Avalanche Advisory: Saturday - Apr 7, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON April 8, 2018 @ 7:13 am
Avalanche Advisory published on April 7, 2018 @ 7:13 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Saturday (4/7) – the avalanche danger has risen to HIGH overnight (Friday) for all elevations, especially from near treeline and above, with continued heavy rain and dense snow forecasted thru Saturday AM. Sensitive dense Wind Slabs will likely be encountered in the upper elevations on NW-NE-SE-S aspects, initially ~11,000 and above but lowering during the day (Saturday) to ~9,000’. Natural Loose Wet are likely, Wet Slab are possible from ~ 11,000’ and below, especially in steep, rocky, or vegetated terrain, triggered releases are very likely. Natural & triggered avalanches from steep open slopes from near and above treeline may run well into forested terrain and be possibly quite destructive. Travel in avalanche terrain not recommended, natural releases likely, triggered releases very likely. 

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Loose Wet
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Today’s forecasts calls for continued heavy rain (snow above ~11,000’ and above, decreasing to 9000’ in the afternoon) further saturating the snowpack, melting the bonds holding the snowpack together with deep loose wet snow on all aspects, up to ~ 11,000’ and possibly higher. Loose wet avalanches are likely today on any steep slopes. Northerly aspects and higher elevation slopes have experienced less melt/freeze cycling during the warm weather with the potential for large loose wet instabilities. Signs of wet snow instabilities: large roller balls, deep ski penetration, and Loose Wet releases. Loose Wet can trigger larger deeper releases, especially in the lower elevations (~10500’ and below) where the snow has become isothermal and saturated. Natural avalanches are likely, triggered releases are very likely, especially in sheltered, steep, or rocky terrain. Natural & triggered avalanches, near and above treeline, may run well into forested terrain and be possibly quite destructive. Travel in avalanche terrain not recommended, natural releases likely, triggered releases very likely.

With the saturated snowpack, some Loose Wet avalanches could erode deeply into the snowpack and be large enough to bury or injure a person, especially when combined with a terrain trap

Avalanche Problem 2: Wind Slab
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The snow level will start out at ~11000 feet decreasing to 9000 feet in the afternoon. Strong to extreme (Gale Force) Southwesterly winds are forecasted above ~10000’ will form dense slabs on NW-NE-SE aspects from near treeline and above. Wind Slabs will most likely be encountered below ridgetops, under cornices, and gully sidewalls. Natural & triggered Wind Slabs avalanches from steep open slopes, near and above treeline, may run well into forested terrain and be possibly quite destructive. Travel in avalanche terrain not recommended, natural releases likely, triggered releases very likely.

 

Avalanche Problem 3: Wet Slab
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Wet Slab potential (~11000' and lower) has risen overnight from  as the snowpack became saturated with water, breaking down bonds between snow grains and layers within the snowpack, and the added water weight stressing an already weakened snowpack. North of Mammoth, early season weaknesses are still present in the mid and deep snowpack could act as weak layers or bed surfaces for water percolation as the snow above becomes saturated with water. Large triggers (Loose Wet, Wind Slabs, rock or cornice fall,) could trigger these underlying weak layers, resulting in a large avalanche. The problem will likely begin to slowly improve this evening as precipitation begins to subside and temperatures begin to fall this afternoon. Natural & triggered avalanches from steep open slopes, near and above treeline, may run well into forested terrain and be possibly quite destructive.  Natural releases possible, triggered releases likely.

advisory discussion

Heavy rain Friday and through the night has saturated an already loose wet snowpack. Widespread, potentially large, Loose Wet avalanches are expected today up to ~11000’, possibly higher. These may erode deep into the snowpack. Deep ski or boot penetration, rollerballs, and pinwheels are signs of loose saturated unstable snow. Loose Wet avalanches will be most wide spread avalanche problem for Saturday.

Snow overnight above ~11,000’ and strong SW winds have likely produced thick moist , Wind Slabs on NW-NE-SE aspects, some of these slabs could be large. Anticipate dense drifts under ridgetop cornices, tops of steep chutes, and sidewalls of gullies. As temperatures begin to cool this afternoon, Wind Slabs may begin to form down to ~9000’ in open and exposed terrain.

The snowpack is transitioning to a spring regime. South of Mammoth, the lower and middle elevation snowpack has mostly transitioned to a dense spring snowpack. North of Mammoth, on northerly aspects above ~10,000’, the snow is still cold and dry below the surface snow. The snowpack depth and colder nature can absorb more water and become saturated, weakening bonds between snow grains and adjacent layers and increasing the threat of Wet Slab avalanches, especially middle and upper elevations where Loose Wet or Wind Slabs avalanche could trigger a large avalanche.

 

recent observations

Sherwins: Wet Conditions (4/6)

Upper Mammoth Rock Bowl: Cornice Fall (4/6)

Negatives: Solar Warming & Cold Snow (4/5)

Rock Creek (4/5)

Red Cone (4/3)

Lundy (4/2)

Temperatures @ 0400

Loc                                          Highs/Lows

Virginia Ridge, 9409’:           47 / NA
Tioga Pass, 9972’:                 37/36 deg F. no freeze
Agnew Pass, 9355’:               49/36 deg F, no freeze
June Mt., 9148’:                     52/42 deg F, no freeze
Mammoth Pass, 9500’:         40/33 deg F, no freeze
Sesame Plot, 9014’:              41/33 deg F, no freeze
Ch. 22-MMSA, 10,067’:         38/33 deg F, no freeze
Summit-MMSA, 11,053’:      38/28 deg F, below freezing AM hours, 4/6
Rock Creek, 9600’:                45/37 deg F, no freeze
South Lake, 9580’:                            53/41 deg F, no freeze
Sawmill, 10,200’:                   49/37 deg F, no freeze

 

 

Weather and CURRENT CONDITIONS
weather summary

Saturday - Showers will continue in the Sierra this morning with the cold front is still on track to move through this morning (11 AM to Noon). The coverage of rainfall will increase ahead of the cold front with a brief 3-hr period of heavy rainfall. Rates of up to 1/2" per hour are possible in the mountains, especially near the Sierra Crest. In the lower elevations to the east, rates will be lighter, but may approach 1/4" per hour.  Precipitation through noon could reach 2+" along the Sierra Crest, 1" west of Highway 395. Behind the front, a strong post-frontal surface gradient produce winds of around 30 mph with gusts to 50-55 mph for much of    the area with a brief 1 hour period of very strong winds just behind the front, centered around late morning. Tonight, the winds will gradually diminish this evening with mainly scattered showers in the Sierra.

Sunday and Beyond - Drier and warmer conditions will prevail on Monday as high pressure briefly builds across the Sierra. Temperatures will warm into the 50’s and 60s for the Sierra valleys. The pattern remains active through next week with multiple storms on track to impact the Sierra. The first trough that drops into the Pac NW on Tuesday appears to brushes northern California with increasing clouds, light showers possible along with gusty winds (35 to 45 mph) on Tuesday. The next stronger wave drops in Wednesday afternoon thru Thursday, with much tighter thermal and pressure gradients that will produce strong winds Wednesday afternoon and evening. The main trough along with the cold front passes through the area early Thursday, bringing much colder air to near 5000 feet by early Thursday morning. Several inches of snow will be possible for the higher elevations. It will be fairly cold behind the front as the trough drops as far south as southern NV and northern AZ. By Friday there is shortwave ridging across the Sierra. Extended model simulations are showing ANOTHER cold trough dropping into the Pacific NW by Saturday and Sunday.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Rain in the morning, then chance of showers in the afternoon. Partly cloudy then becoming clear. Sunny.
Temperatures: 43 to 49 deg. F. 23 to 28 deg. F. 47 to 55 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 20 to 35 mph with gusts to 65 mph. 20 to 30 mph with gusts to 55 mph. 10 to 15 mph with gusts to 50 mph in the morning becoming light.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Rain and snow in the morning, then slight chance of snow showers in the afternoon. Partly cloudy then becoming clear. Sunny.
Temperatures: 37 to 42 deg. F. 19 to 24 deg. F. 42 to 47 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 45 to 65 mph with gusts to 95 mph decreasing to 35 to 55 mph with gusts to 75 mph in the afternoon. 30 to 50 mph with gusts to 75 mph. 30 to 40 mph with gusts to 70 mph decreasing to 15 to 25 mph with gusts to 45 mph in the afternoon.
Expected snowfall: 4 to 10 in. 0 in. 0 in.
Disclaimer
This Avalanche Advisory 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.

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