Eastern Sierra Snowpack Summary - 2016-01-25 06:57

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THIS ADVISORY EXPIRED ON January 27, 2016 @ 6:57 am
Avalanche Advisory published on January 25, 2016 @ 6:57 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.

Windslabs from the latest storm (1/22 – 1/23) are slowly gaining strength but moderate winds are forecast for the snow summary period with winds forecasted from the North Monday then shifting to the Northwest Monday evening before veering back to the North for Tues. There will be sufficient wind to continue building localized windslabs into Tuesday. Cross-loaded features will be of concerns given the wind direction (N-NE). This is mostly confined to the mid and upper elevations but possible in localized lower elevations where upwind fetches provide ample snow for windslab formation. These wind slab avalanches could be large enough to cause injury or death. On steeper terrain, attempt to avoid freshly drifted snow, be looking for signs of windslabs (hollow sounding slabs, shooting cracks, small slab failures). Constantly reassess conditions and objectives as more information is gained. 

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.

Deep - Depth Hoar: present throughout much of the eastern Sierras. In the Mammoth and June Mountain region, the depth of the snowpack is mostly compensating for this weakness.  However, as you travel north or south of this region, the snowpack is much shallower (less than 5 feet) and the possibility of triggering a deep release is more of a concern, especially when approaching steeper terrain.

 

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.

Mid-snowpack: Isolated pockets of Surface Hoar have been reported in the Mammoth Basin and June Lakes regions. Where present (~1.5’ - 2.5’ down in the snow pack) can potentially be very weak layer.  Favored locations: sheltered open areas below treeline, open spaced trees, creek beds, road banks. While traveling in these areas, regularly assess the snowpack for localized changes or isolated weaknesses. 

Snowpack Discussion

The latest storm (Fri.-Sat.) has exited the region with dense snowfall in its wake. Snotel sites reported amounts ranging from 4’ to 16” with snow densities of 11% and greater in many locations. This storm system generated a significant natural avalanche cycle with plenty of evidence of avalanche activity during the storm cycle. (Sherwin Trees, Mt. Gibbs, Dana Plateau, etc.). The combination of dense snow and strong Southwest winds deposited storm snow over less dense snow creating wide spread windslab issues in the mid to higher elevations on leeward slopes (NW-N-NE-E) as well as storm snow instabilities (Mammoth Mountain - R2-D2s, 1-3' crowns, all in storm wind slab snow). The storm snow instabilities have begun to subside as mild temperatures have set in. However, sensitive windslabs still exist throughout the mid – higher elevations, especially where a significant downwind fetch exists or terrain feature promote cross-loading (rock ribs, tree fingers, sides of gullies or depressions, etc.) on slopes of 35 degrees and steeper.  

Prior to the arrival of this latest storm, temperatures spiked throughout the region with significantly above freezing temperatures reported throughout much of the region (Mid 40’ to low 50’s) with some lower elevations areas (up to 7,800’) the snowpack briefly went isothermal where the snowpack was shallow. As the storm moved off to the east, temperatures have fallen to near normal and the snowpack has cooled with the low elevation areas re-freezing and forming a well consolidated and supportive base. However, the snowpack depth in these areas was greatly reduced with extensive natural obstacles resurfacing. Caution is strongly advised.

Deep (Depth Hoar) continues to be found widely throughout the forecast area while persistent instabilities (Surface Hoar) continue to be found in more isolated pockets (shelter trees, gullies, creek beds). These persistent weak layers have shown little reactivity in recent stability tests but should be factored in when approaching any steep terrain, especially where the consequences of a slide are severe.

During the next few days, ahead of the next storm, winds are forecast to be moderate (NW>N>NE), which will continue to form fresh windslabs prone to human triggering in the mid-upper elevations. Windslab formation and location will change as the wind directions moves around the compass rose. Natural avalanches are unlikely but triggered releases are possible on slopes of 35 degrees and steeper.  Regularly reassess conditions throughout the day and be willing to adjust the objectives to the conditions.

 

recent observations

Dana Plateau

(1/21/16) Snowpack had gone isothermal and unsupportable (below 7800')

(1/24/16) Currently, re-frozen and supportive. Storm (1/22 -1/23) deposited ~5+/- cm of new snow at the lowest elevations (~6,500’), and up to 35cm up higher (~ 9,300’). Semi-supportive melt-freeze crust present under new snow up to  ~ 8000'.  Observed unconsolidated snow to relatively soft windslabs at the surface (2 - 8cm thick). Stability tests produced inconclusive results at various levels (non-propagating Q2-Q3). Bottom 15cm of snowpack composed of Depth Hoar, unreactive in stability tests even tough large columns can fail when isolated. Observed sensitive windslabs from 1cm-10cm thick in the mid to upper elevations. Evidence of numerous recent natural avalanches in chutes and below rock bands below Dana Plateau, which occurred during the recent storm cycle. Received reports of skier triggered releases in the Sherwin Forest and the Negatives. Mammoth Mountain control work produced: R2-D2s, 1-3' crowns, all in storm wind slab snow.

Storm Totals: Agnew Ps 1.5” Water, 11”Snow, Mammoth Ps 2.16” Water, 16” Snow, Rock Creek 1.19” Water, 4” Snow, South Lake 1.32” Water, 11” Snow, Tuolumne Meadows 1.2” Water, 11” Snow, Virginia Lakes .7” Water, Dana Meadow 1.3 Water, 6" Snow.

weather

A rather benign weather pattern Monday through Thursday as a ridge of high pressure builds over the west bringing dry conditions, light winds, and periods of clouds. Expect inversions in the valleys to develop as temperatures warm quicker aloft with poor vertical mixing. Temperatures will warm above seasonal averages into the mid/upper 40s for Sierra valleys. High cloud cover begins to increase later Wednesday as the next system approaches the California coast. The next potential winter storm is poised to impact the region next weekend with confidence med-high but details for this system are in question with differences amongst models with the timing and evolution. 

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 SUNNY. CLEAR. SUNNY.
Temperatures: 27 TO 34 deg. F. 16 TO 22 deg. F. 35 TO 42 deg. F.
Wind direction: NORTH NORTHWEST NORTH SHIFTING TO THE EAST IN THE AFTERNOON
Wind speed: 15 TO 25 MPH WITH GUSTS TO 40 MPH. 15 TO 20 MPH WITH GUSTS TO 35 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY THEN BECOMING SUNNY. CLEAR SUNNY
Temperatures: 26 TO 32 deg. F. 9 TO 16 deg. F. 34 TO 40 deg. F.
Wind direction: NORTH NORTH NORTH BECOMING NORTHEAST
Wind speed: 25 TO 35 MPH. GUSTS UP TO 55 MPH DECREASING TO 40 MPH IN THE AFTERNOON. 15 TO 25 MPH WITH GUSTS TO 40 MPH. 20 TO 25 MPH WITH GUSTS TO 35 MPH, 10 TO 15 MPH WITH GUSTS TO 25 MPH IN THE AFTERNOON.
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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