Avalanche Advisory: Tuesday - Mar 13, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 14, 2018 @ 7:00 am
Avalanche Advisory published on March 13, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

A complex set of avalanche problems exist today due to increasing precipitation and rain up to 9000’, and SW wind.  A narrow band of avalanche danger exists at low elevations between ~8500’- 9000’, where rain falling on a deep enough snowpack that isn’t anchored by shrubbery could cause a large natural wet slab avalanche, especially where deeper weak facet layers are found.  At mid to upper elevations above ~9500’, as 2-8" of new snow falls with strong SW winds avalanche danger will increase by late afternoon and evening due to fresh wind slabs and isolated storm slabs sensitive to human triggering.  Overall MODERATE avalanche danger exists. 

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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As snowfall intensity increases this afternoon with 2-8” of dense new snow expected by dark, strong SW winds gusting up to 75mph over ridge-tops will form new wind slabs on leeward slopes and features above ~9500’ that will likely be sensitive to human triggering.  This concern will be greatest for areas that receive greater amounts of new snow.  Be on the lookout for fresh dense wind deposits just below ridgelines on SE to NE to NW slopes, in the sidewalls of gullies, and around other features that promote drifting.  Watch for signs such as shooting cracks, and avoid steep convexities.  This danger will increase and become more widespread overnight as snowfall intensity increases and snowline drops. 

Avalanche Problem 2: Wet Slab
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As precipitation increases today before temperatures begin to drop tonight, rain is possible up to 9000’.  While the snowpack is generally thin below this elevation, for a narrow band where the snowpack is deep enough, rain falling could cause a large natural wet slab avalanche, especially on slopes where weak loose sugary facet layers are found that aren’t anchored by shrubbery.  If you are out today and it is raining, stay out from under such slopes that are greater than 30 degrees. 

Avalanche Problem 3: Storm Slab
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For sheltered slopes above ~9000’ that receive more than ~5” of new snow by evening, it could be possible for storm slabs to be human triggered.  Be on the lookout for shooting cracks, and pay attention to convexities which are the most likely trigger points. Many areas were found to have a layer of surface hoar yesterday above treeline.  While this was destroyed on any slope with sun exposure (E-S-W), and the increasing winds last night likely destroyed it on most other slopes above tree line, it is possilbe that it could have persisted in an isolated sheltered open area on a NE-N-NW facing slope.  Such as area would be much more sensitive to a human trigger.  Storm slabs will become much more widespread tonight as snowfall intensity increases.   

advisory discussion

Just as enough time has passed since our last big storm at the beginning of the month to gain confidence in our snowpack, high pressure has ended with the arrival of more winter storms.  The high snow-level associated with the first storm today will add a new concern that we haven’t seen this season, in the form of wet slabs.  This will be very limited to a narrow elevation range at the upper end by the upper rain level, and the lower end by the lower extent that slopes are actually covered by enough snow that isn’t anchored by bushes.  The most concerning range will be even narrower where rain falls on slopes with enough snow coverage that also have deep faceted weak layers.  The rest of our avalanche concerns with this first storm are the more typical wind slab and possible storm slab for areas that receive greater amounts of new snow, which will be especially for the northern half our our forecast area.  On the positive side for stability, this storm will be very right-side-up, being dense and heavy to start, and then becoming light overnight as snow levels and temperatures drop dramtically.  The deep weak facet layers that we have been discussing that has been found to be most concerning between 9000-10500’ on E-N-W facing slopes will once again become a very valid concern as more snow accumulates overnight. 

recent observations
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-3/12 - Mammoth - Pika Peak: surface hoar and snowpack

-3/12 - Convict - Old Man's Bowl - Weak Layer, surface hoar

-3/12 - June - Devils Slide:  Deep facet layer

3/11- Snowpack tests, Lake Mary Ridge

3/11- Red Lake Bowl, Virginia Lakes

3/11- Banana Chute, Solitude

3/11- Earthquake Dome Snowpack Tests

3/11- Redcone: Warming and Weak Layers

3/11- Convict Area: Persistent Weak Layer, Loose Wet Instability

Weather and CURRENT CONDITIONS
weather summary

*Winter weather advisory is in effect from 1pm this afternoon thru 11am Wednesday morning. 

A series of winter storms will impact our area thru Saturday.  Light precipitation started last night, and will continue thru today, increasing considerably this evening thru tonight as snow levels drop from 9000’ today to below 6000’ after midnight.  This will be accompanied by strong SW winds.  Snow is expected to continue at lighter rates thru Wednesday, until a stronger colder storm moves in Thursday night thru early Saturday morning.  Sunday into early next week is looking dry. 

 

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: Partly cloudy then becoming cloudy. Chance of snow through the day. Chance of rain in the afternoon. Cloudy. Rain in the evening. Snow through the night. Cloudy. Snow likely.
Temperatures: 37 to 45 deg. F. 20 to 26 deg. F. 27 to 35 deg. F.
Wind Direction: S S shifting to SW after midnight SW
Wind Speed: 15 to 25 mph with gusts to 60 mph. 20 to 30 mph with gusts to 55 mph decreasing to 15 to 20 mph with gusts to 45 mph after midnight. 10 to 15 mph with gusts to 40 mph.
Expected snowfall: 1-5 in. 6 - 12 in. 2-6 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Chance of snow in the morning, then snow in the afternoon. Cloudy. Snow. Cloudy. Snow likely.
Temperatures: 30 to 35 deg. F. 13 to 18 deg. F. 20 to 26 deg. F.
Wind Direction: SW SW SW
Wind Speed: 30 to 45 mph with gusts to 75 mph. 30 to 45 mph with gusts to 70 mph. 20 to 35 mph. Gusts up to 60 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 2-6 in. 8-14 in. 2-6 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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