Avalanche Advisory: Wednesday - Mar 21, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 22, 2018 @ 7:00 am
Avalanche Advisory published on March 21, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

CONSIDERABLE avalanche danger exists today due to a complex mix of loose wet, wet slab, and windslab avalanche problems.  Rain on snow below 9000’ and very warm humid air will cause loose wet avalanche activity to be likely up to ~11,000’ on all aspects, and wet slabs possible at lower elevations.  3-7” of sierra cement snow since yesterday afternoon and up to 5” more today accompanied by moderate to strong SW winds means sensitive wind slabs will be likely at treeline and above.  As precipitation intensity increases dramatically tonight, avalanche danger will increase at least to HIGH.    

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Loose Wet
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Rain below 9000’, and very warm humid air with high temperatures reaching the mid 30s around 10,000’ will cause loose wet avalanche activity to be likely up to ~11,000’ on all aspects.  Yesterday evening steeper north facing slopes in the Sherwins were covered by small rollerballs up to the ridgeline at 10,000’, and ski turns at lower elevations easily created large pinwheels.  With the increase in precipitation last night and today, and even warmer temperatures, natural and human triggered loose wet avalanche activity will rise even more.  Even wet surface sloughs can entrain enough snow to take a skier or rider off their feet and could end in a burial especially if the slope ends in a terrain trap.  These could step down and trigger a larger and much more dangerous wet slab release.      

Avalanche Problem 2: Wet Slab
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Rain on snow is likely today up to 9000'.  If slopes with enough snow at these lower elevations become saturated, large dangerous wet slab avalanches are possible.  Avoid being on or under slopes steeper than ~30 degrees that are becoming saturated.  As precipitation increases dramatically tonight, this concern will rise. 

Avalanche Problem 3: Wind Slab
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Above 9000’ 3-7” of sierra cement snow fell since yesterday afternoon and up to 5” more is expected today.  SW winds have been moderate at mid to upper elevations, and are expected to increase this afternoon with gusts up to 80mph over ridge tops.  Wind slabs began forming last night, and will continue to form today on leeward slopes at treeline and above.  Look for dense wind deposited snow just below ridges on NW-NE-SE facing slopes, in the sidewalls of gullies, and around other features that promote drifting.  Pay attention to signs such as shooting cracks.  Human triggering is likely, and natural triggering is possible.  As precipitation intensity increases dramatically tonight, this concern along with storm slabs in sheltered locations will rise dramatically as well.

advisory discussion

With the precursor to our first real warm significant Atmospheric River storm today, we are seeing different avalanche concerns on northerly facing slopes than we are used to in the form of significant loose wet and possibly wet slab problems.  Rain on snow can quickly weaken a snowpack and create dangerous avalanche conditions.  The question is, how intense does the rain need to be, and where will the rain overlap with lower elevation slopes that have enough snow not to be anchored by underlying vegetation?  WIth this season's low amount of low elevation snow, these areas of concern are more limited, but they exist, especially where deep buried facet layers exist!  As rain levels creep higher, especially as intensity dramatically increases tonight, these areas where a larger wet-slab release is possible will become more widespread.  With the heaviest precipitation yet this season tonight and tomorrow, big avalanches will become quite likely, and a very real possiblity exists very deeply buried weak layers could be triggered resulting in VERY large destructive avalanches.

Weather and CURRENT CONDITIONS
weather summary

*Winter Storm Warning in effect from 5pm Wednesday to 5am Friday*

Today will be cloudy with snow showers above 8500’ and rain below, with up to 5” of new snow by dark at higher elevations.  Moderate SW winds are expected to increase this afternoon with gusts up to 80mph over ridgetops, along with high temperatures in the mid 30s around 10,000’. 

Snow intensity will increase dramatically tonight thru Thursday afternoon/evening as a moderate to strong Atmospheric River brings heavy tropical moisture centered on southern Mono County with 2-4’ of snow expected around 10,000’, and 5’+ for locations along the crest.  Snowline will remain high above 8500’ for most of this storm, until the very end when it will drop below 6000’.  

Conditions should clear thru Friday afternoon, and then a cold front moves in dropping temperatures thru the weekend with chances of light snow showers.  Expect high pressure with rising temperatures for next week. 

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. Chance of rain and snow. Cloudy. Snow and rain. Cloudy. Snow and rain
Temperatures: 39 to 45 deg. F. 31 to 36 deg. F. 33 to 39 deg. F.
Wind Direction: SW S SW
Wind Speed: 15 to 25 mph with gusts to 45 mph. 15 to 30 mph with gusts to 50 mph. 15 to 25 mph with gusts to 45 mph.
Expected snowfall: 1-5 in. 7-15 in. 11-19 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy. Chance of snow in the morning, then snow in the afternoon. Cloudy. Snow. Cloudy. Snow.
Temperatures: 31 to 36 deg. F. 25 to 30 deg. F. 27 to 32 deg. F.
Wind Direction: SW SW SW
Wind Speed: 30 to 45 mph with gusts to 70 mph increasing to 40 to 55 mph with gusts to 80 mph in the afternoon. 40 to 55 mph with gusts to 85 mph. 50 to 70 mph with gusts to 105 mph decreasing to 45 to 60 mph with gusts to 95 mph in the afternoon.
Expected snowfall: 1-5 in. 12-20 in. 19-27 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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