Eastern Sierra Avalanche Advisory - 4/24/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 26, 2017 @ 4:23 am
Avalanche Advisory published on April 24, 2017 @ 4:23 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problem for Monday thru Tuesday – will focus on isolated Wind Slabs and Loose Wet avalanches in the mid to low elevations.

Wind Slabs – Moderate to strong Westerly (Mon) to Northwesterly (Tues) winds with the potential for light snowfall (Mon) will form isolated shallow Wind Slabs primarily on exposed N-E-S-SW aspects above ~ 9000’. Natural avalanches unlikely, triggered releases possible below ridgelines, adjacent to terrain features that promote drifting, and in gullies and shallow depressions.     

Loose Wet will be primarily a concern in the low elevations (possibly including mid elevations) as the surface snow thaws on NW-W-SW-S-SE-E aspects, especially south of Mammoth. Temperatures are forecasted to climb into the upper 30’s to upper 40’s below 10,000’. A modest overnight freeze and warming daytime temps will elevate the concern for Loose Wet releases as the surface snow thaws.

Mon – Natural avalanches unlikely, triggered release possible in low elevations, possibly creeping upward into the mid elevations below ~9000’. Tue – Natural and triggered releases unlikely.

Caution – Potential Slide For Life conditions may exist due to firm spring snow conditions prior to thawing. A minor slide into hazardous terrain can have serious consequences.

How to read the advisory

Bottom Line

Primary avalanche problem for Monday thru Tuesday – will focus on isolated Wind Slabs and Loose Wet avalanches in the mid to low elevations.

Wind Slabs – Moderate to strong Westerly (Mon) to Northwesterly (Tues) winds with the potential for light snowfall (Mon) will form isolated shallow Wind Slabs primarily on exposed N-E-S-SW aspects above ~ 9000’. Natural avalanches unlikely, triggered releases possible below ridgelines, adjacent to terrain features that promote drifting, and in gullies and shallow depressions.     

Loose Wet will be primarily a concern in the low elevations (possibly including mid elevations) as the surface snow thaws on NW-W-SW-S-SE-E aspects, especially south of Mammoth. Temperatures are forecasted to climb into the upper 30’s to upper 40’s below 10,000’. A modest overnight freeze and warming daytime temps will elevate the concern for Loose Wet releases as the surface snow thaws.

Mon – Natural avalanches unlikely, triggered release possible in low elevations, possibly creeping upward into the mid elevations below ~9000’. Tue – Natural and triggered releases unlikely.

Caution – Potential Slide For Life conditions may exist due to firm spring snow conditions prior to thawing. A minor slide into hazardous terrain can have serious consequences.

Avalanche Problem 1: Wind Slab
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Westerly (Mon) to Northwesterly (Tues) winds are forecasted to be at threshold in the mid elevations (15 to 25 mph) and above threshold in the upper elevation (Mon, 35 to 60 mph > Tuesday, 25 to 35 mph) with Gusts of 60 to 80 mph with snow showers over the upper elevations, which will result in drifting on leeward slopes. Anticipate isolated shallow Wind Slabs on N-E-S-SW aspects in the mid to upper elevations above ~ 9000’. Riders will likely encounter these below ridgeline, adjacent to terrain features that promote drifting, crossloaded gullies and depressions. Be wary of hollow, drum-like sounding snow or shooting cracks.​ Changing wind directions can alter windloading and making it difficult to discern previous drifting patterns.  

Avalanche Problem 2: Loose Wet
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    Decreasing Danger

Overnight temperatures varied from a modest freeze Mammoth north to above freezing south of Mammoth. Mon – Tues: the mix of wind, clouds, and cooler temperatures will slow surface snow thawing in the low to mid elevations. Southern regions of the forecast area (south of Mammoth) will thaw more quickly due to warmer temps and less cloud cover. 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. Watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases. 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. Pay special attention near rock bands, outcroppings, and near exposed vegetation. 

Mon – Natural avalanches unlikely, triggered release possible in low elevations, possibly creeping upward into the mid elevations below ~9000’. Tue – Natural and triggered releases unlikely.

- 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.

advisory discussion

Spring continues to dominate the weather picture with warm daily temperatures and cool nights. A fast moving spring storm is moving through the region today (Mon) thru Tues with moderate to strong winds, cooler temperatures, increasing cloud cover (primarily Mammoth north), and light precipitation over the upper elevations, which will hinder good corn skiing until the sun returns Wed. The last storm to sweep 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 

Monday ~0500 Observations   Temp  High (Sun)  

Virginia Lakes (Elev. 9445’)           N/A      44                 

Ellery Lake (Elev. 9645’)                28       43                   

Agnew Pass (Elev. 9355’)              33       53                  

June Mountain (Elev. 9148’)          35        47                  

Mammoth Pass (Elev. 9500’)         29        51

Sesame Study Site (Elev. 9014’)    30       46                

Rock Creek Lakes (Elev. 9600’)      36       43                 

Sawmill (Elev.10200’)                    37       44                   

South Lake Cabin (Elev. 9580’)      40       49                 

 

weather

Monday – the first in the series of the 3 waves will skirt northern CA early Monday with a definitive break now showing up behind it on Tuesday morning bringing light to isolated snow showers to the central Sierra. Breezy winds Monday as the jet stream moves across northern California into Nevada, with wind speeds of 40 to 50 across the Sierra with ridge gusts peaking 75-90 mph. Temperatures will be 5-10 degrees cooler today than Sunday.

Tues thru Wednesday - a shortwave ridge will build over northeast CA and western NV for drier conditions and lighter winds.

Thursday thru … - a backdoor cold front is expected to move through late Thursday into Friday as the trough over the Intermountain West deepens. Below average temperatures are expected both days with the coldest day Friday. Highs in the 40s and 50s will feel even colder with a brisk north wind. Any precipitation along the front would be very light and looks isolated. Moisture is rather shallow and the best forcing will be much further east. The flow eventually turns northeast Friday night with good support for a gusty NE winds over the Sierra Crest. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy. Scattered snow showers. Mostly cloudy. Isolated snow showers in the evening. Mostly cloudy.
Temperatures: 37 to 47 deg. F. 24 to 29 deg. F. 42 to 52 deg. F.
Wind direction: West West Northwest
Wind speed: 15 to 25 mph with gusts to 45 mph. 15 to 20 mph with gusts to 40 mph. 10 to 15 mph. Gusts up to 40 mph.
Expected snowfall: up to 1 inch in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy. Scattered snow showers. Mostly cloudy. Isolated snow showers in the evening. Mostly cloudy.
Temperatures: 29 to 37 deg. F. 19 to 24 deg. F. 36 to 42 deg. F.
Wind direction: West West Northwest
Wind speed: 35 to 60 mph. Gusts up to 80 mph. 25 to 35 mph with gusts to 60 mph. 25 to 35 mph with gusts to 70 mph decreasing to 15 to 25 mph with gusts to 50 mph in the afternoon.
Expected snowfall: up to 1 inch 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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