Avalanche Advisory - Thu, Jan. 12, 2017

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THIS ADVISORY EXPIRED ON January 14, 2017 @ 6:59 am
Avalanche Advisory published on January 12, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

*update Friday 7:30am:  in addition to points mentioned below, increased concern exists for sensitive windslabs due to Moderate winds out of the NE at upper AND MID elevations (9,000' and above).  Human triggered avalanches will be very likely on leeward terrain features where newly wind transported snow is deposited.  

For Thursday, there will be areas where Natural avalanches will be likely, and human triggered avalanches very likely due to strong winds and significant snowfall over the past week, and continued snowfall with moderate accumulation throughout today.  Wind slab avalanches are the biggest concern in exposed areas that receive wind deposited snow, followed by storm slabs in more sheltered areas.  For Friday Avalanche danger will decrease somewhat as we will finally get a break from snowfall and winds both Thursday and Friday are forecasted to be light, with a shifted direction out of the northeast.  Natural avalanches will still be possible Friday, and human triggered avalanches will still be likely!  There still exists the possibility that one of these avalanches could trigger a deep persistent weak layer that could result in a very large destructive avalanche.

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.

*Friday 7:30am update:  Increased concern exists for sensitive windslabs due to Moderate winds (25-35mph) out of the NE at upper AND MID elevations (9,000' and above).  Human triggered avalanches will be very likely on leeward terrain features where newly wind transported snow is deposited.  These will most likely be found just below ridges, leeward sides of ridgeline, and gully sidewalls that face NW-W-S-SE.  

Strong winds out of the SW and multiple feet of new snow over the past week have made large wind drifts in exposed areas at all elevations.  Yesterday (Wednesday) thru most of last night, all though lower mountain winds significantly decreased, winds at mid to upper elevations above 9,000’ were still ripping.  We will finally get a break from winds today and Friday, as winds are forecasted to be light with moderate gusts over ridgetops in the 30s shifting from out of the S to out of the NE Thursday night.  What this means is that there will be windslabs that will likely remain sensitive to human triggering on exposed leeward facing terrain features at all elevations, mostly likely to be found on slopes facing NW-N-NE-E-SE.  As time passes thru today and tomorrow, stability will increase, but just how fast is not know.  With the shift in wind direction, it is likely that new smaller windslabs will form at upper elevations where moderate gusts over ridgetops into to 30s out of the NE are expected.  These new windslabs will likely be very sensitive to human triggering, and will most likely be found just below ridgelines on slopes facing NW-W-SW-S-SE.  Be very cautious, and avoid exposed terrain steeper than 30 degrees where windslabs with varying levels of sensitivity exist until they have a little more time to stabilize.   

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.

Multiple feet of snow has fallen over the past week, with varying density changes, and periods of graupel.  Continued snowfall is expected Thursday, with up to another foot at upper elevations before days end especially for Mammoth and north.  Human triggered avalanches will remain possible on sheltered slopes on all aspects and elevations through Thursday, and will still likely be possible through Friday especially on slopes 35 degrees and steeper.  These slopes will likely stabilize quickly as snowfall diminishes.  It is best to avoid these slopes for the moment, and then do your own stability assessments / test pits, and keep an eye out for signs such as shooting cracks and whoomphing before deciding to commit to steeper terrain. 

Avalanche Character 3: Deep Slab
Deep Slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to Storm or Wind Slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

Early season storms In October and early December formed a combination of Melt/Freeze crusts and facets between 9,000 and 10,500', primarily found on NW thru N thru East aspects forming a Persistent Deep Weaknesses that has been found throughout the forecast region. The recent heavy snows have added significant stress to the snowpack. An avalanche failure in the upper snowpack could potentially trigger a step down failure deep in the snowpack triggering a potentially very large and destructive avalanche.  While this is unlikely, it is still a possiblity, especially in areas with shallower snowpacks south of Mammoth and North of June. It is worth doing your own investigation by digging deep and checking out if these weak layers exist, and how sensitive they are with your own stablity tests.

Snowpack Discussion

The New Year has brought us a return to some real old-fashioned Sierra winter weather, with some big storms and lots of snow!  It’s been precipitating since January 4th, with only a couple of small breaks.  Over 16” of WATER has fallen in the Mtns around Mammoth in January so far, bringing total snow depths reading well over 100”.  Lower elevations have finally gotten some coverage, even south past Bishop, making it look like approaches down to 6,000’ can finally be done again over snow!  Besides the high amount of snowfall associated with this past week’s Atmospheric River, the warm temperatures and high rain line brought avalanche dangers related to rain-on-snow in to the equation for a couple of days, as snowpacks became moist to the ground in many areas up to elevations at least up to 9,500’.  This was most clearly seen with the Tele-Bowls in Mammoth having a huge wet-slab release.  Cold temperatures will continue to re-freeze the underlying snowpack, and will likely end up making the lower snowpack between 8,000-10,200’+ stronger than it was before.

recent observations

Snowfall depths (1/12/17 5am):

Virginia Lakes (9,945’): 76” (7” new past 48 hrs)

Tioga Pass (9,943’): 166” (22” new past 48 hrs)

June Mountain (9,148’): 65” (10” new past 48hrs)

Gem Pass (10,750’): 126” (12” new past 48hrs)

Mammoth Pass (9,500’): 137” (13” new past 48hrs)

Rock Creek (9,600’): 74” (6” new past 48hrs)

Sawmill near Big Pine (10,200’):  69” (5” new past 48hrs)

Field Observations:

-Mammoth Mtn Ski Patrol avalanche control results (1-11)

-Mammoth Mtn Ski Patrol avalanche control results (1-10)

 

 

weather

*Winter Storm Warning extended thru 4am Friday Morning.

Thursday:  Moderate Snowfall to continue throughout the day, with 4-8” of light new snow expected, and up to a foot at higher elevations (less south of Mammoth).  Temperatures will be COLD with highs in the teens to single digits above 10,000’, typical of a Tonopah Low weather pattern.  Expect light winds, with moderate gusts up to 40mph over ridgetops. 

Friday:  After a few more inches of snow fall overnight, and light winds with moderate gusts in the 30s over ridgetops shifting out of the northeast, Friday will bring cloudy skies, a slight chance of snow with little new accumulation, slightly warmer temperatures reaching into the upper teens to mid 20s, and continued light winds out of the northeast with gusts in the 30mph range over ridges. 

Long-term:   Clear weather and high pressure is expected through the weekend and early next week, with warming temperatures into the mid 30s.  Winds look to start increasing again on Sunday.  Another weaker Atmospheric River event is forecasted to begin mid week, but it is too early yet to speculate on details of this event. 

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. CLOUDY. SNOW LIKELY. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING, THEN SLIGHT CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 16 TO 22 deg. F. 9 TO 14 deg. F. 21 TO 26 deg. F.
Wind direction: S N NE
Wind speed: LIGHT LIGHT WINDS 10 TO 15 MPH WITH GUSTS TO 25 MPH AFTER MIDNIGHT 10 TO 15 MPH WITH GUSTS TO 25 MPH
Expected snowfall: 4-8 in. 1-4 in. UP TO 1 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SNOW. CLOUDY. SNOW LIKELY IN THE EVENING, THEN SNOW AFTER MIDNIGHT. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING, THEN SLIGHT CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 9 TO 15 deg. F. 2 TO 10 deg. F. 16 TO 22 deg. F.
Wind direction: S SE SHIFTING TO NE NE
Wind speed: 15 TO 20 MPH WITH GUSTS TO 40 MPH 10 TO 15 MPH WITH GUSTS TO 35 MPH AFTER MIDNIGHT 15 TO 25 MPH WITH GUSTS TO 35 MPH
Expected snowfall: 5 TO 8 in. 1 TO 4 in. UP TO 1 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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