Eastern Sierra Snowpack Summary - 2015-12-24 07:51

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THIS ADVISORY EXPIRED ON December 26, 2015 @ 7:51 am
Avalanche Advisory published on December 24, 2015 @ 7:51 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: 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.

Low-density snow, wind, and colder temps will likely produce fresh storm slabs in the mid to upper elevations. The cold temps will likely slow the bonding process and extend the time needed to strengthen and form bonds to adjacent layers. Approach steep slopes, convex rolls, drifted slopes with caution. Be aware of potential terrain traps both ascending and descending. 

Avalanche Character 2: 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.

The anticipated low-density snow will easily be transported by the forecasted strong winds creating extensive windloading on N-E-S aspects, especially the mid to upper elevation. The cold temperatures will likely produce brittle windslabs that may be easily triggered. Watch for red flags, which may include shooting cracks, hollow sounding wind slabs, smooth rounded drifts, dense snow over soft snow below, snow plumes, to name a few.

 

Snowpack Discussion

The most recent system has moved off to the east and in its wake has left behind supportive (mostly) wind buff in the more exposed areas to dense powder in sheltered terrain.  Many stations reported receiving nearly 2 feet of dense new snow over two days with Mammoth Mountain reporting 22”. Mammoth ski patrol reported extensive avalanche activity from avalanche control missions over the past couple of days with several stepping down into the lower faceted snowpack just above ground.  Explosive control efforts triggered many avalanches that ran to the ground in a depth hoar layer.  These include an 8ft+ crown in Climax, and an avalanche mid-slope in "throat" of Avy 3 on Lincoln Mtn. with a 3' crown.  These slopes had been open to public skiing prior to this storm and were extensively skier compacted.  

Tests and observations from Mammoth Crest (12/23/15) indicate that the recent new snow has begun to settle and bond to adjacent layers. The potential for natural avalanche has subsided but triggered releases are possible in the mid to upper elevations where fresh windslabs have been deposited in thin areas, which can fail and possibly propagate deeper into the snowpack on adjacent slopes, especially on terrain 35 degrees or steeper. A persistent weak layer, which formed December 10th, consisting of a 1cm (+/-) weak melt-freeze crust on top of well developed facets directly underneath is still observable but has become less reactive as it recedes deeper into the snowpack. However, it could become more sensitive with this next system as additional load and stress is applied. The next system due to move in Christmas Eve is forecasted to deposit another 10 to 18” of snow and usher in colder temperatures with strong West to Southwest winds. This combination will likely produce extensive wind loading with brittle wind and storm slabs on leeward slopes (N-E-S aspects), which may be easily triggered.

 

recent observations

Snow stability tests from Red Cone Bowl (aspect North, Elevation 10,400, slope 36 degrees) indicate that overall the new snow has bonded to adjacent layers. Test Results: CTH21 @155cm, Q1 on 2-3mm stellars, CTH22@155cm, Q2 on 2-3mm stellars, CTH23 @132cm, Q2, ECTN 27 @180cm, ECTN 28 @ 5cm. Ski cuts on test slopes produced very little action and supports the stability test findings. Further tests on descent produced weaker results but little propensity for propagation (aspect East, elevation 10,000, slope 25 degrees) Results: ECTP 22 @ 22cm from surface, ECTP 26 @ 51 from surface. The melt-freeze layer that was the old snow surface prior to the Dec 11-14 storms was easily identified on this east facing slope, as well as the weak faceted layer just under it. A propagation saw test on this weak layer resulted in PST 48/100(End). A score of less than 50/100 going to the end of the column indicates that propagation of this layer is likely if it is triggered.  

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 10 deg. F.
Max. temperature in the last 24 hours: 21 deg. F.
Average wind direction during the last 24 hours: E -NE
Average wind speed during the last 24 hours: 5-15 mph
Maximum wind gust in the last 24 hours: 23 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 47 inches
weather

Winter storm warning is in effect from 10 am Thursday to 4 am PST Friday.

- Timing: snow will begin Thursday morning and increase through Thursday afternoon and evening. Peak travel impacts due to snow are expected from about 10 am to 10 pm Thursday.

 - Snow accumulations: 3 to 6 inches along highway 395 with 10 to 18 inches in the high Sierra west of highway 395 including Mammoth Mountain, 1 to 4 inches east of highway 395, with an inch or less along highway 6. Heavy snow showers will result in amounts varying considerably over short distances.

- Winds: Southwest 15 to 25 mph with gusts to 45 mph with gusts of over 100 mph along Sierra ridges.

- Impacts: snow and blowing snow due to winds will create periods of near white out conditions along with icy roadways. Travel is likely to be impacted with delays and chain controls likely. Watch for rapid changes in visibility and road condition. The possibility of avalanches will increase especially in the backcountry.

Details: An approaching winter storm will move through the region Christmas Eve and bring much colder temperatures. A combination of a cold unstable airmass with supporting shortwave energy aloft will help create high-ratio (low-density) mountain snows and heavy snow showers in the valleys Thursday and into early Friday. The favored models, the North American Mesoscale  (NAM) and Global Forecast System (GFS), are fairly consistent with snowfall amounts for the Sierras, with some differences. NAM shows a pronounced decrease in precipitation as you move east of the Sierra Front, while the GFS more readily spills precipitation over the Crest. GFS is the preferred model for this cycle which forecasts heavy convective snow showers outside the Sierra and possibly significant variations in snow amounts over short distances with storm totals of 10 to 18”. Strong Westerly to Southwest winds with speeds of 25 to 35 with gusts of 40 to 50 are forecast with blizzard conditions likely over the passes. Temperatures Friday into Saturday will be cold with lows near zero to -10F in the Sierra valleys.

Long-term:  an upper level ridge will bring some warming to mountain tops Sat-Sat night while valleys will remain very cold. Overnight lows will be coldest Sunday morning with many locations dropping to single digits above and below zero. Valley inversions are going to remain strong for much of the period with the only chance of brief mixing Monday as first shortwave feature passes. Temperatures will be slow to warm early next week.

The first half of next week will see a couple of weak shortwaves working through the region, Monday looks to be the strongest. First shortwave will approach late Sunday and move across the area Sunday night into early Monday. The flow begins to split as this shortwave moves inland, limiting the precipitation potential. Confidence is low in how much snow may fall in the Sierra, leaning toward just a few inches. Another weaker shortwave is possible Tue-Tue night but confidence remains low for this second system.

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.SCATTERED SNOW SHOWERS IN THE MORNING...THEN SNOW SHOWERS IN THE AFTERNOON. CLOUDY. SNOW SHOWERS IN THE EVENING...THEN SCATTERED SNOW SHOWERS AFTER MIDNIGHT. MOSTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING...THEN SLIGHT CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 17 TO 23 deg. F. 3 TO 9 deg. F. 12 TO 18 deg. F.
Wind direction: SOUTHWEST WEST NORTHWEST
Wind speed: 25 TO 35 MPH WITH GUSTS TO 50 MPH. 30 TO 35 MPH WITH GUSTS TO 50 MPH DECREASING TO 15 TO 25 MPH WITH GUSTS TO 35 MPH AFTER MIDNIGHT. 15 TO 25 MPH WITH GUSTS TO 35 MPH.
Expected snowfall: 6 TO 10 INCHES in. 4 TO 8 INCHES in. NO ACCUMULATION in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SNOW SHOWERS THROUGH THE DAY. CLOUDY. SNOW SHOWERS IN THE EVENING...THEN SCATTERED SNOW SHOWERS AFTER MIDNIGHT. MOSTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING...THEN SLIGHT CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 11 TO 17 deg. F. 2 BELOW TO 4 ABOVE ZERO deg. F. 2 TO 8 deg. F.
Wind direction: WEST WEST NORTHWEST
Wind speed: 55 TO 65 MPH WITH GUSTS TO 95 MPH. 60 TO 65 MPH WITH GUSTS TO 95 MPH DECREASING TO 45 TO 50 MPH WITH GUSTS TO 60 MPH AFTER MIDNIGHT. 40 TO 45 MPH WITH GUSTS TO 65 MPH DECREASING TO 30 TO 35 MPH WITH GUSTS TO 55 MPH IN THE AFTERNOON.
Expected snowfall: 6 TO 10 INCHES in. 4 TO 8 INCHES in. NO ACCUMULATION in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. 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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