Avalanche Advisory: Thursday - Mar 22, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 23, 2018 @ 6:57 am
Avalanche Advisory published on March 22, 2018 @ 6:57 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

*update 9:45am*

Avalanche danger has risen to EXTREME today!  Intense snowfall and high winds last night will intensify more today.  Widespread sensitive wind slabs and storm slabs will exist at elevations above 8500’, with a good possibility that deep slabs could be triggered.  Large to very large natural and human triggered avalanches are certain.  Remote triggering of large slides have been reported this morning.  Avoid all avalanche terrain!  Areas north of Mammoth where snow intensities may be less, HIGH avalanche danger will exist at minimum. 

5. Extreme

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Above Treeline
Avoid all avalanche terrain.

5. Extreme

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Near Treeline
Avoid all avalanche terrain.

5. Extreme

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Below Treeline
Avoid all avalanche terrain.
    Avoid all avalanche terrain.
Avalanche Problem 1: Wind Slab
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Up to 2ft of heavy snow with 2.5” of water content has fallen overnight with strong southerly winds.  Snowfall intensity reached 3” an hour at times.  2-3ft more of heavy snow is expected before this evening, with even more intense periods of loading than last night peaking around noon.  Strong winds shifting from the S to SW, with gusts over 90mph over ridgetops will continue today.  Keep in mind that wind can deposit snow up to 10X faster than it falls out of the sky!  A large natural avalanche outside of Bishop and large cornice failures on the Sherwins were observed yesterday with relatively light amounts of new snow fall compared to the much more intense snowfall of last night and today.  E-N-W facing slopes under ridgelines will be the most dangerous, and slopes of any aspect down lower could have dangerous wind slabs due to swirling winds.  Avoid being on or under slopes with wind deposited snow.   

Avalanche Problem 2: Storm Slab
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Up to 2ft of heavy snow with 2.5” of water content has fallen overnight with strong southerly winds.  Snowfall intensity reached 3” an hour at times.  2-3 more feet of heavy snow is expected before this evening, with even more intense periods of loading than last night expected around noon.  Heavy dense snow will form slabs easily, and snow bonding can’t keep up with these intense loading rates.  Natural and human triggered avalanches are very likely.  Avoid being on or under sheltered slopes greater than ~30degrees.      

Avalanche Problem 3: Deep Slab
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The snowpack structure throughout the season has been bad.  Areas of weak sugary faceted snow have been found deeply buried under more dense snow throughout the forecast area above 9000’ on E-N-W facing slopes.  These layers have failed only during bigger storm events this season.  As they get buried deeper and deeper, they become less and less concerning.  This is the biggest storm yet this season, and it is likely that intense snow loading could once again trigger these deeper weak layers.  The force of a wind slab or storm slab avalanche could also trigger these deeper layers.  A resulting avalanche would certainly be very large, destructive and deadly.           

advisory discussion

Don’t let the low or even no amount of new snow in town this morning fool you!  This is a warm Atmospheric River storm, with snow lines wavering between 8000 and 8500’.  Above this elevation it has been DUMPING all night, and it will continue to dump through most of today.  The snow is being deposited much faster than it can adjust to hold its own weight, especially where it is being deposited even faster by the wind.  The buried weak layers could very well come back alive.  AVOID avalanche terrain today! 

Snow sensors north of Mammoth on the eastern side of the mountains are indicating that lesser amounts of new snow have fallen than around Mammoth, but sensors deeper in the mountains are still showing high amounts.  In areas where significantly less snow has fallen, avalanche danger may only be HIGH.  

As snowfall tapers off this afternoon and evening, snowline will drop, but the timing isn’t certain.  Could this mean 3” of snow down to 7000’, or a foot?  We will see.      

 

Weather and CURRENT CONDITIONS
weather summary

*Winter Storm Warning in effect now thru 5am Friday morning*

Heavy snow will fall above 8000’-8500’ throughout the day with rain below this elevation.  Intensity should peak around noon, with up to 3” an hour of snowfall before tapering off this evening and tonight.  Heaviest snowfall will be focused on the Mammoth area and south, with up to 2.5’ more expected before tapering off this evening.  Snow levels will gradually drop throughout the day, reaching below 8000’ in the early afternoon, and down to 6000’ by midnight.  Strong S to SW winds will continue through the day with gusts up to 95mph over ridgetops. 

Conditions will clear for Friday before a cold front moves in with chances of lighter precipitation for the weekend.  High pressure and rising temperatures back to seasonal norms are expected for next week.   

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Cloudy. Snow and rain through the day. Slight chance of thunderstorms in the afternoon. Cloudy. Snow and slight chance of thunderstorms in the evening, then slight chance of snow after midnight. Partly cloudy then becoming sunny
Temperatures: 31 to 36 deg. F. 18 to 23 deg. F. 32 to 40 deg. F.
Wind Direction: S SW SW
Wind Speed: 20 to 30 mph. Gusts up to 55 mph. 15 to 25 mph with gusts to 45 mph. 20 to 30 mph with gusts to 50 mph.
Expected snowfall: 14 to 22 in. 2 to 6 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Cloudy. Snow through the day. Slight chance of thunderstorms in the afternoon. Cloudy. Snow and slight chance of thunderstorms in the evening, then chance of snow after midnight. Partly cloudy
Temperatures: 25 to 30 deg. F. 15 to 20 deg. F. 25 to 30 deg. F.
Wind Direction: S shifting to SW in afternoon SW SW
Wind Speed: 45 to 65 mph with gusts to 95 mph decreasing to 40 to 55 mph with gusts to 85 mph in the afternoon. 30 to 45 mph with gusts to 65 mph. 30 to 45 mph with gusts to 70 mph.
Expected snowfall: 21 to 29 in. 3 to 7 in. 0 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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