The recent storm system to move through the region Tuesday thru Thursday with snowfall rates 1” to 2” an hour, depositing 2’- 3’+ of new snow down to 7,500’ in the Mammoth Basin, somewhat lesser amounts north and south. Warm air moved in during the storm cycle Wednesday morning (temperatures jumped 3 degrees in an hour) created a density inversion (upside-down snowpack) within the new snow and formed a sensitive slab. The new snow rests on a variety of snow surfaces depending on aspect, elevation, and whether the slope is shelter from wind. Sheltered locations the old snow surface is a mix of Facets/ Surface Hoar (in exceptionally sheltered areas). Solar aspects saw warm temperatures and sunny skies the past week, which has resulted in melt-freeze crust on slopes with some southerly exposure (SE-S-SW, mid to low elevations). Elsewhere, wind board in the exposed mid to upper elevations. All of these interfaces can act as a bed surface and when combined with Storm Slab conditions is a recipe for widespread instability. Thursday - Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Natural avalanches likely; human- triggered avalanches very likely. Friday – Storm Slab avalanche conditions should improve slowly but dangerous avalanche conditions still exist. Careful snowpack evaluation required, cautious route-finding and conservative decision-making essential. Natural avalanches are possible; human- triggered avalanches likely. Cold temperatures, settlement, and time will help ease the Storm Slab threat somewhat but careful snowpack evaluation will be required, cautious route-finding and conservative decision-making essential. Natural avalanches are possible; human- triggered avalanches likely on slopes of 32 degrees and steeper, all aspects and elevations. As snowfall diminishes and the snowpack settles and gains strength over the next 24 to 48 hours, the risk of natural Storm Slab avalanches will begin to ease but triggered release will remain very likely throughout the valid Advisory period.
Moderate Southwesterly to Westerly winds have deposited new snow at a much greater rate and higher density than the natural snowfall forming sensitive Wind Slabs, primarily on exposed NW-N-E-S aspects, mid to upper elevations (above 9,000'). These can most likely be encountered on exposed slopes 32 degrees and steeper, near treeline and above, below ridgelines, cross-loaded slopes and sidewalls of gullies. Natural avalanches will remain likely; human- triggered avalanches very likely. These may become large enough to bury a person, especially if associated with a terrain trap.
Several underlying weak facet/melt-freeze crust layers exist in the snowpack in many areas throughout the forecast area between 9,200’ and 11,200’, primarily E-N-W aspects. These weak faceted layers formed during the warm storms in October and early December where rain observed up to 11,200’. In some areas these layers are buried over three feet deep (Mammoth Basin), less than a foot (south of Mammoth or north of June Lake). The recent new heavy snow load and new snow avalanches could produce enough stress to these underlying layers to cause them to fail, especially areas where these Persistent Weaknesses are not deeply buried (Rock Creek, McGee, Convict, Bishop and north of June). If triggered, the resulting avalanches could likely be very large and destructive, failing well above a human trigger.
Because of the widely varying conditions, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.
The latest in a series of storms moved into the region Tuesday with light snow, increasing steadily throughout the day. By late afternoon it was snowing 1” to 2” an hour through Wednesday. Snow accumulations - Mammoth Pass 28", Virginia Lks 21", Rock Crk: 29", Gem Pass 25", June Mtn Summit 15". Snow Water Equivalent ranged from just over 6” at Rock Creek to 1.62” recorded at June Lakes Snow Study Area. Temperatures were more moderate than originally forecasted, with temps mostly in the mid 20’s. As warm air moved in Wednesday morning, temperatures jumped 3 degrees in an hour creating a density inversion (upside-down snowpack) and briefly raising freezing levels to above 7,500’.
The new snow rests on a variety of snow surfaces depending on aspect, elevation, and whether the slope is shelter from wind. Sheltered locations the old snow surface is a mix of Facets/ Surface Hoar (in exceptionally sheltered areas). On Solar aspects warm temperatures and sunny skies the past week has created melt-freeze crust on slopes with some southerly exposure (SE-S-SW, mid to low elevations). Elsewhere, exposed mid to upper elevations the snowpack is capped by wind board/Wind Slab. All of these interfaces can act as a bed surface and when combined with Storm Slab conditions is a recipe for widespread instability, all aspects and elevations. As the system moves off to the east and cold artic air moves into the region Thursday evening into Friday, the combination of time, settlement, a cold temperatures will help strengthen the Storm Slab weaknesses but Wind Slabs will remain a concern due to moderate prevailing Westerly winds continuing through Friday for the mid to upper elevations (above 9,000'). Persistent Deep Slabs will remain a concern for the foreseeable future as an Atmospheric River event takes aim at the region for the weekend.
Because of the widely varying conditions along the length of the Range, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.
Mammoth Pass: Storm snow - 28"; Snow WC - 3.6. Virginia Lks: Storm snow - 21"; SWC - 3.1. Rock Crk: 29"; SWC - 6.05. Gem Pass: 25"; SWC - 1.51. June Mtn Summit: Ave Winds - 20.48; Ave Direction - 191 deg. June Study Plot: Storm snow - 15"; SWC - 1.62
Mammoth Mountain, Mammoth Basin (1/4/17) - Hand charges and ski cuts producing widespread avalanche activity at low to mid mountain. 12-18" crowns, wind slabs and storm slabs in sheltered locations, some running quite far, up to R4-D3. 18-22" new snow early this morning, with upside-down structure (denser snow on top). Strong SW winds, loading leeward features quickly. Evidence that natural slides occurred before control efforts as well. Several patrollers caught in slides while ski cutting, no serious injuries but multiple lost skis.
Northeast Face, South Peak, Virginia Lakes (1/2/17) - Shallow snow cover mid to lower elevations (12-20"). As you move into more exposed terrain, snow depth is variable because of wind transport. Snow surface - .5 cm new, wind board, patches of faceted snow. Plenty of evidence of wind effect from New Years Day, elevated tracks in places. Foot penetration - 50cm. Wind westerly, blowing snow SW-W. Surface temp -12.2 C, Aspect N, Angle 27 degrees, Elevation 10320'. Lite Melt/Freeze Crust on Solar Aspects.
Stability Tests - CTM 2 X2 @ 143cm, CTE0 @ 143 (failed on isolation), CTM19, CTH20, CTM15, CTM 11 all at 20 cm facet layer. Deep persistent layer not currently active but may become overstressed by avalanche activity in the upper snowpack or with rapid loading resulting in failures stepping down deep into the snowpack. Will need to monitor with approaching system.
Chicken Wing, June Lake (1/2/17) - Route: From 395 up Glass Flow Rd, up NE slope of chicken wing, back down. Thin snow, but supportable and skiable! 6-8" down low (8,100'), up to 12-20" up high (9,600'). North exposed slope wind-effected with areas of firm Wind Slab (no shooting cracks observed). NE slope more wind sheltered, pit dug at 9,500' ENE facing, see attached profile. Of note, rain crust 10cm down with some faceted snow underneath (CT test failed under this crust with moderate force, Q1-2 result (in side of pit near shallow rocks), ECT test failed but did not propagate on this layer). Also, bottom 20cm of snowpack well faceted and loose, but ECT test or CT test did not fail at all in this layer. Moderate SW winds, no visible snow transport today. No sensitive Wind Slabs found near ridgeline here (often this ridge does have small cornices and slab formation ... surprised not too find any today). Windy in exposed areas (the flat rd in, the ridgetop and more the ridgy north slope), NE slope wind sheltered. Upper part of snowpack is faceting, good skiing/riding conditions, save for the shallowness and obstacles to be weary of.
* ...Winter Storm Warning now in effect until 10 am PST Thursday...
- Moderate to heavy snow will continue until early Thursday morning.
- Snow levels: 6000 to 7000 feet.
- Additional snow accumulations: 6 to 12 inches above 7000 feet with up to 6 inches between 6000 and 7000 feet.
- Impacts: dangerous conditions for travel and outdoor activities. Whiteout conditions are likely over mountain passes.
Thursday thru Friday - Snowfall rates will slowly diminish as moisture becomes shallower after daybreak as the cold front pushes south with enough lift to keep some snow going through mid-late morning. Some warming has occurred with a short-lived rain-snow mix in Mammoth Lakes (weighing down tree branches and power lines), colder air will change precip back to snow by daybreak. Isolated to scattered snow showers will linger through this afternoon and early evening. A break in the precipitation later this evening through Friday night with clearing skies and very cold temperatures.
Sat thru Monday -
- Moderate to Strong Atmospheric River likely Saturday-early Monday.
- Flood Impacts Likely Sunday-Monday.
- Poor conditions for travel and outdoor activities this weekend.
Snow levels should rise rather quickly Saturday afternoon/evening as deeper moisture and warmer air arrives. Snow levels are forecast to peak Sunday morning through Sunday evening, then edge slightly lower later Sunday night. The highest snow levels will coincide with the most intense precipitation, 9000-10000 feet. Liquid amounts for this event Saturday through Monday could approach 12 inches along the entire length of the Sierra Crest, with 6-12 inches between the Sierra Crest and foothills west of US-395. Snow totals above the snow line will be quite large (3-6 feet), with snow to liquid ratios at roughly 7-8:1. Both the GFS/ECMWF are on board now with lowering snow levels and a winding down of precipitation Monday by early to mid afternoon.
This Snowpack Summary is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.