Avalanche Advisory: Saturday - Apr 21, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON April 24, 2018 @ 7:00 am
Avalanche Advisory published on April 21, 2018 @ 7:00 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Sunny skies, weak overnight freezes, and above seasonable temperatures will elevate the potential for Loose Wet activity thru Monday. Natural Loose Wet avalanches are possible, especially in steep rocky terrain, triggered releases likely. Moderate to strong NE winds during the day (4/20) and early evening formed Wind Slabs on NW-SW-SE aspects, which have begun to settle and strengthen. Wind Slabs will become increasingly unlikely as more time elapses. Anticipated Wind Slab activity will generally be limited to small isolated pockets in steep or complex terrain from ~ 9500’ and above. Natural Wind Slab avalanches unlikely, triggered release possible.

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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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Forecasted sunny skies, above seasonable temperatures, and poor overnight freezes will maintain the threat of Loose Wet avalanches thru Monday. The recent storm snow (4/16 & 4/19) is expected to warm rapidly during the day and will likely become loose and saturated from ~8500’ and above. This is especially true in the Mammoth and the Alpine regions (~10,500 and above) that received the greatest amount of new snow. Below ~8500’, the snowpack is thinning and receding rapidly, especially outside of the Mammoth/June region where the snowpack is generally thinner and the snowline higher. What remains is becoming largely patchy and well anchored and is not likely to have enough snow to generate slides of significant size. Generally, solar aspects will become will thaw and become saturated first with shaded aspects to follow. As the new snow on northerly aspects begin to heat-up significantly for the first time since storm passage, Loose Wet activity will likely become increasingly touchier. Loose Wet avalanches can trigger larger deeper releases, especially in the lower elevations (~10500’ and below). Small natural Loose Wet avalanches are possible, triggered releases likely, especially in sheltered, steep, or rocky terrain. Signs of wet snow instabilities include: pinwheels, rollerballs, deep ski or boot penetration, and point releases. If it’s becoming increasingly hard to turn, it’s likely past time to move onto to shadier, less steep slopes. Use extra caution near rocky or in vegetated areas. 

Avalanche Problem 2: Wind Slab
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A quick moving spring storm moved through the region 4/16 and another quick impulse on 4/19 deposited 2” to 9” (~5 to ~20cm) of new snow in the Mammoth/ June region and the higher elevations. Moderate Northeasterly winds yesterday (4/20) continued into the evening hours, forming Wind Slabs on NW-SW-SE aspects near and above treeline (~9500’ and above). As temperatures warm during the day, these Wind Slabs may become saturated with free water percolating down and weakening the bonds to the underlying snowpack, maintaining the concern through the forecast period, though will become increasingly unlikely as more time elapses. Natural avalanche are unlikely, triggered releases possible in steep or complex terrain from approximately 9500’ and above, especially areas that promote drifting (rock outcrops, gullies, leeward side of ridges, etc.) Use additional caution in and around terrain features that promote drifting, under cornices, tops of chutes, and gully sidewalls. 

advisory discussion

A quick moving spring storm moved through the region 4/16 and another quick impulse on 4/19 deposited 2” to 9” (~5 to ~20cm) of new snow in the Mammoth/June region and the higher elevations. The storm was accompanied by moderate to strong SW winds initially forming Wind Slabs on NW-NE-SE aspects. These Wind Slabs have had sufficient time to bond and are not a concern beyond isolated pockets in steep complex terrain. As the last storm (4/19-/20) exited the region, winds shifted to the NE, which formed fresh tender Wind Slabs on NW-SW-SE aspects. Winds eased overnight and are forecasted to remain mostly light through the forecast period, which will limit new slab formation and allow the recently formed Wind Slabs to begin strengthening and bonding to the underlying snowpack. Wind Slabs will become increasingly unlikely as more time elapses. However, the forecasted warm temperatures and sunny skies may generate enough free water to percolate down and weaken the bonds to the underlying snowpack and increase the potential for triggered releases, though generally small. 

The latest storm to pass through the region (4/16) left 2’ to 9” in its wake (primarily in the Mammoth/June region and the Alpine terrain). Temperatures quickly rebounded as the storm moved off to the east and are forecasted to remain above seasonable through Monday. The recent storm snow (4/16 & 4/19) is expected to warm rapidly during the day and will likely become loose and saturated from ~8500’ and above. This is especially true in the Mammoth and the Alpine regions (~10,500 and above) that received the greatest amount of new snow. As the new snow on northerly aspects begins to significantly heat-up for the first time since storm passage will likely increase the potential for Loose Wet activity on northerly aspects. Loose Wet avalanches can potentially trigger larger deeper releases, especially in the lower elevations (~10500’ and below), especially on solar aspects. Below ~8500’, the snowpack is thinning and receding rapidly, especially outside of the Mammoth/June region where the snowpack is generally thinner and the snowline higher. What remains is becoming largely patchy and well anchored and is not likely to have enough snow to generate slides of significant size. 

 

recent observations
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Packed powder on South Peak (4/20)

Red Cone: Wind Slabs and Loose Wet (4/20)

Packed powder on Dunderberg (4/19)

Rock Creek: Skier Triggered Wind Slab (4/18)

Gaylor Peak: New Snow Stability (4/18)

Gaylor Peak: Isolated Wind Slabs & Small Loose Wet Avalanches (4/18)

Temperatures & New Snow Accumulation @ 0400

Loc                                          New”   Highs/Lows                                            

Virginia Ridge, 9409’:             0          48/33 deg F., 0 hrs below freezing, 
Tioga Pass, 9972’:                 0          42/29 deg F., 8 hrs below freezing.
Agnew Pass, 9355’:               0          53/29 deg F., 4 hrs below freezing
June Mt., 9148’:                    0          45/33 deg F., 0 hrs below freezing 

Mammoth Pass, 9500’:          0          51/31 deg F., 5 hrs below freezing
Sesame Plot, 9014’:              0          50/32 deg F., 2 hrs below freezing
Ch. 22-MMSA, 10,067’:          0          54/34 deg F., 0 hrs below freezing
Summit-MMSA, 11,053’:        0          36/35 deg F., 2.5 hrs below freezing 9-1130PM
Rock Creek, 9600’:                0          48/27 deg F., 8 hrs below freezing
South Lake, 9580’:                0          56/27 deg F., 7 hrs below freezing

Sawmill, 10,200’:                   0          45/26 deg F., 8 hrs below freezing

Weather and CURRENT CONDITIONS
weather summary

Sat thru Monday- High pressure will be the dominant weather feature into the beginning of next week. Conditions will be dry and characterized by unseasonably warm temperatures into next week with breezy winds Sunday. Daily highs will generally be 10-15 degrees above seasonal averages. Expect increased snowmelt over the weekend with rivers running fast and cold. Winds slow Sunday night with light northerly flow for Monday. Temperatures will be marginally cooler Monday, but still above seasonal averages in the 60s. 

Tuesday Onward- Well above normal temperatures will continue into the middle of the week with southerly flow aloft. Limited moisture and instability will limit the potential for Thunderstorm to the Sierra where localized convection is possible. Better chances exist for Wednesday through Friday as an upper level low approaches the coast from the eastern Pacific bringing increased moisture and upper level divergence. Confidence remains low for the end of the week with this closed low due to a wide range of ensemble forecast solutions and the inherent problem of springtime closed lows being notoriously tricky to forecast. Models show the Low trending farther north when it comes inland next weekend. This would put the region under southwest to westerly flow, and limit any shower and thunderstorm chances and a slight cooling trend. Temperatures will likely be near or slightly above normal. 

 

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
Temperatures: 54 to 62. deg. F. 34 to 39 deg. F. 53 to 63 deg. F.
Mid Slope Winds: Light winds Light winds Light winds
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: 47 to 53 deg. F. 29 to 35 deg. F. 47 to 53 deg. F.
Ridge Top Winds: Light Winds Light Winds Southwest
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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