Avalanche Advisory: Sunday - Apr 8, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON April 10, 2018 @ 7:00 am
Avalanche Advisory published on April 8, 2018 @ 7:00 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Sunday thru Monday – the overall avalanche danger has declined to where natural avalanches are unlikely, triggered releases are possible from ~8000’ and above. Loose Wet avalanches maybe encountered as the surface melt/freeze crust thaws on steep E-S-W-NW aspects from ~8000’ and above, especially in steep, rocky, or vegetated terrain. Recently formed (4/6-4/7) Wind Slabs are strengthening and are becoming increasingly stubborn to triggering, especially by Monday, but may be encountered on NW-NE-SE aspects from ~10000’ and above. Evaluate snow and terrain carefully, identify features of concern. Natural releases unlikely, triggered releases possible.

* Slide For Life conditions may exist on steep slopes where the snow is firm, especially during the AM hours. Ice Axes, Crampons, and a Helmet are recommended if traveling in steep or complex terrain. 

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Below Treeline
Avalanche Problem 1: Loose Wet
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The recent Atmospheric River enhanced storm has moved off to the east with sunny skies and warming temperatures forecasted thru Monday, which will thaw and saturate the surface snow, primarily on solar aspects. Loose Wet avalanches are possible today on E-S-W aspects solar aspects (E-S-W) today (Sunday) and possibly expand to include NW aspects by Monday. Loose Wet activity will be mostly confined between 8000' and 10500' Sunday but may begin to creep up into the upper elevations (above ~10500') by Monday. Signs of wet snow instabilities include: pinwheels, rollerballs, deep ski or boot penetration, and point releases. Loose Wet avalanches can trigger larger deeper releases, especially in the lower elevations (~10500’ and below) where the snow has become isothermal and saturated. Natural avalanches are unlikely, triggered releases are possible, especially in sheltered, steep, or rocky terrain.

* Slide For Life conditions may exist on steep slopes where the snow is firm, especially during the AM hours. Ice Axes, Crampons, and a Helmet are recommended if traveling in steep or complex terrain. 

Avalanche Problem 2: Wind Slab
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The storm system that passed through the region 4/6-4/7 was accompanied by gale force Southwesterly winds, which formed sensitive Wind Slabs in the upper elevations (~10000’ and above), primarily on NW-NE-SE aspects. These freshly formed Wind Slabs are strengthening and will become increasingly stubborn and difficult to trigger as time passes, especially by Monday. Use additional caution in and around terrain features that promote drifting, under cornices, tops of chutes, and gully sidewalls. Natural avalanches unlikely, triggered releases possible but becoming increasingly unlikely by Monday. 

Avalanche Problem 3: Wet Slab
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advisory discussion

An unusual late season Atmospheric River enhanced storm passed through the region (4/6-4/7) with significant moisture, mostly in the form of rain (snow confined to ~10000’ to 11000’ and above) and accompanied by gale force winds. The heavy rain saturated the snowpack, resulting in extensive Loose Wet and numerous Wet Slab avalanches in the mid and lower elevations from McGee Creek north, and a few Wind Slab avalanches in the upper elevations Friday thru Saturday AM. The most avalanche activity occured primarily on N-NE aspects that had not gone through extensive freeze/thaw cycles and retained colder dry snow under the snow surface. The cold dry snow quickly became saturated by 1’ to 2.5” of rain, resulting in #’s D2-3 and a few isolated D3.5’s. As the rain eased off Saturday afternoon: excess free water began to leech out of the snowpack, while new inter-grain bonds began to form as temperatures began to fall and the snow began to refreeze. This allowed the avalanche hazard to fall back quickly Saturday as the snow began to adjust to the new load and stressors.

The 4/6-4/7 storm also was accompanied by strong to gale force winds in the upper elevations forming sensitive Wind Slabs on NW-NE-SE aspects with a D3 reported in the Sherwins. As time passes, these Wind Slabs are strengthening and are becoming increasingly stubborn to triggering but may still be encountered on NW-NE-SE aspects from ~10000’ and above. However, they’re becoming increasingly stubborn to trigger, especially by Monday.

The Wet Slab hazard has declined as heavy rains decreased Saturday, allowing the snowpack to begin consolidating and excess free water to drain, combined with a strong overnight freeze, this has essentially locked up the snowpack, preventing additional Wet Slab avalanche activity for the forecast period.

* Slide For Life conditions may exist on steep slopes where the snow is firm, especially during the AM hours. Ice Axes, Crampons, and a Helmet are recommended if traveling in steep or complex terrain. 

recent observations

June Lake > Mammoth > Convict Lake > McGee Creek > Rock Creek: Avalanche Observations (4/7)

Little Morrison: Wet Slab Avalanche (4/7)

Sherwins: Wet Slides (4/7)

Sherwins: Wet Conditions (4/6)

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

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

Rock Creek (4/5)

Temperatures @ 0400

Loc                                          Highs/Lows

Virginia Ridge, 9409’:             44/28 deg F., 8 hrs below freezing
Tioga Pass, 9972’:                 33/16 deg F., 23 hrs below freezing.
Agnew Pass, 9355’:               43/31 deg F., 8 hrs below freezing
June Mt., 9148’:                    43/32 deg F., 7 hrs below freezing
Mammoth Pass, 9500’:          40/24 deg F., 8 hrs below freezing
Sesame Plot, 9014’:              42/28 deg F., 8 hrs below freezing
Ch. 22-MMSA, 10,067’:         40/24 deg F., 12 hrs below freezing
Summit-MMSA, 11,053’:        36/18 deg F., 16 hrs below freezing
Rock Creek, 9600’:                40/30 deg F., 9 hrs below freezing
South Lake, 9580’:                43/33 deg F., 7 hrs below freezing (8PM-3AM)
Sawmill, 10,200’:                   42/32 deg F., 9 hrs below freezing

 

Weather and CURRENT CONDITIONS
weather summary

Sun thru Monday - Gusty West to Northwest winds early this morning should subside through the day as Saturday’s trough moves east of the forecast area. A ridge begins to build over the area Sunday thru Monday then slides east with near normal temperatures today and 6-10 degrees warmer on Monday.

Tuesday - another low pressure will brush by the forecast area with limited moisture tap with mid level winds increasing well ahead of the jet and a tightening low level temperature/pressure gradient will create strong surface winds of 20-30 mph with Ridge gusts near 90-95 mph.

Wed thru Thursday - Windy conditions are expected Wednesday afternoon and evening ahead of the next cold front with most of the moisture behind the front. A secondary wave on the back side of the low may produce another burst of snow Thursday afternoon. High temps will be well below average with temperatures in the 30s and 40s area-wide. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Sunny Clear Sunny
Temperatures: 47 to 55 deg. F. 24 to 29 deg. F. 55 to 63 deg. F.
Wind Direction: Southwest Light winds. Light winds.
Wind Speed: 15 to 20 mph with gusts to 40 mph in the morning becoming light.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Sunny Clear Sunny
Temperatures: 42 to 47 deg. F. 23 to 28 deg. F. 49 to 54 deg. F.
Wind Direction: West Northwest Light winds
Wind Speed: 25 to 40 mph with gusts to 65 mph decreasing to 15 to 25 mph with gusts to 40 mph in the afternoon. 10 to 15 mph.
Expected snowfall: 0 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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