Eastern Sierra Avalanche Advisory - 4/23/17

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Avalanche Advisory published on April 23, 2017 @ 6:29 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Primary avalanche problem for Sunday will focus on Loose Wet avalanches, primarily on NW-W-SW-S-SE-E aspects as the surface snow thaws. Overnight lows were above freezing in most locations. Temperatures are forecasted to climb into the 40’s and mid 50’s below 10,000’ and into the mid 30’s to low 40’s above 10,000’ during the day. With the poor overnight freeze and warm temps, anticipate the surface snow thawing quickly. Natural avalanches 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 near exposed vegetation. - Loose Wet slides are dense and heavy, which can make it difficult to extract yourself if entrained and are capable of carrying a rider into hazardous terrain or result in possible burial if combined with terrain traps. 

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


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

High pressure will dominate today's (Sunday) weather with spring conditions found throughout the region. Overnight lows were above freezing in most locations last night (Saturday), which kept the snow from setting up and freezing thoroughly. Highs forecasted for today (Sunday) will be almost 10 degrees cooler over yesterday, which will slow thawing somewhat. Cooler temperatures and moderate winds are forecasted for the upper elevations today (Sunday), which will slow the AM thaw in the upper elevations, the mid to lower elevations will not benefit from the winds as much and will thaw more rapidly, especially as the cloud cover disapates. NW aspects in the mid and upper elevation are beginning to warming extensively for the first time this season, heightening the risk of Loose Wet avalanches in those areas. Small point releases can be a sign that larger avalanches are possible. Loose Wet avalanches typically involve the snow near the surface of the snowpack but can trigger larger deeper releases. As the snow surface warms throughout the day natural Loose Wet avalanches will become increasingly possible, especially around rock outcrops and below cliff bands, triggered releases likely. Timing critical for avoiding Loose Wet 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. Large pinwheels or ski penetration of boot top or greater are signs the snow is loosing internal cohesion and is becoming increasingly unstable. Starting early and being out of steep, rocky terrain before things get too warm is the best strategy for avoiding loose wet avalanches.

- 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. 

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

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

advisory discussion

Spring continues to dominate the weather picture with warm daily temperatures and cool nights. A fast moving spring storm swept though the region Tuesday (4/18/17) with 3” to 12” inches of new snow reported across the forecast area above ~8500’. However, snow levels fluctuated considerably during the storm with many areas receiving rain Monday before turning to snow in the early morning hours on Tuesday. Snow levels rose once again during the day, Tuesday, with rain up to ~ 9,000’ then easing back down to ~8000’ by Tuesday PM. Loose Wet avalanches were prevalent during the storm throughout the mid elevations as the surface snow becoming saturated with water and internal bonds began to dissolve. Moderate to strong SW winds during the storm formed Wind Slabs in exposed locations throughout the mid and upper elevations, primarily above ~ 9000’ on NW-NE-SE aspects with several avalanches observed. 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 a new round of Wind Slabs throughout the upper and mid elevations, primarily on N-E-S aspects. Since then, the Wind Slabs have had a couple of days to strengthen but moderate SW winds are forecasted for the upper elevations today (Sunday), which may form very isolated shallow Wind Slabs in the upper elevations on NW-NE-SE aspects where upwind fetches still have snow available for transport. Northwesterly aspects in the mid to upper elevations are starting to heat-up significantly for the first time this season with the dry snow becoming wet, elevating the possibility of loose wet avalanches somewhat on this aspect.


recent observations

4/22/17 - Avalanche On Mt Whitney Mountaineers Route

4/22/17 - Avalanche On East Face Mt. Morrison

4/22/17 - Week of Obs

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

Sunday ~0500 Observations       Temp  High (Sat)  

Virginia Lakes (Elev. 9445’)                 40       53                 

Ellery Lake (Elev. 9645’)                      35       51                   

Agnew Pass (Elev. 9355’)                    37       53                  

June Mountain (Elev. 9148’)                 37       54                  

Mammoth Pass (Elev. 9500’)                32       60                 

Sesame Study Site (Elev. 9014’)           36       53                

Rock Creek Lakes (Elev. 9600’)             34       66                 

Sawmill (Elev.10200’)                           33       50                   

South Lake Cabin (Elev. 9580’)              34      56                 

weather summary

Sun thru Monday – A short-lived ridge is shifting eastward as a weak system passes through the northwest Sunday. This will bring increased winds and high-level clouds to the region, but precipitation should be limited to near the Oregon border, with meager totals. Sunday, surface gradient is stronger but less of a speed/gust spread. Monday, as the jet moves over the eastern Sierra wind speeds will increase with surface wind gusts of 30 to 45 mph and ridge gusts of 80 to 90 mph likely. Highs Sunday will return to near Normal, a few degrees cooler Monday

Tuesday - The trajectory of the jet stream and main area of low pressure is well to the north and are fairly progressive but lacking organization this far south, so precipitation totals should be minimal. Generally, Mono County totals of up to a few hundredths are likely for some locations. Temps a few degrees cooler than normal. 

Wednesday thru … - The large scale pattern next week will feature a trough developing in the Great Basin and over the Rockies with ridging building into the southeastern Pacific. The pattern will become more amplified through the week as additional shortwaves dig into the Great Basin deepening the trough into the Southern Rockies. Wednesday and Thursday will remain cool with lingering showers possible Wednesday. Wednesday and Thursday could also be quite breezy as cold fronts push through from the northwest. Daytime high temperatures will remain below average through most of the week with highs in the upper 40`s to low 50`s in the Sierra. The storm track continues to stay active in the Pacific Northwest next weekend which will keep the ridge from amplifying over the western U.S. Currently looks like next weekend could be similar to this weekend with above average temperatures limited to just a few days. Precipitation is looking unlikely next weekend, however, the ridge is showing signs of retrograding west which could allow fast moving cold fronts from the north back into the region by the first week of May. 


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: Partly cloudy then becoming sunny. Partly cloudy. Mostly cloudy. Scattered snow showers in the morning. Isolated snow showers in the afternoon.
Temperatures: 46 to 54 deg. F. 31 to 36 deg. F. 39 to 49 deg. F.
Wind Direction: Southwest West West
Wind Speed: 10 to 20 mph. Gusts up to 40 mph. 10 to 15 mph with gusts to 35 mph. 15 to 25 mph. Gusts up to 40 mph increasing to 55 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0-1 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming sunny. Clear then becoming partly cloudy. Mostly cloudy. Isolated snow showers.
Temperatures: 36 to 44 deg. F. 26 to 32 deg. F. 29 to 37 deg. F.
Wind Direction: Southwest West West
Wind Speed: 25 to 45 mph with gusts to 60 mph decreasing to 20 to 30 mph with gusts to 50 mph in the afternoon. 15 to 25 mph with gusts to 40 mph. 30 to 40 mph with gusts to 65 mph increasing to 30 to 50 mph with gusts to 85 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0-1 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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