the 30-50mph range, resulting in areas of visible snow transport.  This means that even this morning before the new snowfall there could be small very isolated pockets of fresh wind slab just below ridgelines and sides of gullies in higher elevation steep exposed E-NE-N facing slopes that could be sensitive to human triggering.



recent observations

Snowpits and stability tests in the Sherwins yesterday revealed that a similarly weak snowpack structure exists there as in the Mammoth Lakes Basin and Virignia Lakes.  See Recent Observations section under the homepage for more details.

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 20 deg. F.
Max. temperature in the last 24 hours: 25 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 40 mph
Maximum wind gust in the last 24 hours: 56 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 32 inches

A modest winter system moves into the Eastern Sierra today.  The southerly orientation of the flow is mainly shadowing things east of the Sierra crest.  A jet streak and vort max at the base of the trough are acting to back the flow even more southerly, producing a strong shadowing effect, which reduces the chance of precipitation reaching western Nevada during the day. In Mono County, the snow will likely hold off until late morning. With the southerly flow, the heaviest snowfall will be confined to the mountains west of highway 395, with only light snow reaching along and east of highway 395. Winds will be locally gusty as the system moves through in the AM, which should diminish with frontal passage. There will be a brief break between systems Saturday night into early Sunday before more moisture begins to work into northern CA/NV ahead of the next system with the strongest push of moisture Monday afternoon into Tuesday. Winds will be increasing ahead of this wave, helping with mixing and bringing snow levels up to the 5500-7000 foot mark.


Long term (Tuesday through Christmas day): An active pattern will continue through the week.

Tuesday will be breezy with relatively high snow levels at 6500-7500 feet. The models continue to show a good shadow signature and the winds aloft support this idea. Perhaps another 1/2-3/4" of water near the Sierra crest with amounts rapidly diminishing further east. Models are projecting a weak short wave ridge for Wednesday with reduced chances for precipitation and cloud cover. However, a semi-moist upslope west flow will continue to produce showers over the Sierra as colder air filters in during the day. Christmas eve/Christmas Day, models continue to show Thursday being wet with snow levels falling to near 4500 feet. This system also looks like it would have the best forcing so amounts of a foot or more are possible in the Sierra.


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: Snow before 10am, then snow showers after 10am. Chance of precipitation is 90%. Total daytime snow accumulation of 4 to 8 inches possible. Snow showers before 10pm. Chance of precipitation is 80%. New snow accumulation of less than one inch possible. A 20 percent chance of snow showers after 10am. Mostly cloudy.
Temperatures: High near 25 deg. F. Low around 9 deg. F. High near 31 deg. F.
Wind direction: Southwest West West
Wind speed: 30-35mph, gusts to 55mph 15-20mph, gusts to 35mph 20-25mph, gusts to 35mph
Expected snowfall: 4-8" in. <1" in. <1" in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Snow. Chance of precipitation is 90%. Total daytime snow accumulation of 5 to 9 inches possible. A 50 percent chance of snow showers before 10pm. Mostly cloudy. A 20 percent chance of snow after 10am. Mostly cloudy.
Temperatures: High near 25 deg. F. Low around 10. deg. F. High near 30 deg. F.
Wind direction: Southwest West Southwest West
Wind speed: 25-35mph, gusts to 45mph 15-20mph, gusts to 25mph 20-25mph, gusts to 35mph
Expected snowfall: 5-9" in. <1" in. <1" in.

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 Eastern Sierra Avalanche Center who is solely responsible for its content.

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