Eastern Sierra Snowpack Summary - 2015-12-17 08:12

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THIS ADVISORY EXPIRED ON December 19, 2015 @ 8:12 am
Avalanche Advisory published on December 17, 2015 @ 8:12 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: 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.

Test results indicate that the new snow has bonded well to the old/new snow surface. Additionally, the previously noted weak layer within the recent new snow continues to settle/strengthen and shows signs it has bonded to the adjacent layers. However, test results continue to show a persistant weak layer of well preserved facets mid-pack under a melt/freeze crust that formed prior to the recent new snow load. Currently it is not highly reactive but has the potential to be triggered by rapaid loading (e.g. cornice fall, small slides) or possibly triggered from adjacent slopes. 

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.

Snow observations continue to show a snowpack that is primarilly comprised of facets in the lower depths and new snow toward the surface. Test results indicate that the new snow has bonded well to the old/new snow surface. Additionally, the previously noted weak layer within the recent new snow continues to settle/strengthen and shows signs it has bonded to the adjacent layers. However, small pockets of instability remain on slopes of 35 degrees and steeper where shallow slabs have formed over facets. By themselves, these pockets are capable of producing slides with serious consequences, if caught, but more importantly they have the potential to propagate and trigger larger releases on adjacent slopes with similiar snowpack structure. These pockets can primarily be found on Northwest thru East aspects in the mid to upper elevations on slopes of 35 degrees and steeper where the snow is not uniformly deep. The underlying early season snowpack remains relatively shallow, variable and predominantly weak.

 

Snowpack Discussion

Storm snow and wind slabs from this past week’s two storms (Dec 11 - Dec 13) appear for the most part to have settled and bonded well with the underlying snowpack. However, there is a small possibility that isolated pockets of wind slab in steeper higher elevation alpine terrain may still be prone to human triggering. Of greater concern is pockets of persistent deep layer instability due to weak sugary facets that have grown mid-snowpack just under the old firm pre-storm snow surface. This instability, along with pockets of even deeper weak facets just above the ground have been found in recent snow pits and stability tests in the Mammoth Lakes Basin and Virginia Lakes. The reactiveness of these layers was observed to vary widely across slopes, even within areas of very close proximity to each other. Be cautious of areas of shallow snowpack on slopes over 35 degrees, near rocks and on the sides of gullies where it may be more likely for a skier or rider to trigger a failure in these deeper layers. While the cold temperatures have been keeping the surface snow skiing well, it is still early season with rocks and stumps lingering just under the surface waiting to snipe you!

recent observations

A significant skier triggered avalanche that occurred in a closed area on Mammoth Mountain on 12/14 was found to have failed within the deeper old snow layer of facets mentioned above (see the Recent Observations section on the homepage for more detailed information on this).

Snow data (12/16/15) from Virginia Lakes region shows quite a different picture than the Mammoth Crest. Overall, the snowpack in the Virginia Lakes region is shallower and more unevenly distributed with facets and even some well-developed depth hoar throughout the lower snowpack. The snow from the recent storms has strengthened and bonded to each other as well as to the deeper old/new snow interface. However, stability test results continue to show a persistent weak layer of facets directly below the old/new snow interface (approximately 32cm from the surface, 2 to 3mm grain size) that is capable of failing and then eroding deep into the snowpack. This layer has consistently revealed itself in the Mammoth Crest region as well as today’s tests in Virginia Lakes.  Additionally, two extended column tests failed within the facets and depth hoar directly above the ground while isolating the columns (ECTPV).  No natural avalanche action was noted but a party did report whumpfing and visible collapse of the underlying depth hoar.

Virginia Lakes: Elevation 10,300, Aspect NNE, Slope Angle 28 degrees, HST 80cm, ECTP 16 @ 35 cm, stepping down to 25 cm as it slide off the column).  Same location: ECTP 21 @ â35cm. An adjacent site with slightly less total snow consistently failed while isolating the column, ECTPV @12cm (X2).  Further testing nearby with a deeper wind drifted snowpack (110cm), extended column tests produced:  ECTN 14 @ 84cm (within new snow), ECTN 17 @ 80cm (within new snow), ECTN @ â65cm (within weak facet layer below melt freeze crust that was the old snow surface before this past week’s storms). 

Winds between Mammoth and Virginia Lakes have been observed to have decreased significantly over the past two days resulting in little snow transport and the development of few, if any, new wind slabs.

 

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 28 deg. F.
Max. temperature in the last 24 hours: 40 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 10 -20 mph
Maximum wind gust in the last 24 hours: 33 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 30 inches
weather

Clouds are beginning to increase from the north as moisture spills over the high-pressure ridge into northern California with periods of partly to mostly cloudy skies persisting into the weekend as the next low pressure approaches. Warmer temperatures are expected Thursday and Friday ahead of a shortwave ridge due in on Friday, which will bring light winds to the valleys while ridge-top winds will increase to around 50-60 mph as the next shortwave approaches northern California. Precipitation is forecast to move into the northern Sierra Friday afternoon, spreading south into Mono county Saturday morning. Models indicate the system will split and weaken slightly as it moves onshore, decreasing the dynamics and potential for heavy snow than if it held together. Snow amounts forecasted for the Sierras do not look very impressive given the recent storms. Snow levels with this system should be fairly low, starting at 5500-6000 feet and falling to near 4000-4500 by Saturday morning with 2” to 4” for Mono county and the surrounding mountains Friday night into Saturday with less overall impact than previous storms.  

Long-term: the weather pattern remains open to a wet on-shore flow as a strong 200 knot upper level jet moves into the northern Cascades with rain and snow at the higher elevation beginning Monday on the heels of the previous exiting wave. Models continue to favor an active weather pattern leading up to the Christmas holiday. Snow levels start around 6000 feet and slowly rising to around 7500 feet by midweek. Snowfall totals of a couple of feet above 7000 feet would be a conservative estimate at this time. Snow totals below 7000 feet will be less and vary widely due to the rising snow levels through Wednesday. Expect breezy conditions to develop early in the week as the surface gradient begins to tighten Tuesday as a strong cold front moves into the Pacific Northwest with temperatures warming to above seasonal averages by Tuesday. Models are hinting at a colder, more dynamic system approaching the west coast late Wednesday to early Thursday. 

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: MOSTLY CLOUDY MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY.
Temperatures: 32 TO 39 deg. F. 23 TO 29 deg. F. 33 TO 40 deg. F.
Wind direction: WEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 25 MPH WITH GUSTS TO 35 MPH 15 TO 20 MPH 20 TO 30 MPH. GUSTS INCREASING TO 55 MPH IN THE AFTERNOON.
Expected snowfall: NO ACCUMULATION in. NO ACCUMULATION in. NO ACCUMULATION in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY CLOUDY MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY.
Temperatures: 27 TO 34 deg. F. 20 TO 26 deg. F. 29 TO 35 deg. F.
Wind direction: WEST WEST SOUTHWEST
Wind speed: 20 TO 30 MPH. GUSTS UP TO 45 MPH IN THE EVENING. 15 TO 20 MPH WITH GUSTS TO 30 MPH. 20 TO 25 MPH WITH GUSTS TO 40 MPH INCREASING TO 30 TO 35 MPH WITH GUSTS TO 60 MPH IN THE AFTERNOON.
Expected snowfall: NO ACCUMULATION in. NO ACCUMULATION in. NO ACCUMULATION 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 Avlanche Center who is solely responsible for its content.

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