Avalanche Advisory - Tue, Jan. 03, 2017

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THIS ADVISORY EXPIRED ON January 5, 2017 @ 6:58 am
Avalanche Advisory published on January 3, 2017 @ 6:58 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

*Winter Storm Warning in effect for our forecast area from 4pm Tuesday through 4am Thursday*

Tuesday:  Lingering small isolated pockets of wind slab are likely to exist on exposed slopes facing SE-E-N-NW at tree line and above that may be possible for humans to trigger.  If snowfall rates increase in the afternoon, new windslabs will begin forming on these slopes that will likely be sensitive to human triggering.   

Wednesday:  As heavy snowfall accompanied by strong SW winds begins Tuesday night and continues through Wednesday night avalanche danger will continually increase at all elevations and aspects.  Greatest concern will be for sensitive wind slab formation on slopes facing SE-E-N-NW where these strong winds deposit this new snow at a much greater rate and higher density than the natural snowfall.  Natural avalanches will become likely and human triggered avalanches almost certain.   

Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Tuesday: High southwesterly winds on New Years day resulted in a good deal of visible snow transport at mid to upper elevations (above 9,500’) across slopes and over ridges and peaks easily visible from 395.  While previous moderate winds did not result in nearly any snow transport, the intensity increased enough on New Years to dig into and whip up the snow that was in areas that were previously not wind affected.  Sensitive windslabs were likely formed, but as the winds decreased back to moderate levels yesterday and today (1/2-3/17), these new windslabs were given a chance to start stabilizing.  As such, there may still be small pockets from these New Years winds that remain sensitive to human triggering.  These could likely be found on exposed slopes >35deg facing SE-E-N-NW at tree line and above, below ridgelines, cross-loaded slopes and sidewalls of gullies.  If snowfall begins Tuesday afternoon, new sensitive windslabs will begin forming quickly, and human triggered avalanches will become likely.  These could easily knock a skier or rider off their feet and possibly drag them through dangerous terrain.  These may become large enough to bury a person, especially if associated with a terrain trap.

Wednesday: As heavy snowfall accompanied by strong SW winds begins Tuesday night and continues through Wednesday night avalanche danger will continually increase at all elevations and aspects.  Greatest concern will be for sensitive wind slab formation on slopes facing SE-E-N-NW where these strong winds deposit this new snow at a much greater rate and higher density than the natural snowfall.  Natural avalanches will become likely and human triggered avalanches almost certain.  Avoid slopes >32 deg with denser wind deposited snow on Wednesday.    

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

Tuesday: Very unlikely.

Wednesday:  As snowfall rates increase dramatically Tuesday night through Wednesday night with 2-3ft+ of new snow forecasted (slightly lesser amounts south of Mammoth), storm slab avalanche danger will increase on all aspects and elevations in areas sheltered form the wind.  Several factors during this storm will increase the likelihood and danger of these soft-slab avalanches.  First, much of the underlying upper old snow surface in wind-sheltered locations has become weak and faceted over the past week, making for a weak layer for this new snow to fail on.  Also feathery surface-hoar has been found to be growing on many slopes over this past week, which is a very fragile weak layer.  While this surface hoar is easily destroyed by winds, some may have persisted in more sheltered locations.  Second, sunny skies on many days over the past week have resulted in slick melt-freeze sun crusts forming on slopes with some southerly exposure (SE-S-SW) that could act as a slick bed surface for new slabs to fail on.  And third, the incoming storm is forecasted to come in quite cold, but undergo a warming period on Wednesday resulting in denser snow falling on top of lighter snow (upside-down snowpack), which is not a good recipe for stability.  Natural avalanches will become likely, and human triggered avalanches very likely on slopes >32 degrees, especially as the storm warms and denser snow falls.  These avalanches could easily be large enough to bury a person. 

Avalanche Character 3: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

Tuesday: Very unlikely.

Wednesday:  Several underlying weak layers exist in the snowpack in many areas throughout the forecast area between 9,200’ and 11,200’.  These are layers of faceted weak snow surrounding rain-crusts that developed during warm storms in October and also on Dec 9th where rain fell on the snowpack up to 11,200’.  In some areas these layers could be buried over a meter deep (more typical around Mammoth), and in other areas less than a foot (more typical south of Mammoth and north of June).  Widespread avalanche activity resulted during heavy storms in mid to late December, many of which failed in some of these deeper weak layers.  While these weak layers of concern have already failed in many avalanche prone areas, it is likely that they still exist in some as well.  While these layers have not been too concerning since the last major Christmas Eve storm, a new heavy snow load and new snow avalanches could produce enough stress to these underlying layers to cause them to fail.  Resulting avalanches would likely be very large and destructive, and could fail well above a human trigger.  This threat is mostly of concern on slopes >32 deg facing E-N-W between 9,200’ – 11,200’, and even more so for areas with shallower snow packs such as Rock Creek, McGee, Convict, Bishop and north of June. 

Snowpack Discussion

Over a week of high pressure and sunny skies has ended as a new winter storm is dropping into our forecast area.  Mostly stable snow conditions have existed.  Cold nights have led to widewspread surface and near-surface faceting of the snowpack in areas that don't have dense windslabs, making for some great skiing conditions this past week in sheltered locations.  Quite a bit of feathery surface-hoar has been observed to be growing during this periods when the winds were calm.  However, much of this surface hoar was likley destroyed with the increase in heavy SW winds which occured on New Years day.  Deep persistent weak layer sourounding early season rain crusts found to be present still in many areas, not reactive at moment, but may re-awaken with additional snowload.         

weather

*Winter Storm Warning in effect for our forecast area from 4pm Tuesday through 4am Thursday -  A significant winter storm originating in Canada will finally begin impacting our forecast area Tuesday afternoon with light snow which will become heavy in the evening and persist at heavy rates through Wednesday night. 

For Tuesday, expect scattered light snow showers through the day, intensifying in the afternoon and evening, with 1-4” of snow before nightfall.  Winds will be out of the Southwest at moderate levels, with gusts up to 60-70mph over ridgetops, with temps in the low to mid 20s.

Expect snowfall throughout the night Tuesday, with 1-1.5ft of new snow expected. Southwest winds will become stronger with gusts reaching into the 90s over ridgetops, and temperatures staying in the low to mid 20s.

For Wednesday, continued snowfall will result in another 1-1.5ft of new snow, with continued strong southwesterly winds with gusts exceeding 100mpg over ridgetops.  Temperatures will warm slightly, especially at elevation below 10,000’, with highs expected in the low to mid 30s. 

Snow is expected to continue through Wednesday night and tapper off through out the day on Thursday.  Friday looks to be a break with some sunny skies, before a potentially heavy atmospheric river event impacts our area through the weekend with more heavy snowfall. 

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: CLOUDY. SCATTERED SNOW SHOWERS IN THE MORNING, THEN SNOW LIKELY IN THE AFTERNOON. CLOUDY. SNOW. CLOUDY. SNOW THROUGH THE DAY.
Temperatures: 25 TO 31 deg. F. 23 TO 28 deg. F. 30 TO 36 deg. F.
Wind direction: SW SW SW
Wind speed: 15 TO 20MPH WITH GUSTS UP TO 50MPH INCREASING TO 60MPH IN THE AFTERNOON 15 TO 25MPH WITH GUSTS UP TO 55MPH INCREASING TO 75MPH AFTER MIDNIGHT 20 TO 30MPH WITH GUSTS UP TO 65MPH
Expected snowfall: UP TO 4" in. 6 TO 12 in. 10 TO 15 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: CLOUDY. SCATTERED SNOW SHOWERS IN THE MORNING. SNOW LIKELY IN THE AFTERNOON. CLOUDY. SNOW. CLOUDY. SNOW.
Temperatures: 20 TO 26 deg. F. 19 TO 24 deg. F. 22 TO 28 deg. F.
Wind direction: SW SW SW
Wind speed: 30 TO 45MPH WITH GUSTS TO 75MPH DECREASING TO 25 TO 35MPH WITH GUSTS TO 65MPH IN THE AFTERNOON 30 TO 45MPH WITH GUSTS TO 80MPH INCREASING TO 35 TO 55MPH WITH GUSTS TO 95MPH AFTER MIDNIGHT 40 TO 60MPH WITH GUSTS TO 105MPH
Expected snowfall: 1 TO 4 in. 6 TO 18 in. 10 TO 15 in.
Disclaimer

This Snowpack Summary 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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