Avalanche Advisory: Tuesday - Feb 27, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON February 28, 2018 @ 6:57 am
Avalanche Advisory published on February 27, 2018 @ 6:57 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The avalanche danger for Tuesday (2/27) is Considerable. The combination of moderate to strong winds Monday afternoon and through the night with ~3” to 6” (more in favored locations) of low-density new snow have likely produced tender Wind Slabs on all aspects near and above treeline. Natural avalanches possible, triggered releases are likely.

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

 

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The storm currently passing through the region has deposited ~3” to 6” (more in favored locations) of very transportable low-density snow from Mammoth Lakes north with lesser amounts to the south. Monday saw moderate to strong Southwesterly winds durng the afternoon and light to moderate snowfall forming Wind Slabs on NW-N-NE-SE aspects from treeline and above. With frontal passage (early evening), snowfall rates increased while winds decreased to moderate while shifting to North to Northeast, producing a new round of Wind Slabs on NW-SW-S-SE aspects. Northeasterly winds are forecasted to remain moderate to strong through the day, forming additional fresh tender Wind Slabs through today. Temperatures have dropped precipitously since frontal passage and will slow the normal strengthening and bonding process, allowing the Wind Slab threat to linger through the forecast period. With the wide swings in wind direction during the storm, optimal wind speeds for snow transport,  and plenty of transportable snow, all aspects near and above treeline should be treated as suspect. Wind Slabs will most likely be encountered near and above treeline: below ridgelines, in steep or complex terrain, under cornices, crossloaded gullies, and terrain features (i.e. rock outcrops) that promote drifting. Natural avalanche possible, triggered releases likely near and above treeline. Pay attention to changing snow density and dense snowdrifts and deposits. Careful snowpack evaluation, cautious route-finding and conservative decision-making is essential if traveling in avalanche prone terrain.

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

 

advisory discussion

The latest storm to impact the region will begin to move off to the east today (Tuesday) with a weak ridge forming before 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 ushered in another round of unseasonably cold weather, which will slow bonding and strengthening allowing the Wind Slab hazard to linger longer. The wide swings in wind direction, optimal wind speeds for snow transport, and plenty of transportable snow have increased the avalanche hazard to Considerable for all aspects, near and above treeline for the forecast period.

Caution – though unlikely, areas that have received the upper snowfall amounts forecasted with this most recent storm may see some localized Storm Slab concerns, near and above treeline.  

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, which formed Wind Slabs and drifts primarily near treeline and above on all aspects. The temperatures have remained below seasonable norms since post-frontal passage, slowing the normal strengthening and bonding process. These older Wind Slabs are stubborn in nature to fail 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.

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

 

 

 

Weather and CURRENT CONDITIONS
weather summary

Tues thru Wednesday - Tuesday, snow showers (light accumulation) will continue, mainly south of Bridgeport. Northeast flow will keep temperatures chilly as a weak ridge briefly transits the region. Wednesday will be a relatively quiet with increasing clouds and breezy conditions developing by afternoon ahead of the next storm.

Thurs thru Friday - Simulations have a deep and cold low pressure system just off the Pacific NW coast, which is a known to favor heavy snow in the Sierra with cold polar air paired with the dynamics of the low and substantial Pacific moisture will be directed into the Sierra by a very strong Pacific jet. Strong gusty winds will develop on Thursday ahead of and along the cold front with wind gusts approaching 100 mph along the Sierra Crest. Winds will decrease once the cold front pushes through Thursday afternoon for the lower valleys and foothills but    winds will continue in the Sierra with localized blowing snow and limited visibility throughout the storm. The initial wave on Thursday will be the heaviest period of snow, although this storm will continue through Friday and Saturday with additional shortwaves rotating through the trough with multiple rounds of moderate to heavy snow in the Sierra with storm totals of around 3 to 5 feet in the High Sierra above 7000 feet.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy then becoming partly cloudy. Chance of snow showers in the morning. Partly cloudy then becoming clear. Partly cloudy then becoming mostly cloudy. Slight chance of snow showers in the afternoon.
Temperatures: 21 to 26 deg. F. 5 to 10 deg. F. 29 to 37 deg. F.
Mid Slope Winds: North Light winds. Light winds becoming southwest.
Expected snowfall: up to 1 in. 0 in. up to 1 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy then becoming partly cloudy. Chance of snow showers in the morning. Partly cloudy then becoming clear. Partly cloudy then becoming mostly cloudy. Slight chance of snow showers in the afternoon.
Temperatures: 17 to 22 deg. F. 4 to 9 deg. F. 23 to 28 deg. F.
Ridge Top Winds: Northeast North West
Expected snowfall: up to 1 inch in. 0 in. up to 1 inch 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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