Eastern Sierra Avalanche Advisory - 4/26/17 | Eastern Sierra Avalanche Center

THIS AVALANCHE ADVISORY EXPIRED ON April 29, 2017 @ 7:11 am
Avalanche Advisory published on April 26, 2017 @ 7:11 am

Issued by Doug Lewis - Eastern Sierra Avalanche Center

Bottom Line

Primary avalanche problem for Wednesday thru Friday will focus on isolated Wind Slabs in the mid to upper elevations and Loose Wet avalanches in the mid to low elevations.

Wind Slabs – Moderate to strong Westerly (Wed) to Northwesterly (Thurs-Fri) winds with the potential for light snowfall (Wed) will form isolated shallow Wind Slabs primarily on exposed N-E-S-SW aspects above ~ 9500’. 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 on NW-W-SW-S-SE-E aspects in the mid and all aspects for the low elevations as the surface snow thaws. Temperatures are forecasted to climb into the mid 40’s to mid 50’s below 10,000’. Modest overnight freezes and warm daytime temps will elevate the concern for Loose Wet releases as the surface snow thaws. Moderate to strong Westerly to Northwesterly winds will slow thawing but is a weak counter to the intense April sun. Wed thru Fri - Natural avalanches unlikely, triggered releases possible on steep slopes as the snow surface warms and thaws in the Low to Mid elevations.

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.

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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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Overnight temperatures struggled to get below freezing, especially south of Mammoth. Wed: a mix of wind, clouds, and cool temperatures will tend to impact areas north of June Lakes, which will slow surface snow thawing in the low to mid elevations somewhat while southern portions of the forecast area (south of Mammoth) will thaw more quickly due to warmer temps and less cloud cover. Moderate to strong Westerly (Wed) to Northwesterly winds will slow thawing of the surface snow somewhat but is a weak counter to the intense April sun. As the snow surface warms throughout the day triggered Loose Wet avalanches will become increasingly possible, natural will remain unlikely. Extra caution is recommended in and around rock outcrops and below cliff bands where triggered releases are more likely. Timing is critical for avoiding Loose Wet releases. Easterly aspects thaw first, followed by southerly, then westerly, and finally northwesterly aspects as the spring sun moves across the sky. Lower elevations 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 increasingly possible. Loose Wet avalanches typically involve the snow near the surface of the snowpack but can trigger larger deeper releases.

Wed thru Fri – Natural avalanches unlikely, triggered release possible in low elevations, possibly creeping upward into the mid elevations below ~9000’.

- 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 lead to 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.

Avalanche Problem 2: Wind Slab

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Westerly (Wed) to Northwesterly (Thurs - Fri) winds are forecasted to be at or above threshold for snow transport in the mid to upper elevation with Gusts of 75 to 80 mph and light snow accumulation over the upper elevations will result in light drifting on leeward slopes. Anticipate isolated shallow Wind Slabs on N-E-S-SW aspects in the mid to upper elevations above ~ 9500’. Riders will likely encounter these below ridgeline, adjacent to terrain features that promote drifting, along 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.

advisory discussion

Spring continues to dominate the weather picture with warm daily temperatures and cool nights interrupted by the occasional glancing blow by weak fast moving spring storms as the storm track has shifted north into the Pacific Northwest.The most recent spring system to move through the region Mon thru Tues with moderate to strong winds, cooler temperatures, and increasing cloud cover (primarily Mammoth north) with light precipitation over the upper elevations forming isolated shallow Wind Slabs in the upper elevations primarily on N-E-S-SW aspects. The last significant storm to sweep though the region was 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.

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Wednesday ~0500 Obs Crnt Temp High (Tues)

Virginia Lakes (Elev. 9445’) 42 46

Ellery Lake (Elev. 9645’) 34 50

Agnew Pass (Elev. 9355’) 34 55

June Mountain (Elev. 9148’) 40 49

Mammoth Pass (Elev. 9500’) 37 57

Sesame Study Site (Elev. 9014’) 40 53

Rock Creek Lakes (Elev. 9600’) 38 63

Sawmill (Elev.10200’) 37 46

South Lake Cabin (Elev. 9580’) 37 56

Weather and CURRENT CONDITIONS

weather summary

Wed - An anomalous moisture plume is sitting just off the northern California coast this morning. However, while this is impressive from a magnitude perspective, the plume is "more bark than bite" with forcing for heavy precipitation weak over northern California with an upper disturbance near the Oregon and Washington border. Warm air advection lift combined with the already moist atmosphere will still serve to bring a shield of light rain and high elevation snow to the region this morning with snow levels are slowly rising above 8500’ with best chances along the Sierra Crest north of June Lake. Elsewhere, mostly cloudy to cloudy, scattered snow showers along the Crest, windy, with slightly below normal temperatures. Winds, are expected to increase substantially by this afternoon with sustained winds between 20 and 35 mph for many areas with the highest winds expected in southern Mono County near Highway 395 (which is susceptible to northwest flow). Winds in those areas could reach 35-40 mph sustained with gusts to 55 mph.

Thurs thru Friday - Surface gradients will remain strong with gusty conditions continuing. Northeast California will see the end of shower chances but remain under northwest to north flow aloft for below average afternoon temperatures. Lows, near or below freezing temperatures look to return Thursday and Friday. Afternoon highs, 50s to low 60s.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS

For 8,000 ft. to 10,000 ft.

	Wednesday	Wednesday Night	Thursday
Weather:	Mostly cloudy. Chance of snow and rain in the morning. Scattered showers in the afternoon.	Partly cloudy then becoming clear.	Sunny then becoming partly cloudy.
Temperatures:	47 to 57 deg. F.	28 to 33 deg. F.	44 to 54 deg. F.
Mid Slope Winds:	West	West	Northwest
Expected snowfall:	0-1 in.	0 in.	0 in.

Over 10,000 ft.

	Wednesday	Wednesday Night	Thursday
Weather:	Mostly cloudy. Chance of snow in the morning. Scattered snow showers in the afternoon.	Partly cloudy then becoming clear.	Partly cloudy then becoming clear.
Temperatures:	37 to 47 deg. F.	23 to 28 deg. F.	34 to 44 deg. F.
Ridge Top Winds:	West	Northwest	Northwest
Expected snowfall:	0-1 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.