Avalanche Advisory: Wednesday - Mar 14, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 15, 2018 @ 7:00 am
Avalanche Advisory published on March 14, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Up to a foot of dense snow with 1.5” of water content fell last night accompanied by strong SW winds.  As snowfall and moderate SW winds continue today avalanche danger will remain HIGH due to fresh wind slabs at mid and especially upper elevations.  Human triggering of storm slabs, and the potential to trigger a deep weak layer resulting in a large persistent slab avalanche are very real concerns as well today.  Avoid wind-loaded slopes, and carefully evaluate snowpack on all other slopes, and use cautious route finding and conservative decision-making. 

4. High

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Above Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

3. Considerable

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

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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Dangerous sensitive wind slabs will continue to build on leeward slopes and features today.  These will most likely be found just below ridgelines on SE to NE to NW slopes, and in the sidewalls of gullies and around other features that promote drifting on any aspect.  Watch for signs such as shooting cracks, and avoid slopes greater than about 30 degrees where denser wind slabs may exist.  Human triggered avalanches will be very likely, and natural avalanches likely, especially at higher elevations in areas that received greater amounts of new snow. 

Avalanche Problem 2: Storm Slab
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Another 4-8” of lighter snow is expected today on top of the dense snow that fell last night.  This is enough new snow for storm slabs to be of concern, and while natural avalanches aren’t likely, human triggering is quite possible, especially for areas that receive the upper end of these snowfall amounts.  Be on the lookout for shooting cracks and pay attention to steep convexities which are the most likely trigger points.  While this storm came in very right-side-up (lighter snow ontop of denser snow), periods of graupel early in the storm could act as a sliding layer, and at mid to upper elevations hard melt-freeze crusts on solar aspects could act as smooth surfaces for slabs to slide on.  Evaluate the snowpack carefully, use cautious route-finding and conservative decision making.        

Avalanche Problem 3: Persistent Slab
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The structure of the existing snowpack in many areas is still very suspect.  Loose weak sugary facets can be found buried on many E-N-W facing slopes, especially at mid-elevations.  Tests continue to show that these weak layers are still reactive and can propagate fractures easily.  While the snowpack before this storm seems to have adjusted well and triggering these weak layers became very unlikely, the stress of this new snow-load could make human triggering of these layers possible again.  Even more likely is for a smaller wind or storm slab avalanche to step down and trigger a deep weak layer.  A resulting avalanche would likely be very large, destructive, and deadly.  Keep this in mind when choosing slopes to travel on.        

advisory discussion

Less snow than expected fell during the day yesterday, with drizzle up beyond 9000’.  After midnight, temperatures began dropping dramatically as snowfall intensity increased.  While many snow sensors are reporting light amounts of new snow, several reported up to 1.5” of water (especially around Mammoth and June), which could mean quite a lot more snow at higher elevations where snow was less dense.  Yesterday, rain on snow was a concern for lower elevation slopes, but now that temperatures have dropped, these slopes which became quite moist yesterday will begin refreezing and stabilizing quickly.  This will shrink the band of concern slightly for persistent weak loose buried layers, by bringing the lower elevation up a bit higher.  Field tests yesterday at 8800’ showed that the upper snowpack was moist, but the lower buried facets were still loose and dry.  As this upper snowpack refreezes, it will make it even harder to trigger the deeper facets, but they will still be there.  At higher elevations, tests are still showing fractures to propagate in these deep weak layers, but consistently they are taking more and more force.  With new snow-load, these layers will become a concern again until some time passes and the conditions settle, and they will become even less of a concern than before.  

Weather and CURRENT CONDITIONS
weather summary

*Winter weather advisory is in effect until 1am Thursday morning.

A series of winter storms will impact our area thru Saturday.  The first major wave of snowfall occurred last night into this morning as snowline dropped significantly after midnight.  Snow showers down to 6000’ are expected to continue thru today (with some thunder possible) and tonight with another 4-8” of snow in the mountains likely by Thursday morning.  Thursday should be a break in snowfall, until the strongest wave of the system brings more substantial snow from late Thursday night thru Friday night. 

Long Term:  Conditions begin to dry thru the weekend, until models are showing potential for another warm winter storm Tuesday night.  

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: Cloudy. Snow showers likely. Mostly cloudy. Snow showers likely in the evening, then slight chance of snow showers after midnight. Cloudy. Slight chance of snow showers in the morning, then chance of snow in the afternoon.
Temperatures: 27 to 35 deg. F. 11 to 17 deg. F. 25 to 31 deg. F.
Wind Direction: SW SW SW
Wind Speed: 10 to 15 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon. 10 to 15 mph with gusts to 30 mph. 15 to 20 mph with gusts to 35 mph
Expected snowfall: 1-5 in. 1-4 in. up to 1 inch in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy. Chance of showers. Cloudy. Snow showers in the evening, then chance of snow showers after midnight. Cloudy. Slight chance of snow showers in the morning, then chance of snow in the afternoon.
Temperatures: 20 to 26 deg. F. 4 to 9 deg. F. 18 to 24 deg. F.
Wind Direction: SW SW SW
Wind Speed: 25 to 35 mph with gusts to 55 mph decreasing to 15 to 25 mph with gusts to 40 mph in the afternoon. 20 to 30 mph with gusts to 45 mph. 20 to 30 mph with gusts to 45 mph.
Expected snowfall: 2-6 in. 2-6 in. up to 1 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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