Eastern Sierra Avalanche Advisory - 2/17/17

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THIS AVALANCHE ADVISORY EXPIRED ON February 18, 2017 @ 6:59 am
Avalanche Advisory published on February 17, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
bottom line

A winter weather advisory is now in effect thru 10am Saturday morning.

Avalanche danger will rise throughout the day today as new snow accumulates and increasing winds out of the South will form new fresh sensitive wind slabs on non-southerly facing slopes exposed to the wind.  Natural avalanches will become possible and human triggered avalanches will become likely, especially in steep wind-drifted locations, but also in sheltered locations as more snow accumulates in the afternoon.  Be careful of denser wind-deposited areas, carefully assess the snowpack, and choose your route to avoid areas steeper than about 35 degrees that could have unstable snow. 

How to read the advisory

Bottom Line

A winter weather advisory is now in effect thru 10am Saturday morning.

Avalanche danger will rise throughout the day today as new snow accumulates and increasing winds out of the South will form new fresh sensitive wind slabs on non-southerly facing slopes exposed to the wind.  Natural avalanches will become possible and human triggered avalanches will become likely, especially in steep wind-drifted locations, but also in sheltered locations as more snow accumulates in the afternoon.  Be careful of denser wind-deposited areas, carefully assess the snowpack, and choose your route to avoid areas steeper than about 35 degrees that could have unstable snow. 

Avalanche Problem 1: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Increasing Danger

Snowfall is expected to continue throughout the day today and thru tonight, with increasing winds out of the south gusting into the 60mph range around 10,000’.  Sensitive wind slabs will form mostly on non-southerly facing leeward side of ridgelines, sidewalls of gullies, and around other features that promote drifting.  Remember, winds can cause snow to accumulate 10x faster than it can fall from the sky.  Be on the lookout for denser, potentially hollow sounding snow.  Do your own stability assessments with quick hand-pits, and avoid areas greater than about 35 degrees that could have more than a few inches of fresh wind slab.  Also avoid being under slopes that are being actively wind-loaded that could avalanche naturally from rapid snow deposit and/or cornice failures.         

Avalanche Problem 2: Storm Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Increasing Danger

Snowfall is expected to continue throughout the day today and thru tonight, with 6-12” of new snow expected before dark, and up to 2’ along the crest.  Danger of storm slab avalanches will increase at all elevations and aspects throughout the day as more soft snow accumulates in areas that are sheltered form the wind.  More snow means more potential danger.  Assess the snowpack carefully, and be cautious of slopes greater than about 35 degrees where even soft unconsolidated sloughs could knock a skier or rider down and end up burying them in a terrain trap or pinned on the uphill side of a tree.

advisory discussion

The significant several-day heavy wet storm that occurred during the first half of last week brought widespread wind slab and wet avalanche activity throughout the region, many of which could have destroyed buildings (luckily these did not run near developments.)  Rain fell almost to 9,500’, but not before a layer of light cold snow fell in the beginning of the storm, resulting in an up-side-down snowpack structure.  Much of the larger activity occurred during the storm or just after when high winds continued to blow out of the SW.  As winds calmed and temperatures and snowline dropped Thursday night into Friday, cold dry snow fell leaving the alpine covered with a fresh blanket of 1-3 feet of light right-side-up powder snow on-top of a fairly stabilized snowpack.  Finally we got a break in storms, and several days of blue skies and lightish NE winds allowed for mostly stable conditions and the high country was fully exploited, and some epic alpine ski descents were had.  During the initial days of clear skies and warm temperatures, however, widespread smaller wet-point releases were seen throughout the region anywhere where rocks were warming up from the sun on steep slopes.  Some of these point releases entrained enough snow that a person could have been buried had they been caught (again, luckily no incidents).  Parties ice-climbing in Lee Vining Canyon reported seeing some of these avalanches happen, burying their approach boot packs in several feet of snow within an hour of their boots actually being in the pack.  Winds then increased unexpectedly on Wednesday out of the SW, creating dangerous wind slabs and closing the window on epic alpine powder riding conditions.  At least 2 skier-triggered wind slab avalanches were reported, one of which caught a skier and her dog (luckily only resulting in minor injury).  A good reminder how quickly the winds in the Sierra can change safe slopes into dangerous ones.  Yesterday (Thursday) morning about 2 more inches of snow fell with strong SW wind, creating some new isolated wind slabs.  Winds decreased throughout the day, to only moderate SW winds over more exposed ridge, and blue skies popped out by the afternoon.

*There have also been recent reports of deep wide creep cracks.  Many of these will be covered by new snow, could be very hard to see, and could be very dangerous to fall into.  A report came in this week of one skier falling 15' into one that he did not see, lodging himself, unable to move.  He was able to use his cell phone and got help.  Without a cell phone, this could have been a fatal accident.    

weather

A winter weather advisory is now in effect thru 10am Saturday morning.

The first of another series of moderate Atmospheric Rivers has begun to impact our region this morning.  Thankfully snowline will stay low for this initial storm, dropping below 7000’ this morning and continuing to drop down to 6000’ by early Saturday morning. 

Friday:  Snow all day, with ½ to 1’ of accumulation expected before dark, and up to 2’ over the crest.  Winds will be increasing out of the south gusting up to 60mph over 10,000’ by afternoon.  Expect highs in the mid 20s around 10,000’.  Another ½ to 1’ of snow is expected Friday night with continued South winds before snow tapers off Saturday morning.

Saturday: Snow in the morning before 10am, then scattered flurries with up to 2” of new accumulation.  Winds will continue to be out of the South, but decreasing with gusts only expected up to 30mph by afternoon.  High temperatures are again expected in the mid 20s around 10,000’.

Long-term: Heavier storms are looking likely early next week.  After a break in precipitation from Saturday late morning through Sunday, the next storm is expected to begin to impact our area late Sunday night / early Monday morning through Tuesday morning.  This one looks to be heavier and warmer, with liquid amounts in the 3-5” range possible at higher elevations but snowlines creeping up into the 8000’ range.  Yet another storm is expected to move in Wednesday morning. 

  

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Friday Friday Night Saturday
Weather: Cloudy. Snow. Cloudy. Snow. Cloudy. Snow likely in the morning, then chance of snow showers in the afternoon.
Temperatures: 28 to 33 deg. F. 21 to 26 deg. F. 30 to 36 deg. F.
Wind direction: S S S
Wind speed: 15 to 25 mph. Gusts up to 30 mph increasing to 45 mph in the afternoon. 15 to 25 mph. Gusts up to 45 mph. Gusts up to 30 mph in the morning.
Expected snowfall: 3 -9 in. 2-7 in. 1-2 in.
Over 10,000 ft.
  Friday Friday Night Saturday
Weather: Cloudy. Snow. Cloudy. Snow. Cloudy. Snow likely in the morning, then chance of snow showers in the afternoon.
Temperatures: 22 to 28 deg. F. 16 to 22 deg. F. 23 to 29 deg. F.
Wind direction: S S S
Wind speed: 20 to 30 mph. Gusts up to 45 mph increasing to 60 mph in the afternoon. 20 to 35 mph with gusts to 60 mph. 10 to 15 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon.
Expected snowfall: 5-10 in. 3-8 in. 1-2 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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