Avalanche Advisory: Monday - Feb 26, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON February 27, 2018 @ 6:57 am
Avalanche Advisory published on February 26, 2018 @ 6:57 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The avalanche danger for Monday (2/26) will begin to rise to Considerable, near and above treeline, by this afternoon into the evening as another storm moves into the region with potentially 3’ to 12” of snow and moderate to strong Southwesterly to Southerly winds by the PM, veering Northeast after midnight. Prior to snow onset, moderate to strong Southwesterly winds will likely continue to form new small Wind Slabs on leeward terrain (NW-N-NE-E-SE). Early during the day natural avalanches unlikely, small triggered are possible, with snow onset and as snow begins to accumulate this afternoon/evening natural avalanches will be possible, triggered releases 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

Last Thursday’s storm produced (2/22) 3”-15” of low-density snow primarily from Mammoth Mtn and north with moderate to strong SW winds forming Wind Slabs on NW-N-NE-SE aspects from treeline and above. As the system moved out of the region, winds swung around to the North to Northeast, producing a new round of Wind Slabs on NW-SW-S-SE aspects on all but the most sheltered areas. The cold temperatures post-frontal passage have slowed the normal strengthening and bonding process, allowing the Wind Slab threat to linger. Although little recent natural avalanche activity has been reported, overnight moderate to strong winds have likely formed new small isolated Wind Slabs on NW-N-NE-E-SE aspects prone to triggering today along with older more stubborn deposits. The hazard will begin to increase and become more widely distributed this afternoon to early evening with the 3 to 12" storm totals forecasted and moderate to stron SW winds. With the wide swings in wind direction and steady moderate to strong winds, combined with plenty of easily transportable snow (especially after snow onset), all aspects near and above treeline should be suspect. Today, sensitive Wind Slabs will most likely be encountered on NW-N-NE-E-SE facing slopes near and above treeline, especially: below ridgelines, in steep or complex terrain, under cornices, crossloaded gullies and terrain features (i.e. rock outcrops). By this evening Natural avalanche possible, triggered releases likely near and above treeline. Pay attention to changing snow density and dense snowdrifts and deposits. Perform your own snow assessments.

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

advisory discussion

The cold front that passed through the region last week (2/22) brought 3” to 15” of low-density, very transportable snow primarily from Mammoth Lakes north to Virginia Lakes. The storm system was accompanied by periods of moderate to strong SW - S winds, veering to N - NE, which formed Wind Slabs and drifts primarily near treeline and above, near favored features on many aspects throughout the alpine. The temperatures have remained below seasonable norms since post-frontal passage, slowing the normal strengthening and bonding process. Fresh Wind Slabs will begin to form this afternoon into the evening near and above treeline as another storm takes aim at the Sierras with 3" to 12" forecasted this afternoon into the evening with moderate to strong SW winds. This will create a new round of potentially tender Wind Slabs on NW-N-NE-E-SE aspects. Winds are forecasted to switch to the Northeast late tonight, which may further complicate the Wind Slab distribution, near and above treeline. As a result, additional caution is required in steep or complex terrain near and above treeline where recent drifting is evident. Wind Slabs will most likely be found in the upper elevations under cornices, below ridgelines, at the top of chutes and couloirs, and on steep convexities. Hollow sounding drifts, or dense cracking snow may indicate rider triggered failure is still possible.

If the approaching storm produces the upper snowfall amounts forecasted, we may see some Storm Slab concerns, near and above treeline, by tomorrow AM.  

Below treeline, in very sheltered areas, the snow is mostly soft with thin Wind Slabs dispersed throughout and the occasional recently exposed old melt/freeze crust.

The patchy persistent weaknesses from early season that was primarily confined to NE-N-NW aspects above ~9500’ is showing signs of sintering and strengthening with test results show slowly improving cohesion. However, Thursday’s approaching storm is forecasted to produce up to 3’ of snow. 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, to step 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 coverage (below threshold).​

 

 

 

Weather and CURRENT CONDITIONS
weather summary

Monday thru Wednesday – Snow showers developing over the higher elevations today with the bulk of the snowfall pushing in afternoon into early evening as the upper low closes off over central California. Northeast, upslope flow along with instability aloft could result in snow totals to be 2 to 3 times higher for northern Mono County than surrounding areas. Residual snow showers will likely continue much of Tuesday for portions Mono County. Wednesday, shortwave ridging builds across the region with a brief break in precipitation.

Thursday and beyond - A strong winter storm for Thursday and Friday as a upper low drops southward off the Pacific Northwest coast. The front will work southeast across the area late in the day with showers spilling over behind the front as snow levels drop to valley floors Thursday night. A couple of shortwave features will slide into the Sierra thru Friday night with periods of moderate to heavy snow in the Sierra.

 

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: Cloudy. Chance of snow in the morning, then snow in the afternoon. Cloudy. Snow showers in the evening, then snow showers likely after midnight. Mostly cloudy then becoming partly cloudy. Chance of snow showers in the morning.
Temperatures: 15 to 20 deg. F. 4 to 10 deg. F. 21 to 26 deg. F.
Wind Direction: Southwest becoming South South shifting to the Northeast after midnight. North
Wind Speed: 35 to 55 mph with gusts to 85 mph, 20 to 30 mph with gusts to 55 mph in the afternoon. 10 to 15 mph. Gusts up to 30 mph. 10 to 15 mph. Gusts up to 45 mph decreasing to 35 mph in the afternoon.
Expected snowfall: 1 to 5 in. 2 to 6 in. up to 1 in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Cloudy. Chance of snow in the morning, then snow in the afternoon. Cloudy. Snow showers in the evening, then snow showers likely after midnight. Mostly cloudy then becoming partly cloudy. Chance of snow showers in the morning.
Temperatures: 15 to 20 deg. F. 1 to 6 deg. F. 17 to 22 deg. F.
Wind Direction: Southwest becoming South South shifting to the Northeast after midnight. Northeast
Wind Speed: 35 to 55 mph with gusts to 85 mph, 20 to 30 mph with gusts to 55 mph in the afternoon. 10 to 15 mph. Gusts up to 30 mph. 20 to 30 mph. Gusts up to 55 mph decreasing to 45 mph in the afternoon.
Expected snowfall: 1 to 5 in. 2 to 6 in. up to 2 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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