Eastern Sierra Avalanche Advisory - 2/18/17

The backcountry is more accessible than it has been in years. Please help!  ... Click Here to Find Out How

THIS AVALANCHE ADVISORY EXPIRED ON February 19, 2017 @ 7:00 am
Avalanche Advisory published on February 18, 2017 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
bottom line

With 1 to 2.5’ of new snow since yesterday morning (6”-12” overnight), heightened avalanche danger exists in many areas.  Natural wind slab avalanches are possible in isolated areas at upper elevations that continue to recieve wind deposition or as a result of cornice failure, and human triggered wind slab and storm slab avalanches will remain likely on steep slopes at all elevations.  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

With 1 to 2.5’ of new snow since yesterday morning (6”-12” overnight), heightened avalanche danger exists in many areas.  Natural wind slab avalanches are possible in isolated areas at upper elevations that continue to recieve wind deposition or as a result of cornice failure, and human triggered wind slab and storm slab avalanches will remain likely on steep slopes at all elevations.  Carefully assess the snowpack, and choose your route to avoid areas steeper than about 35 degrees that could have unstable snow. 

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

1’-2.5’ of light new snow has fallen since yesterday morning, with greater amounts centered around Mammoth and June, and lesser amounts south and north.  Winds were surprisingly light with the majority of this storm, at least at lower and mid-mountain locations, resulting in widespread low-density storm slab formation.  These were very sensitive and easy to trigger yesterday afternoon resulting in 10-12”+ crowns in the Mammoth area on slopes greater than 35 degrees.  With an additional 6-12” of light snow overnight, resulting crowns will likely be larger today.  As the day progresses and these slabs settle, they will become less sensitive, but a denser slab release could be more consequential.  Assess the snowpack carefully, do your own localized stability assessments, and realize that even in areas that don’t have slab-like tendencies, large soft sloughs can still result in a burial if they end in a terrain trap or pin a person against a tree. 

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

1’-2.5’ of light new snow has fallen since yesterday morning, with overall light winds.  At mid and upper elevations however where SW to S winds were slightly stronger, this light snow could easily be transported into soft sensitive wind slabs.  Wind can transport snow much faster than it can fall from the sky, and soft wind slabs several feet thick plus are conceivable.  These are likely to be found on non-southerly facing leeward sides of ridgelines, sidewalls of gullies, and around other features that promote drifting.  Be on the lookout for denser 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.

advisory discussion

Mostly stable backcountry conditions ended this past Wednesday as SW winds increased significantly, ending an epic window of alpine powder skiing.  Sensitive wind slabs became widespread, and several reports came in from surprised skiers who triggered and/or got caught in resulting avalanches.  Luckily no major injuries resulted.  This was a good wake-up call to how quickly the winds in the Sierra can change safe slopes into dangerous ones.  About 2” of snow fell Thursday morning with some strong SW winds throughout the forecast area, blue skies popped out by Thursday afternoon.  Early yesterday morning, heavy snowfall began again as the first in another series of Atmospheric River storms impacted our area.  Snowline remained below 7000’ for this storm, which dropped 1-2.5’ of light new snow.  This storm was not accompanied by the typical strong winds, resulting in less wide spread wind slab formation limited to exposed mid to upper elevation locations.  This storm has now mostly ended, leaving us with widespread tender storm slabs, less widespread but potentially more consequential wind slabs, and risks of significant sloughing.  There were many reports of widespread soft storm slab releases and sloughing in steeper mid to low elevations yesterday around the Mammoth and June area, as well as from Ice Climbers in Lee Vining Canyon who got significantly sloughed on from slopes above.

Snowpack history leading up to this week:  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. 

recent observations

-2/17 - Earthquake Dome: Widepspread storm slab and loose dry avalanche activity

-2/17 - Sherwins - Widespread storm slab avalanches, natural and triggered

-2/17 - Lee Vining Canyon Ice - Natural loose dry activity

-2/17 - Punta Bardini - Weak layer under recent thin melt freeze crust, Storm snow, avy activity 

-2/17 - Mammoth Mountain - Ski Patrol afternoon ski cut results

Storm Totals (Friday early morning to Saturday 5am)

Location -- Storm Snow -- Storm Water

VA Lakes (9445’) -- n/a -- 0.5-0.7”H2o

Tioga Pass (9798’) -- 12” -- n/a

Ellery Lake (9645’) -- n/a -- 0.6”H20

June (9148’) -- 20” -- 1.9”H20

Gem Pass (10750’) -- n/a -- 1.2”H20

Mammoth Sesame St (9014’) -- 25” -- 2.1”H20

Rock Creek (9600’) -- 7” --n/a

Saw Mill-Big Pine (10200’) -- 10” --n/a

Big Pine Creek (10000’) -- 16” -- n/a

 

weather

The bulk of this past Atmospheric River storm event is over as snow showers tapper off early this morning.  We get a small break from stormy weather now until Sunday night/early Monday morning, when the next in a series of Atmospheric Rivers begins to impact our area , this one stronger with heavier snowfall and much stronger winds thru Tuesday.

Saturday:  Cloudy skies with lingering snowfall in the morning before 10am, then scattered flurries with up to 2” of new accumulation by day’s end.  Winds will continue to be light out of the South, with gusts over 10,000’ up to 40mph, and decreasing through the day.  High temperatures are expected in the mid to upper 30s between 8000’ and 9000’, and in the upper 20s above 10,000’.

Sunday:  Partly cloudy skies with occasional flurries leading to no new accumulation.  Winds look to increase slightly with gusts up to 55mph above 10,000’ and shift slightly out of the SW and W.  Temperatures will warm slightly with highs in the upper 20s to low 30s above 10,000’. 

Long-term:   “For Sunday night through early Tuesday, a powerful Pacific storm fueled by a wide and moderate-strong atmospheric river is on track to slam into the Sierra and eastern CA.”  Strong winds are expected Monday morning, with ridgetop gusts exceeding 125mph.  This next storm also brings heavier amounts of moisture with liquid amounts in the 3-6” range at higher elevations (several feet + of snow), and also slightly higher snow lines, but likely still under 8,000’.  Then yet another AR storm looks likely to roll-in Tuesday evening thru Wednesday night, this one much colder, with another couple of feet of snow possible at upper elevations.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Cloudy. Chance of snow in the morning, then scattered snow showers in the afternoon. Mostly cloudy then becoming partly cloudy. Isolated snow showers through the night. Partly cloudy. Slight chance of snow showers.
Temperatures: 31 to 37 deg. F. 20 to 25 deg. F. 33 to 39 deg. F.
Wind direction: SW SW SW
Wind speed: Light winds. Gusts up to 30 mph in the morning Light winds. Gusts up to 25 mph after midnight 10 to 15 mph with gusts to 40 mph
Expected snowfall: up to 2 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Cloudy. Chance of snow in the morning, then scattered snow showers in the afternoon. Mostly cloudy then becoming partly cloudy. Isolated snow showers through the night. Partly cloudy. Slight chance of snow showers.
Temperatures: 25 to 31 deg. F. 14 to 20 deg. F. 27 to 33 deg. F.
Wind direction: SW W W
Wind speed: 10 to 15 mph. Gusts up to 40 mph in the morning. 10 to 15 mph. Gusts up to 30 mph after midnight 20 to 30 mph with gusts to 55 mph
Expected snowfall: 1-2 in. 0 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.

ESAC receives support from ...