Eastern Sierra Snowpack Summary - 2016-01-28 07:32

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THIS ADVISORY EXPIRED ON January 30, 2016 @ 7:32 am
Avalanche Advisory published on January 28, 2016 @ 7:32 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 generated during the latest storm cycle (1/22 – 1/23) have slowly become less reactive but the pre-frontal Westerly winds forecasted for Thursday into Friday will build fresh windslabs on N-E-S aspects thru the forecast period. 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: 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 increase late Friday into Sunday, even in wind-protected areas on slopes steeper than 35 deg. at all elevations and aspects due to forecasted 1’-2’ of new dense snow along the Sierra Crest, 3-6" or more below 8,000’ that is expected by late Saturday. This warmer denser snow will fall on lower density faceted snow, which will likely create a weak layer as the new snow forms a slab above. This significant amount of new snow, natural avalanches will be possible, triggered avalanches will be likely.  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.

Deep - Depth Hoar: present throughout much of the eastern Sierra. 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.

Additional considerations: Surface Hoar - 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 recent bout of warm/clear conditions is coming to a windy (Thurs) and snowy end (late Fri - Sun). The onset of the warm temperatures and light winds this past few days (Sun – Wed) have helped the latest snowfall to build strength and from bonds with the adjacent layer without adding additional stress. However, windslabs formed during the latest snow cycle (1/22-1/23) have continued to show reactivity in favored steep terrain (leeward slopes and cross-loaded: depressions, gullies, chutes, rock fingers, tree fingers, etc.) with reports of pockets failing with crowns of ~1’ foot to 2’ (1/24). Mid to lower elevations have been impacted the most by the warm temperatures with melt freeze crusts on SE-S-W-NW aspects. Depth Hoar continues to persist throughout the region with it most prominent outside of the Mammoth Crest. An example, while performing stability tests in Solitude Canyon, some columns would fail on isolation on well-developed Depth Hoar. The recent cold clear nights have set the stage for Surface Hoar development and facet formation. Surface Hoar has been observed throughout the Sherwin Trees and in Solitude Canyon as well as other favored areas.

The primary concern for the upcoming 2 days is pre-frontal winds (W-SW, 25-45 mph) and snow to follow. The pre-frontal winds will form new windslabs on NW-N-E-SE aspects, especially in the mid to upper elevations. However, this may break up the Surface Hoar that has formed recently but it could be an issue in more sheltered terrain. As precipitation spreads south along the Sierra Crest the snowline will initially be very high (8,000’). The combination of strong winds and warm temperatures will generate dense snow with the potential for quickly forming slabs over the current surface facets (possibly Surface Hoar) creating storm slab concerns as the weekend unfolds.

An additional consideration as this storm develops, it has the potential for over 2” of water or greater in a fairly short time, when combined with the potential for heavy windloading, this will likely be sufficient to generate natural avalanches in the higher elevations, possibly extending lower.  Stay alert for changing conditions and be alert for red-flags (recent avalanches, collapsing, whumpfing, rapid windloading, etc.) Maintain a flexible agenda and be willing to alter plans. 

recent observations

This last snow cycle produced extensive avalanching throughout the range with observed avalanche activity in the mid to upper elevations with slides observed along the Dana Plateau, Mammoth reported (1/24) large avalanches from previous week's storms: R-3 (medium relative to path), D2-3 (bury injure/kill a person to bury or destroy a car or wood frame house, break a few trees), debris sighted on the flanks of Mt. Gibbs (E Face). Reports of skier triggered releases in the Sherwin Tress (1/23), Additional reports include: Upper Dry Creek ran past the 3-5 Saddle, and Scotty’s ran past Terry’s cliffs. A skier triggered released while on belay outside of June Lakes ski area (3D Chute, 1/24) as well as a ski triggered release with 1 ft crown in a chute next to hole in the wall (Y-Chute) as well as Hole In The Wall.

Recent observations have shown tender windslabs, especially in favored steep terrain (leeward slopes and cross-loaded terrain: depressions, gullies, chutes, rock fingers, tree fingers, etc.). This is evidenced by reports of triggered pockets. The mid-pack continues to strengthen and compensate for deeper weaknesses.

Depth Hoar continues to persist throughout the region, most prominent outside of the Mammoth Crest region. An example, while performing stability tests in Solitude Canyon, some columns would fail on isolation on well-developed Depth Hoar. Yet, Extended Column Tests indicate little tendency for propagation.

The recent cold clear nights have set the stage for Surface Hoar development and facet formation. Surface Hoar has been observed throughout the Sherwin Trees and in Solitude Canyon as well as other favored areas. 

weather

Our relatively benign weather will come to an end Thursday night with a winter storm watch in effect Friday evening thru Saturday afternoon for Sierra.

Details:

includes the cities of...Bridgeport...Mammoth Lakes

 Winter storm watch in effect from Friday evening through Saturday afternoon.

 * Timing: Strong winds with rain and snow will move into Mono County on Friday and continue into Saturday afternoon. Heavy snow is likely Friday night and Saturday.

 * Snow levels: 8000 to 9000 feet Friday evening, lowering to 6000 to 7000 feet Saturday afternoon.

 * Snow accumulations: 10 to 18 inches along the Sierra Crest...including Mammoth Mountain. 5 to 10 inches between 7000 and 8000 feet, including Mammoth Lakes and June Lake. Up to 4 inches possible along highway 395.

 * Winds: southwest 20 to 30 mph with gusts to 60 mph, with Sierra ridge gusts over 100 mph.

 * Impacts: dangerous conditions for recreational activities and travel along highway 395. Strong winds may damage drought stressed trees with power outages possible.

 

Friday - Confidence remains high for a moderate storm system for the Sierra starting to spread along the Sierra Crest Friday morning but some simulations show the front stalling until later Friday afternoon/evening Friday. The initial onset of light precipitation will be warm with dense snow, snow levels 8,000 to 9,000 feet. A rain shadow (An area of reduced precipitation on the lee side of a mountain barrier caused by warming of air and dissipation of cloudiness as air descends the barrier) develops in the Lee of the Sierra Friday as winds veer from the West to South-Southwest and wind speeds increase. Gusts could reach 80 mph in the wind prone areas and foothills along US-395. Sierra ridge gusts may easily exceed 100 mph. Once the front passes winds will shift toward the north and decrease.

Saturday - Model simulations have come into better agreement with the cold front moving from north to south Saturday morning into afternoon. Snow levels should fall to valley floors (4000-5500 feet) behind the cold front with the deeper moisture pushing south with the great snowfall along the Sierra Crest (1”-3” Precipitable water), East of US-395 in Mono County 0.75-1.5 inches with the heaviest precipitation occurring with frontal passage. For locations east of Highway 395, rain should change to snow Saturday with 1 to 4 inches Saturday through Saturday night, with locally higher amounts possible.

 Sunday - active winter weather is likely to continue into Sunday as cold air mass associated with an upper trough (An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.) spreading across the Sierra with highs a good 10+ degrees below average. The potential for additional heavy Sierra snow appears to be decreasing as the bulk of the moisture shifts into Southern CA, moderate snow accumulations are still possible over the Sierra.

Monday - cold conditions will prevail on Monday with highs only in the 30s and some moderate northwest winds enhancing the chill. Highs are expected to remain below normal with overnight lows in the teens to lower 20s except single digits for Sierra valleys. If the cloud cover thins, then areas could end up at least 5-10 degrees colder. 

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: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. MOSTLY CLOUDY. SLIGHT CHANCE OF RAIN AND SNOW AFTER MIDNIGHT. CHANCE OF RAIN AND SNOW IN THE MORNING. RAIN AND SNOW LIKELY IN THE AFTERNOON.
Temperatures: 42 TO 52 deg. F. 29 TO 35 deg. F. 39 TO 49 deg. F.
Wind direction: WEST WEST SOUTHWEST
Wind speed: 10 TO 20 MPH WITH GUSTS TO 30 MPH 15 TO 30 MPH WITH GUSTS TO 45 MPH. 25 TO 45 MPH WITH GUSTS TO 70 MPH.
Expected snowfall: 0 in. 0 in. 0-2 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW AFTER MIDNIGHT CHANCE OF SNOW IN THE MORNING...THEN SNOW LIKELY IN THE AFTERNOON.
Temperatures: 32 TO 42 deg. F. 25 TO 35 deg. F. 28 TO 38 deg. F.
Wind direction: WEST WEST WEST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 45 MPH. 30 TO 45 MPH WITH GUSTS 50 TO 70 MPH. 45 TO 65 MPH WITH GUSTS 75 TO 100 MPH.
Expected snowfall: 0 in. 0 in. 0-2 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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