Avalanche Advisory: Wednesday - Apr 4, 2018

Avalanche Advisory published on April 4, 2018 @ 6:28 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

For Wednesday and Thursday: Natural and human triggered loose wet avalanches are expected to be small and unlikely, but not impossible. Expect wet instabilities to be increasingly possible as temperatures climb in the afternoons. Look for rollerballs and boot-top penetration into wet snow as clues that conditions are deteriorating. Watch for unstable snow on steep E-S-W facing slopes, near rocky areas, in cirques and gullies, and near exposed vegetation.

Hard and fast conditions may exist, especially early in the day, in steep alpine terrain. Use caution and crampons to prevent a slide-for-life.

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

Loose wet activity will be the primary concern for the forecast period. The snow will warm throughout the day making rollerballs and small sluffs increasingly possible by the afternoon. Most instabilities are expected to be small thanks to below freezing temperatures overnight and cloudy skies during the day. However, rocky outcrops, chutes and cirques, and vegetation can intensify solar heating and are likely places for triggering small loose wet avalanches as temperatures climb on steep slopes. Even small sluffs can be dangerous when combined with a terrain trap. Continue to employ appropriate caution in your backcountry travels.

advisory discussion

The spring melt-freeze cycle is in full swing. Snow surfaces are thawing throughout the day into wet corn snow and generally re-freezing overnight into hard crusts. Several factors come into play when it comes to the extent, depth, and speed of thawing at the surface. Sunny skies and warm temperatures are the source of the warming on solar slopes, whereas thin clouds can hold in the heat and increase thawing on northerly aspects, especially overnight where temperatures struggle to get below freezing. That’s the story that played out earlier in the week with daytime highs in the 50s and overnight lows mostly above 32 deg F. Hazy clouds triggered the greenhouse effect and increased the thawing. This generally saturated the snow with melt water and the loss of cohesion at the surface made loose wet avalanches possible. Observations of small natural and skier-triggered rollerballs and sluffs were an indication that larger wet slides were possible during the heat of the day.

Recently however, slightly cooler temps and cold winds in the upper elevations have helped reduce the scope of the melting. As of Wednesday morning, only a few areas below about 9500’ are reporting low temps above freezing. Cloudy skies overnight may have helped keep lower elevations a little warmer. But with mostly cloudy skies forecasted through Thursday, solar radiation will be less intense. At upper elevations cooler temps and increasing winds will help keep surfaces from melting too extensively. Loose wet activity is not impossible in the coming days, but it is expected to be smaller and less widespread.

Terrain and time of day both amplify the risk of encountering wet instabilities. As the sun and temperatures climb throughout the day solar radiation will melt the snowpack more and more. Cirques and gullies receive and retain more heat. Rocks and vegetation can increase local thawing and loose wet avalanches often originate from just below these features. Rollerballs and boot-top penetration into wet snow are good indications that loose wet avalanches are becoming possible. You can avoid the problem by moving to shady and less steep slopes before these signs appear. Even small loose wet avalanches can be dangerous if they carry you into a terrain trap, so continue to use appropriate caution in your backcountry travels.

Slide-for-life conditions early in the day are of equal concern. Using crampons, an ice ax, and a helmet may reduce your exposure to a slip and fall with high consequences when the snow surface is hard and fast.

Finally, an unusually late-season and strong atmospheric river is expected to begin impacting the region Friday. Expect conditions to change rapidly going into the weekend.

recent observations

4/3- Red Cone

4/2- Lundy

Temperatures @ 0400am, Wednesday

Loc                                          Highs/Lows

Virginia Ridge, 9409’:                50/35 deg F, no freeze
Tioga Pass, 9972’:                    47/26 deg F. 9hrs freezing
Agnew Pass, 9355’:                  57/28 deg F, 8hrs freezing
June Mt., 9148’:                       52/34 deg F, no freeze
Mammoth Pass, 9500’:             56/28 deg F, 6hr freezing
Sesame Plot, 9014’:                 50/34 deg F, no freeze
Ch. 22-MMSA, 10,067’:            46/33 deg F, no freeze
Summit-MMSA, 11,053’:           37/27 deg F, 10hrs freezing
Rock Creek, 9600’:                   47/26 deg F, 6hrs freezing
South Lake, 9580’:                   54/27 deg F, 3hrs freezing
Sawmill, 10,200’:                     46/28 deg F, 4hrs freezing

weather summary

Mild weather is expected through Thursday. A strong atmospheric river will impact our area Friday into Saturday, bringing periods of moderate to heavy rain, high elevation snow, gusty winds, and concerns for flooding along rivers and streams. With a strong sub-tropical influence and the jet stream forecast to stay well north of the region during most of the event, precipitation will fall primarily as rain, even in the higher elevations. Snow levels could be as high as 10,000 to 11,000 feet during the heart of the event. Strong atmospheric rivers are quite rare for this time of year.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Mostly cloudy. Mostly cloudy.
Temperatures: 47 to 57 deg. F. 29 to 34 deg. F. 47 to 57 deg. F.
Mid Slope Winds: SW SW SW
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Mostly cloudy. Mostly cloudy.
Temperatures: 36 to 46 deg. F. 24 to 29 deg. F. 34 to 44 deg. F.
Ridge Top Winds: SW SW SW
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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