Avalanche Advisory: Wednesday - Feb 28, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 1, 2018 @ 7:01 am
Avalanche Advisory published on February 28, 2018 @ 7:01 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The avalanche danger for Wednesday (2/28) is Moderate from treeline and above, generally Low below treeline. Forecasted moderate Southwesterly and Westerly winds through the day may produce additional tender Wind Slabs in the Alpine regions. Cold temps continue to hinder the bonding and strengthening of recently formed Wind Slabs (2/26 – 2/27). However, they are generally decreasing in sensitivity. Anticipate Wind Slabs on all aspects from treeline and above, especially on NW-N-NE-E-SE aspects where you may encounter more sensitive freshly formed Wind Slabs. Natural avalanches unlikely, triggered releases are possible from treeline and above. Low below treeline. Triggered Loose Dry avalanches are possible in steep sheltered terrain near treeline where new snow amounts exceeds ~4".

...WINTER STORM WARNING REMAINS IN EFFECT FROM 4 AM THURSDAY TO 10 AM PST SATURDAY...

Below ~9,000’, natural and triggered releases are unlikely due to thin snow coverage (below threshold).​

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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The storm that passed through the region Monday deposited ~3” to 6” (more in favored locations) of very transportable low-density snow from Mammoth Lakes north, lesser amounts to the south. The storm was accompanied by moderate to strong Southwesterly winds, which formed Wind Slabs on NW-N-NE-SE aspects from treeline and above, especially in the Virginia Lakes region where winds were stronger and have pushed down into open trees. Post frontal passage (early evening), snowfall continued and winds decreased to moderate but swung to North to Northeast, which produced a fresh round of Wind Slabs on NW-SW-S-SE aspects, which affected the Virginia Lakes region the most. Temperatures have remained below seasonable since storm passage, which has slowed the normal strengthening and bonding process. This has allowed the Wind Slab threat to linger longer than what might be expected. With the wide swings in wind direction during the storm, optimal wind speeds for snow transport, and plenty of transportable snow, all aspects from treeline and above should be treated as suspect. Wind Slabs will most likely be encountered from treeline and above: below ridgelines, in steep or complex terrain, under cornices, crossloaded gullies, and terrain features (i.e. rock outcrops) that promote drifting. Natural avalanche unlikely, triggered releases possible from treeline and above. Pay attention to changing snow density and dense snowdrifts and deposits. Evaluate snow and terrain carefully, identify features of concern.

Below ~9,000’, natural and triggered releases are unlikely due to thin and well-anchored snow cover (below threshold).​

 

Avalanche Problem 2: Loose Dry
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In sheltered steep terrain where the new snow exceeds ~4” (~10cm), it may be possible to encounter or release Loose Dry slides. Though generally small in nature and unlikely to be a burial threat (unless combined with a terrain trap) certainly could be an issue on steep or complex slopes where it could potentially sweep riders into or over hazardous terrain. These may be encountered on all aspects where there’s sufficient base to act as a bed surface and sufficient amounts of new loose snow, this is especially true for the area surrounding Mammoth Lakes. 

advisory discussion

The latest storm to impact the region (2/26) moved off to the east Tuesday with a weak ridge forming over CA briefly ahead of the next storm takes aim at the Sierras, Thursday thru Saturday. The latest storm deposited 3” to 6” (more in favored areas) of low-density snow and accompanied by unseasonably cold temperatures, which has slowed the bonding and strengthening process, allowing the Wind Slab hazard to linger longer from treeline and above. Below treeline, winds were not as strong, forming isolated Wind Slabs, primarily in exposed or open terrain. Wind Slab sensitivity is improving.  Natural avalanches are unlikely but triggered releases remain possible from treeline and above through the forecast period. Well below treeline, natural and triggered releases unlikely.

The cold front that passed through the region last week (2/22) deposited 3” to 15” of snow primarily from Mammoth Lakes north to Virginia Lakes. The storm system was accompanied by periods of moderate to strong winds and significant wind-loading, which formed stiff Wind Slabs and drifts primarily near treeline and above on all aspects. The cold temperatures since frontal passage have slowed the normal strengthening and bonding process, especially where the Wind Slabs overlay near surface facets that formed prior. These older Wind Slabs are stubborn in nature but are not well bonded to the underlying snowpack. A large trigger (i.e. avalanche, cornice failure) may be capable of triggering these older Wind Slabs in isolated locations.

The patchy persistent weaknesses from early season, primarily confined to NE-N-NW aspects above ~9500’, continues to show signs of sintering and strengthening with test results indicating improving cohesion. However, Thursday’s approaching storm is forecasted to produce ~ 3’+ of snow. This will be the biggest storm since November to impact the region and may produce the biggest avalanche cycle of the season. If the upper amounts are realized we may see enough load to stress the underlying persistent weakness for it to become reactive or for slides, such as a large Wind Slab stepping down into the these deeper weaknesses.

Caution – the mid to lower elevation snow coverage remains thin with plenty of hazards lurking just below the snow surface (i.e. rocks, logs, and stumps). The snow is hiding plenty of hazards while providing little protection.

 

Weather and CURRENT CONDITIONS
weather summary

...WINTER STORM WARNING REMAINS IN EFFECT FROM 4 AM THURSDAY TO 10 AM PST SATURDAY...

Wednesday – will be the last bit of quiet weather before the storm moves in late tonight. Will see increasing cloud cover throughout the day in advance of the approaching system. Generally light to moderate Westerly to Southwesterly winds with below seasonable temperatures.

Thurs thru Saturday - Forecaster confidence remains high for a major winter storm early Thursday morning and continuing through Saturday morning. Strong winds and snow are expected with this storm, especially in the Sierra. WINDS: Increase in the early Thursday ahead of the storm with gusts of over 100 mph can be expected Thursday morning and into Thursday afternoon over the Sierra ridges. PRECIPITATION: Snow will start in the Sierra early Thursday-Thursday night. Heavy snowfall rates in the Sierra will combine with strong winds to create periods of dangerous whiteout conditions. Snow levels will plummet with the cold front with snow down to all valley floors for the weekend. Thursday and Thursday night will see heavy snowfall rates of up to 2-4 inches per hour and strong winds producing very low visibility. A possible brief lull in the storm Friday, but then a second round Friday evening and night as a secondary wave moves through the region with additional heavy snow.  STORM TOTALS THROUGH SATURDAY: Snow amounts of around 2 to 5 feet in the High Sierra

 

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: Partly cloudy then becoming mostly cloudy. Mostly cloudy. Chance of snow showers in the evening, then chance of snow after midnight. Mostly cloudy. Snow.
Temperatures: 31 to 37 deg. F. 18 to 23 deg. F. 28 to 34 deg. F.
Mid Slope Winds: Light winds becoming southwest Southwest Southwest
Expected snowfall: 0 in. up to 2 in. 5 to 11 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy then becoming mostly cloudy. Slight chance of snow showers in the afternoon. Mostly cloudy. Chance of snow showers in the evening, then chance of snow after midnight. Cloudy. Snow.
Temperatures: 24 to 29 deg. F. 14 to 19 deg. F. 21 to 26 deg. F.
Ridge Top Winds: West Southwest Southwest
Expected snowfall: 0 in. up to 2 in. 5 to 11 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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