Avalanche Advisory: Tuesday - Apr 17, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON April 19, 2018 @ 6:34 am
Avalanche Advisory published on April 17, 2018 @ 6:34 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

It will be possible to trigger both wind slab and loose wet avalanches for the forecast period. Triggered avalanches could be large enough to bury or injure a person. You are more likely to encounter wind slabs in exposed, steep terrain near and above treeline. Pay special attention to crossloaded gullies and just below ridgelines. Denser, recently deposited snow that cracks is a direct sign of instability. Wind slab sensitivity may improve by Wednesday morning, but renewed S to SW winds may create new drifts by Wednesday afternoon, especially if the next round of precipitation begins by mid-day. Below about 10,500’ loose wet avalanches will become possible as the new unconsolidated snow becomes saturated under the high April sun. Rollerballs and small sluffs originating from rocks or vegetation and wet surface snow are signs that conditions are deteriorating as the day warms and a clue that it’s time to move to shady, less steep slopes. Loose wet avalanches may be less likely on Wednesday.

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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Soft wind slabs formed during the most recent storm near and above treeline on the leeward sides of ridgelines, convexities, and in gullies. Winds were generally from the SW forming drifts on W-N-NE-SE aspects, However, the wind shifted to the N as the storm exited. That combined with local channeling may have deposited some isolated wind slabs on W-SW-S-E aspects. Tuesday will have lighter winds, but they will pick up again as the next disturbance makes it’s way down the California coast on Wednesday. The renewed SW winds will likely still have soft snow to blow around. Intense spring sunshine and warmer temps may initially make wind slabs more sensitive. Pay attention to where snow has drifted, especially under cornices or just leeward of wind lips. Denser wind deposited snow that cracks around you is a direct sign of unstable wind slabs.

Avalanche Problem 2: Loose Wet
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On Tuesday the temperatures will climb to above freezing, especially below about 10,000’. This will be the first time the soft, unconsolidated new snow is warmed. The high sun angle this time of year will impart lots of solar radiation to the new snow. Loose wet avalanches will become possible during the warmest part of the day. Areas that warm appreciably that already had good snow coverage prior to the Monday storm may become wet and saturated. Cirques, bowls, and gullies will receive and retain more heat and areas with shallower snow cover may become wet first. Loose wet avalanches often originate near rocky areas or vegetated slopes. Look for rollerballs and wet snow at the surface. These are clues that it is time to move to shady, less steep slopes.

Increasing cloud cover and increasing winds may lessen the problem on Wednesday.

advisory discussion

Variable spring weather continues with warm periods interrupted by fast moving, colder storms. The most recent disturbance to pass through our area deposited almost a foot of new snow at upper elevations around the Mammoth area, and around 6” at middle elevations. Strong to moderate SW winds were blowing and soft wind slabs were observed on Monday afternoon. The new snow was very soft and unconsolidated sitting atop a thick, hard melt-freeze layer from earlier in the month. The new snow has not yet bonded well to the underlying crust and loose sluffs were easy to trigger throughout the storm. Temperatures in the low teens and single digits Monday night will have slowed the bonding process for both wind slabs and the loose storm snow.

Quiet weather on Tuesday may allow wind slabs in the upper elevations to slowly heal, but the intense spring sunshine and warmer temps may initially make wind slabs more sensitive. That same warming trend Tuesday may melt some of the loose surface snow and create a loose wet avalanche hazard as well by the warmest part of the day. By Wednesday, with increasing cloud cover and renewed winds, the loose wet problem may improve. However, if there is still loose snow available for transport, new wind slabs my form by Wednesday afternoon.

These periods of frequent change make forecasting challenging over a 48 hour period. The important thing to remember is what to look for in terms of instability. For wind slabs: steep slopes with recently drifted snow near and above treeline are suspect. These drifts can look rounded or pillow-shaped and will most likely be found below ridgelines and on the sidewalls of gullies. Denser snow that cracks around you is a direct sign of instability. For thawing problems: wet snow at the surface heralded by rollerballs or small sluffs is a good clue. Slopes receiving the most solar radiation, especially at middle and lower elevations near rocky or vegetated areas will be of most concern. Since the warming follows the sun across the sky, from E-S-W facing slopes, you can time your day to avoid loose wet avalanches. By the time the surface snow is saturated you should move to shadier, less steep slopes.

recent observations

4/16- Mammoth Rock

4/16- Trees East of TJ Bowl

New Snow @ 0400

Loc                                     New” approximate                    
Virginia Ridge, 9409’:               6”    
Tioga Pass, 9972’:                  9”   
Agnew Pass, 9355’:                6”  
June Mt., 9148’:                      7”       
Sesame Plot, 9014’:                7”
Sawmill, 10,200’:                    2”    

Weather and CURRENT CONDITIONS
weather summary

Chilly weather will continue through Thursday. Dry conditions with light winds are expected today, then a weak storm system will bring another round of light rain and snow showers late Wednesday through Thursday evening, with brisk winds on Thursday. Much warmer temperatures will quickly return for the upcoming weekend.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny. Clear then becoming partly cloudy. Partly cloudy then becoming mostly cloudy.
Temperatures: 34 to 42 deg. F. 16 to 21 deg. F. 35 to 43 deg. F.
Mid Slope Winds: Light winds. Light winds. S
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny. Clear then becoming partly cloudy. Partly cloudy then becoming mostly cloudy. Slight chance of snow showers in the afternoon.
Temperatures: 28 to 33 deg. F. 12 to 17 deg. F. 28 to 34 deg. F.
Ridge Top Winds: N S SW
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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