Avalanche Advisory: Saturday - Apr 28, 2018

Avalanche Advisory published on April 28, 2018 @ 7:01 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

This is the last snowpack summary for the 2017/18 season. We would like to thank everyone for your financial contributions and snow observations throughout the season. We couldn’t provide this valuable service without your support. Enjoy the spring skiing. We are looking forward to serving the Eastern Sierra community next season. 

Loose Wet avalanches continue to be the primary concern through Monday. Timing is key to avoiding Loose Wet avalanches. Plan your travels to be clear of steep slopes before the snow becomes loose and unsupportive. Signs of the snow is becoming increasingly saturated, loose, and potentially unstable: pinwheels or rollerballs, small wet sloughs, and deep ski or boot-top penetration. These are good indicators that the snow is becoming increasingly unstable and Loose Wet avalanches are becoming increasingly possible, time to find another aspect that offers more supportive skiing or retreat to lower angled slopes. Small natural Loose Wet avalanches are unlikely but becoming possible later in the week as temperatures begin to rebound. Triggered Loose Wet (D1) releases are possible, becoming more likely as temperatures begin to warm later in the week in steep or complex terrain, primarily on E-S-W-NW aspects.  

Caution – Slide for Life conditions exist on steep slopes prior to thawing where an unarrested fall can have serious consequences. Ski and (or) boot crampons, ice axe, and helmet recommended. 

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Loose Wet
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Spring is firmly in place with warm temperatures, cool nights, and corn snow. A weak upper level low moving through northern California has ushered in cooler temperatures, which allowed for a modest freeze overnight at most sites, with modest freezes forecasted through the early part of next week. This will be accompanied by more moderate high temperatures during the day. The combinations will improved the corn skiing while reducing the potential for Loose Wet releases. In the upper elevations (~10,500’ and above) on N-NE aspects the snow is still transitioning to corn snow and may be slow to thaw during the day. Anticipate lower elevations warming first and most, though many low elevation slopes lack sufficient snow coverage to produce significant avalanches or is thin and relatively well anchored. Timing your travels is key to avoiding Loose Wet avalanches. Plan on being off of steep slopes before the snow become unsupportive, especially in steep, complex, or vegetated terrain. Use additional caution around steep vegetated or rocky slopes where the snow is often punchy and are likely places for triggering Loose Wet releases. Signs the snow is becoming increasingly saturated, loose, and potentially unstable: pinwheels or rollerballs, small wet sloughs, and deep ski or boot-top penetration are indicators that the snow is becoming increasingly unstable and Loose Wet avalanches are becoming increasingly possible on slopes ~35 degrees and steeper. Small natural Loose Wet avalanches (D1’s) possible, especially on E-S-W-NW aspects though the day. Larger natural releases are unlikely. Triggered Loose Wet (D1) releases are possible and becoming more likely as temperatures begin to warm later in the week in steep or complex terrain, primarily on E-S-W-NW aspects. Small D1 avalanches may trigger a larger releases, especially after an extend thaw or possible rain on snow event. 



advisory discussion

This is the last Snowpack Summary for the 2017/18 season but there’s still plenty of snow in the higher elevations for the motivated skier. As the season moves deeper into Spring, keep in mind that the danger of Loose Wet avalanches will likely linger through May in the mid to upper elevations, especially on steep solar aspects and in rocky terrain. Generally, they’ll be relatively small (D1 most likely, natural and triggered) with a few D2 possible in isolated locations. Larger (D3) releases may be possible in favored locations but will likely require a large trigger (cornice fall, etc.) or heavy rain. The best way to limit the threat of Loose Wet avalanches is to plan your travels around early AM starts and completing your travels before mid-day. N-NE aspects will be slower to transition fully to corn snow and will likely lag behind the rest of the aspects in terms of improving stability until they have had an opportunity to cycle through a few good freeze thaw cycles. 

Spring can bring fast moving storms that are accompanied by quick hit of new snow and strong winds, which can quickly form Wind Slabs on top of the firm spring snow. The firm corn snow can act as a good bed surface for Wind Slabs to slide on but they generally strengthen and bond fairly quickly. Use additional caution if going out into the backcountry during a strom or immediately after it clears. 

Keep in mind the different hazards that spring skiing brings such as: Slide For Life conditions, rock fall, hazardous creek crossing, cornice fall or collapse, and occasional lightening.  

Firm snow conditions are a major hazard should a fall occur in steep or complex terrain. Plan accordingly to either avoid these slopes until they begin to thaw or carry ski and/or boot crampons, ice axe, and a helmet are recommended. These tools can reduce the risk and consequences of a fall but you need to learn how to use them safely and practice self-arrest regularly to maintain proficiency. 

Cornices – slopes with overhanging cornices should be avoided or approached with extreme caution. This is the time of year where they begin to sag and collapse without warning, and triggering an avalanche on the slope below. Also, be cautious while traveling along corniced ridgelines where  cornices may fail without warning and often break much further back from the edge than anticipated. 

Rock fall – can occur anytime but often picks up as the sun begins to heat rocky faces or as water begins to freeze in the cracks. Keep in mind couloirs are great way to ascend but they act as garbage chutes for rocks and potential cornice falls. A helmet can protect you from small rocks but timing and good route planning is your best defense. 

Thunder Storms – not a frequent visitor to the eastern Sierra but not uncommon this time of year when the upper atmosphere is unstable. The weather service is quite good at identifying the conditions for thunderstorm development. Check the forecast to determine the likelihood prior to embarking for the backcountry. Generally, they begin to form over the high country mid-morning and become fully developed by mid afternoon. Plan to be out of the mountains early if thunderstorms are forecasted. 

Creek crossings – most snow bridges have collapsed at this point or are on the verge. This may mean crossing creeks with fast moving water. Do not underestimate the power of swift water. Water approaching knee height is borderline for crossing without the aid of poles or partners. Beware of what’s down stream before any crossing. Avoid crossing with sweepers, dead snags in the water, or rapids down stream. Hypothermia is a real concern during the spring if a person becomes immersed for any length of time.  



recent observations

Black Mtn, E and NE Aspects (4/27)

Rock Creek: Conditions (4/26)

Red Cone (4/26)

Rock Creek: Warming & Avalanche Activity (4/24)

Black Mountain, Red Lake Bowl > North Face (4/23)

Mammoth Crest, Blue Couloir (4/23)

Temperatures & New Snow Accumulation @ 0400

Loc                                      New”     High (Time)/Low                                              

Virginia Ridge, 9409’:             0          49 (1400)/ 30 deg F.,   2 hrs below freezing, 
Tioga Pass, 9972’:                 0          43 (1300)/ 23 deg F.,   9 hrs below freezing.
Agnew Pass, 9355’:               0          52 (1500)/ 30 deg F.,   6 hrs below freezing
June Mt., 9148’:                    0          51 (1500)/ 31 deg F.,   1 hrs below freezing

Mammoth Pass, 9500’:           0          49 (1500)/ 27 deg F.,   7 hrs below freezing
Sesame Plot, 9014’:               0          46 (1500)/ 28 deg F.,   6 hrs below freezing
Ch. 22-MMSA, 10,067’:          0          40 (1630)/ 24 deg F.,   9 hrs below freezing
Summit-MMSA, 11,053’:         0          33 (1415)/ 21 deg F., 11 hrs below freezing 

Rock Creek, 9600’:                 0          50 (1300)/ 31 deg F.,   3 hrs below freezing
South Lake, 9580’:                 0          50 (1500)/ 33 deg F.,   0 hrs below freezing 

Sawmill, 10,200’:                   0          45 (1300)/ 31 deg F., 1 hrs below freezing



weather summary

Sat thru Sunday–A upper low along the Pacific Northwest coast will slowly move into northern California and Nevada with breezy conditions and below average temperatures for the weekend. A few isolated thunderstorms are possible in the afternoons and early evenings as daytime heating maximizes instability. With fairly low snow levels, snow pellets can be expected to occur in many showers down to around 7000 feet. Little accumulation is expected except in the heaviest showers. Gusty winds will continue thru the weekend with gusts of 25-35 mph for the afternoon as thermal gradients rise with daytime heating. 

Monday- another upper wave is expected to move into northern California and western Nevada under a broad western CONUS trough. This is expected to cause showers and thunderstorms to break out in the Sierra with daytime heating near the disturbance`s deformation axis. Monday night, the deformation is shown in simulations to combine with upslope flow into the eastern Sierra to bring some snow showers above 6000-6500 feet. A light accumulation is possible in Mono County by early Tuesday morning given the nighttime arrival of the deformation axis.

Tuesday thru Saturday- Low pressure lingers over southern CA and NV on Tuesday, with isolated to scattered rain/snow showers and snow pellet showers. A few isolated thunderstorms are possible on Tuesday due to good instability from the cold air aloft. Ridging will start building into northern CA/NV from the west. Temperatures remain below average on Tuesday with breezy north winds. High pressure continues to strengthen bringing clearing skies, drier conditions, and warming temperatures to end the week. 

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: Sunny. Clear. Sunny then becoming partly cloudy.
Temperatures: 42 to 52 deg. F. 25 to 30 deg. F. 40 to 50 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Sunny. Clear. Sunny then becoming partly cloudy.
Temperatures: 35 to 41 deg. F. 21 to 26 deg. F. 33 to 39 deg. F.
Ridge Top Winds: Southwest West Southwest
Expected snowfall: 0 in. 0 in. 0 in.
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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