Eastern Sierra Avalanche Advisory - 4/22/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 23, 2017 @ 5:11 am
Avalanche Advisory published on April 22, 2017 @ 5:11 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary problem for Saturday will focus on Loose Wet avalanches as the surface snow thaws. Overnight lows were above freezing in many locations. Temperatures are forecasted to climb into the 50’s and low 60’s below 10,000’ and into the mid 40’s to low 50’s above 10,000’ during the day. With the poor overnight freeze and warm temps, anticipate the surface snow thawing quickly. Natural avalanche possible, triggered releases likely on steep slopes as the snow surface warms and thaws. Watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases. Pay special attention near rock bands, outcroppings, and exposed vegetation.

Caution - Firm snow conditions in the AM can produce slide-for-life conditions. A minor slide into hazardous terrain can have serious consequences. 

 

How to read the advisory

Bottom Line

Primary problem for Saturday will focus on Loose Wet avalanches as the surface snow thaws. Overnight lows were above freezing in many locations. Temperatures are forecasted to climb into the 50’s and low 60’s below 10,000’ and into the mid 40’s to low 50’s above 10,000’ during the day. With the poor overnight freeze and warm temps, anticipate the surface snow thawing quickly. Natural avalanche possible, triggered releases likely on steep slopes as the snow surface warms and thaws. Watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases. Pay special attention near rock bands, outcroppings, and exposed vegetation.

Caution - Firm snow conditions in the AM can produce slide-for-life conditions. A minor slide into hazardous terrain can have serious consequences. 

 

Avalanche Problem 1: Loose Wet
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High pressure continues to dominate Today's (Saturday) weather with spring conditions being found throughout the region. Overnight lows were above freezing in many locations last night, which kept the snow from setting up and freezing thoroughly. With daytime Highs forecasted to rise almost 10 degrees over yesterday’s, anticipate rapid warming today, with most areas rising above freezing quickly in the AM. Moderate Southwesterly winds will slow the daily thaw somewhat but is a weak counter to the strong mid-spring sun. NW-N-NE aspects in the upper and mid elevation have retained some dry winter snow woth mid elevation NW aspects beginning to warming for the first time, which will heighten the risk of loose wet avalanches in those areas. Loose Wet avalanches typically occur within the wet snow near the surface of the snowpack, but can gouge into deeper snowpack layers, possibly triggering larger deeper releases. Small point releases can be a sign that larger avalanches are possible. As the snow surface warms throughout the day natural Loose Wet avalanches are increasingly possible, especially around rock outcrops and below cliff bands. Triggered releases likely. Timing critical for avoiding wet point releases. East aspects thaw first, then south, then west and finally northwest as the spring sun moves high into the sky. Lower elevations will warm more quickly than higher elevations. Watch out for pinwheels, a sign that the surface snow is loosing internal cohesion and is becoming increasingly unstable.

Caution - Firm snow conditions in the AM can produce slide-for-life conditions. A minor slide into hazardous terrain can have serious consequences. Additionally, Loose Wet  slides are dense and heavy, which can make it difficult to extract yourself if entrained and can carry a rider into hazardous terrain or possible burial when combined with terrain traps. 

* Today (Saturday) - Moderate SW winds forecasted for the mid to upper elevations may form very isolated shallow Wind Slabs in the mid to upper elevations on NW-NE-SE aspects where upwind fetches still have snow available for transport.

 

 

 

advisory discussion

Spring is in full swing in the Sierras after a fast moving spring storm swept though the region Tuesday with 3” to 12” inches of new snow reported across the forecast area above ~8500’. Snow levels fluctuated considerably during the storm with many areas received rain Monday before turning to snow in the early morning hours on Tuesday. Snow levels rose once again Tuesday with rain up to almost 9,000’. Snow levels finally eased back down to ~8000 by Tuesday late afternoon. Loose wet avalanches were prevalent during the storm throughout the mid elevations. Upper elevations - strong southwest winds accompanying the storm formed Wind Slabs on leeward slopes above about 9,000’. Mammoth Mountain Ski Patrol reported significant results from avalanche control work on Wednesday morning. Most of these avalanches were triggered in wind slabs with small hand charges and ski cutting.

Westerly winds continued thru Wednesday and Thursday with snow banners and localized drifting observed from Mammoth south to Rock Creek, forming Wind Slabs throughout the upper and mid elevations, primarily on NW-N-NE-SE aspects. Since then, the Wind Slabs have had a couple of days to strengthen but moderate SW winds are forecasted for the mid to upper elevations today (Saturday), which may form very isolated shallow Wind Slabs in the mid to upper elevations on NW-NE-SE aspects where upwind fetches still have snow available for transport. Northwesterly aspects are starting to heat-up for the first time with the dry snow becoming wet for the first time, which will increase the possibility of loose wet avalanches. Rocky outcrops, gullies, and trees will trap the sun’s heat today and reflect in back down onto the snow, also elevating the hazard of wet sloughing. Large pinwheels or ski penetration of boot top or greater are signs the snow is loosing strength and becoming unstable. Starting early and being out of steep, rocky terrain before things get too warm is the best strategy for avoiding loose wet avalanches.

Caution - Firm snow conditions in the AM can produce slide-for-life conditions. A minor slide into hazardous terrain can have serious consequences.

 

recent observations

4/21/17 - Loose Wet Avalanches On University Peak

4/20/17 - Punta Bardini

4/20/17 - Piute Crags

4/20/17 - Morning Wind Transport, Rock Creek Area

4/19- Mammoth Mtn. Ski Patrol Control Results 

Saturday ~0500 Observations    Temp  High (Fri)  

Virginia Lakes (Elev. 9445’)               41         51                 

Ellery Lake (Elev. 9645’)                   38         55                   

Agnew Pass (Elev. 9355’)                 30         59                  

June Mountain (Elev. 9148’)              40         53                  

Mammoth Pass (Elev. 9500’)              32        60                 

Sesame Study Site (Elev. 9014’)         41        59                

Rock Creek Lakes (Elev. 9600’)           39        58                 

Sawmill (Elev.10200’)                         27        54                   

South Lake Cabin (Elev. 9580’)            31        63                 

 

 

weather

Sat thru Sunday - low pressure approaching the far northern California and southern Oregon coasts with a ridge of high pressure shifting into central and eastern Nevada, causing winds to increase over Sierra ridges. By late morning and for the afternoon breezy conditions with gusts to 35 to 40 mph for many areas for the afternoon and/or evening. Late tonight and Sunday, a broad trough will move overhead bringing additional cooling aloft. Highs Sunday are expected to fall 5 to 10 degrees. Winds Sunday should be similar or slightly lower than today as thermal gradients between the Sierra and Basin and Range ease somewhat.

Monday - another fast-moving wave skirts by near the California-Oregon-Nevada border. This will ease temperatures down another 5-10 degrees for many areas, with another boost in winds as flow aloft increases and low-level thermal gradients intensify once again. Monday- precipitation, moderate to strong winds aloft could push a few light showers east of the Sierra, primarily confined to the northern portion of the forecast region.

Tuesday - abundant moisture will continue to stream across Northern California and Nevada. However, the best forcing remains north into Oregon so any precip amounts will be generally light. Snow levels look to remain 7-8000 feet. Winds will continue to be gusty from the west at times especially during the afternoon and evening Tuesday.

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 Partly cloudy Partly cloudy then becoming sunny
Temperatures: 53 to 61 deg. F. 31 to 37 deg. F. 46 to 54 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 5 to 15 mph increasing to 15 to 20 mph in the afternoon. Gusts up to 40 mph. 15 to 20 mph with gusts to 45 mph. 15 to 25 mph with gusts to 40 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Sunny Partly cloudy. Sunny
Temperatures: 46 to 52 deg. F. 27 to 33 deg. F. 36 to 44 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15 to 25 mph with gusts to 45 mph increasing to 20 to 30 mph with gusts to 55 mph in the afternoon. 25 to 40 mph with gusts to 65 mph. 20 to 35 mph with gusts to 55 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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