Eastern Sierra Snowpack Summary - 2016-02-01 08:26

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THIS ADVISORY EXPIRED ON February 4, 2016 @ 8:26 am
Avalanche Advisory published on February 1, 2016 @ 8:26 am
Issued by Doug Lewis - 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.

Sensitive wind slabs will continue to form throughout the forecast period due to strong North to Northwest to West winds (40 to 50 mph, decreasing to 20 to 30 mph by Tuesday). Natural avalanches are possible and human triggered avalanches very likely in wind loaded terrain >30 degrees at mid to upper elevations initially on E-S-W aspects but will shift to N-E-S—SE facing slopes as the wind veers towards the Northwest to West.  Be on the lookout for these fresh dense wind slabs especially below ridgelines and cross-loaded slopes.  These wind slabs will be fairly obvious, denser, hollow sounding snow.  While exposed slopes will be of most obvious concern, high winds can also deposit snow into deeper soft wind slabs that could be sensitive to human triggering in open forested terrain.  These deposits in more protected terrain may only be slightly more dense than snow in un-wind affected areas.  Shooting cracks are an obvious warning sign that these softer wind deposits exist and may be unstable.  These avalanches will likely range from small slabs that could knock someone off balance, to carrying a person into undesirable terrain, to much larger avalanches that could bury, injured and kill a person.

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.

Up to 2 feet of new snow fell through this recent cycle with a foot or more of low-density snow laid down over denser snow with relatively poor bonding. As the new surface snow settles it will become more slab like with the bond to the underlying snow is still forming. This is a weak combination and will need a couple of days to stabilize. In sheltered areas, natural avalanches are unlikely but triggered a possible on slopes of 30 degrees and steeper. Mid to upper elevations are the highest concern, where snowfall amounts are greatest. Be on the lookout for red flag indicators of instability including whumphing, shooting cracks, and recent avalanche activity. The depth of the new snow is sufficient that even a small release may be capable of burying a skier. Caution is strongly advised while traveling in the backcountry and regularly reassess snow conditions and stability as you move into to higher or steeper terrain and new areas. 

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 deep slab instabilities (Depth Hoar) often increases during periods of rapid loading but recent stability test have shown little reactivity and observations have reported little deep slab activity. The mid and upper snowpack is showing signs of bridging over the Depth Hoar in the Mammoth Basin where the snowpack is the thickest. Outside of the Mammoth Lakes Basin, this bridging effect is less well developed and is more suspect. While it is unlikely, there remains a possibility that a smaller avalanche in the upper new snow could produce enough force in a sweet spot that could result in a deeper much larger slope-wide failure.  This problem is more of a consideration in areas with a shallower snowpack (<1.5 meters) north and south of the Mammoth Basin.  The Surface Hoar that formed early last week was likely destroyed by the warm temperatures and strong pre-frontal winds (Thurs-Fri) with no reports of Surface Hoar in recent snow observations.

 

Snowpack Discussion

Heavy snow fell throughout the region with the reports of 20" to 27” at some of the backcountry Snowtel sites, Mammoth Mountain came out on top with 30" new snow with 4” of snow water equivalent. The weather prior to this latest storm was unseasonably warm, which helped to settle & strengthen the snow. The first storm system came through the region with warm temperatures, wet snow and strong SW winds. Some light rain and drizzle and high density snow was reported as this first short wave moved through the region. Temperatures during this initial wave were well above freezing at most location with wet snow and strong SW winds. As this wave eased out of the region to the east, another storm system slipped in behind with cooler temperatures and lower-density snow. The system lingered through Sunday into Monday with winds picking-up from the North to Northwest.

Stability assessments in the area during this storm cycle have yielded widely varying results. Areas that are well protected, such as Earthquake Dome and June Lakes, show little signs of instability. While in areas that are less protected there have been numerous reports of cracking, slabs shifting, ski cut slab failures (4”-16”) throughout the Sherwin Forest, Panorama Dome, and the Mammoth Mountain. These reports show a weak layer within the new snow with a slab overlaid of varied density, slab characteristics, and reactivity. Areas were the wind deposited a surface slab seems to be the most reactive while other areas show more storm slab characteristics. The unseasonably cold temperatures during the early part of the coming weak will preserve the storm slab tenderness for the next couple of days but windslabs will continue to be the primary concern due to the forecasted strong Northerly winds and abundant supply of transportable snow. This combination has the potential to produce large windslabs throughout the mid to upper elevations. The forecasted Northerly winds (40 to 50 mph, gusts of 75 mph) will form fresh windslabs predominately on E-S-W aspects. Monday evening winds will begin to ease (35 to 40 mph, gusts 60 mph, decreasing to 25 to 30 mph, gusts to 50 mph after midnight).  This scenario will continue to form fresh windslabs throughout the mid and higher elevations with plenty of new snow available for transport.  Anticipate windslabs on NE-E-SE-S-SW aspects, especially areas where there is a downwind fetch. As the week begins, in the mid to upper elevations: natural avalanches possible, triggered releases are likely on slopes of 30 degrees and steeper.  As temperatures slowly rebound and the snow has time to adjust to the recent now load, we can expect the storm slab instability to slowly decline but the windslab concerns will continue.

The deep slab instabilities (Depth Hoar) often increases during periods of rapid loading but recent stability test have shown little reactivity and observations have reported little deep slab activity. The mid and upper snowpack is showing signs of bridging over the Depth Hoar in the Mammoth Basin where the snowpack is the thickest. Outside of the Mammoth Lakes Basin, this bridging effect is less well developed and is more suspect. While it is unlikely, there remains a possibility that a smaller avalanche in the upper new snow could produce enough force in a sweet spot that could result in a deeper much larger slope-wide failure.  This problem is more of a consideration in areas with a shallower snowpack (<1.5 meters) north and south of the Mammoth Basin.  The Surface Hoar that formed early last week was likely destroyed by the warm temperatures and strong pre-frontal winds (Thurs-Fri) with no reports of Surface Hoar in recent snow observations. 

recent observations

Recent observations have shown a wide range of conditions. More exposed mid elevations have shown a very sensitive conditions with shooting cracks and triggered releases while approaching convex rolls (About 75-100' across, 12-16" crowns, slid about 800' down slope).  The slab was new low-density snow on a much more dense layer of storm snow that fell before temperatures dropped. The new snow has not had enough time to bond well to the underlying snow, which will take a few days to form and stabilize.     

Elsewhere (V-Bowl, below Dana Plateau) reports of almost a foot deep layer of grauple (pellet like styrofoam balls) from 8500' to at least 9200', likely up to 10,000', in one of the chutes lookers right of V-Bowl which could act as a very dangerous weak layer once buried.  The observers turned around about 9200' due to the fact that the overlying slab above this grauple layer was getting thicker as they ascended (up to 6" where they turned around).  Grauple layers tend to stabilize a day or two after a storm, however, this is an unusually thick layer with a fresh slab formed on top is potentially a dangerous combination.  Distribution throughout the area is unknown but it is something to look for while performing your snow assessments.

June Lakes region (lower Dream Peak) showed a very soft slab over denser snow with little propensity to propagate. It showed a weak slab like characteristics but expect it to become more defined with the potential for increase propagation as it compresses and becomes more slab like. No deep or mid-pack weaknesses noted.

 

weather

A quieter pattern will prevail for this week with an overall cold next few days.

Monday - The center of the surface Low, which brought heavy snowfall across the forecast area, has moved off into Nevada with a few lingering light snow showers continuing across the region. As the surface low deepens over southern Nevada, it will produce strong with wind chills of teens and single digits expected across much of the region. Areas of blowing snow may also limit visibility at times especially across portions of Mono and Mineral counties where wind gusts up to 45 mph are possible through Monday afternoon.

Tuesday – The High Pressure Ridge holds fast over the west coast with winds shifting to the Northwest and decreasing to 35 to 40 mph with gusts of 60 mph.

Wednesday – A quick moving trough is forecast to move into northern CA Wednesday with a chance for light snowfall Wednesday night.

Thursday - Predominant feature will be a significant ridge developing along the west coast. This will block big precip producing storms from the region.

Temperatures will begin to warm Thursday onward with daily highs approaching 50 with increasing gusty Northeast winds for the Sierra.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. BLOWING SNOW IN THE MORNING. SLIGHT CHANCE OF SNOW IN THE MORNING. PARTLY CLOUDY MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS
Temperatures: 11 TO 18 deg. F. 4 TO 10 deg. F. 17 TO 24 deg. F.
Wind direction: NORTH NORTHWEST WEST
Wind speed: 40 TO 50 MPH. GUSTS UP TO 75 MPH DECREASING TO 65 MPH IN THE AFTERNOON. 35 TO 40 MPH WITH GUSTS TO 60 MPH DECREASING TO 25 TO 30 MPH WITH GUSTS TO 50 MPH AFTER MIDNIGHT. 20 TO 30 MPH WITH GUSTS TO 50 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. BLOWING SNOW IN THE MORNING. SLIGHT CHANCE OF SNOW IN THE MORNING. PARTLY CLOUDY. MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS.
Temperatures: 7 TO 14 deg. F. -3 TO +4 deg. F. 11 TO 18 deg. F.
Wind direction: NORTH NORTHWEST NORTHWEST
Wind speed: 50 TO 55 MPH WITH GUSTS TO 80 MPH DECREASING TO 40 TO 45 MPH WITH GUSTS TO 65 MPH IN THE AFTERNOON. 40 TO 45 MPH WITH GUSTS TO 65 MPH DECREASING TO 30 TO 35 MPH WITH GUSTS TO 55 MPH AFTER MIDNIGHT. 25 TO 35 MPH WITH GUSTS TO 55 MPH.
Expected snowfall: 0 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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