Eastern Sierra Snowpack Summary - 2016-01-14 06:11

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THIS ADVISORY EXPIRED ON January 16, 2016 @ 6:11 am
Avalanche Advisory published on January 14, 2016 @ 6:11 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
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.

Strong SW to W winds that began early Wednesday morning and will continue into Friday, have and will continue to form wind slabs on NW-N-NE-E-SE facing slopes below ridgelines and across slopes at all elevations.  Some of these slabs will very likely be sensitive to human triggering on steep slopes >35 degrees.  Likely trigger points include areas below ridgelines, slope convexities, and sidewalls of gullies.  Be on the lookout for denser, hollow sounding snow.  Resulting avalanches could possibly be large enough to bury a person, or carry a person through trees or over cliffs. 

Avalanche Character 2: 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.

A weak basal snowpack still exists throughout much of the forecast area due to shallow early season snow conditions and cold temperatures.  While it is unlikely for a human to trigger an avalanche at this level, the resulting slide would likely be large with bad consequences.  This layer is becoming less and less of a concern as the snowpack deepens.  However, it is still something to be aware of, especially where shallower snowpack exists outside of the Mammoth area.  Regularly reevaluate the local snow stability by digging snow pits and performing stability tests to help you make informed decisions.  This layer will likely become a greater concern right after or during a significant new snow load. 

Snowpack Discussion

The main avalanche concern for Thursday and Friday is freshly formed wind slabs from the strong Southwest to West winds that began blowing early Wednesday morning and are forecast to continue through Friday.  Ridge top gusts are expected over 100mph and winds at mid to lower elevations are expected to continue in the 30-60mph range.  Obvious slabs will be found just below ridgelines on NW-N-NE-E slopes, as well as mid slope and lower on a wide range of aspects due to swirling winds and cross loading.  These slabs will be most dangerous on more open slopes steeper than 35 degrees, on convexities, and sidewalls of gullies.  The strength of these winds could also form potentially dangerous slabs on steep slopes with widely spaced trees.  Up to 4” of new snow could fall tonight (Thursday), and 2” Friday, which will add more loose snow available to this wind transport / wind slab formation.  Currently early this morning (Thursday) winds are down, but don't let these short periods of calmer winds fool you into thinking there are not fresh wind slabs sensitive to human triggering.

The secondary avalanche concern is still the weak basal facets that are found throughout the region.  These are most concerning in areas of shallow snowpack where the weight of a skier or snowboarder (and even more likely the greater weight of a snowmobiler) could possibly trigger this deeper weak layer.  While a failure in these deep basal facets is unlikely at this point in time, it still remains a possibility, and the resulting avalanche would likely be large with severe consequences.  If and when a more powerful zonal flow impacts our area with heavy snow fall, such as the “atmospheric river” which is being talked about as a possibility for early next week, this layer will be of much greater concern.  

recent observations

High winds were observed and reported throughout the forecast area yesterday (Wednesday), with visible snow transport at all elevations, especially over ridges.  These winds were predominantly out of the Southwest.  Mammoth Mountain anemometers reported gusts up to 95mph over 10,000’ consistently out of the SW.  Mammoth Mountain Ski Patrol is planning on doing avalanche control work early this morning (Thursday) as a result of these winds.  June and Mammoth upper mountain lifts were closed all day yesterday due to high winds. 

Observations were made behind June Mountain on Hemlock Ridge yesterday (Wednesday).  Snow was being transported even in the widely spaced trees.  Isolated denser wind slabs were forming in more exposed areas on NE to E facing slopes, which failed easily during hand shear tests 4-8” deep.  Ski cuts on steeper protected slopes did not produce results, but given more time to build, even softer less dense slabs could become reactive.  A pit showed a thick layer of basal facets, which reacted to Compression tests, but not Extended Column tests.  The upper 10-15cm dense wind slab failed and propagated consistently in ECT tests upon isolation or a very light tap.  This is a heads-up result; think hard before entering committing steep terrain where wind slabs might exist.      

Observations made on Punta Bardini on Tuesday included a 1.4m deep pit dug near a shallowly buried rock outcrop.  Extended Column tests failed and propagated consistently on the basal facets at this particular location (which is representative of a possible trigger point, but may not be representative of the greater slope).  

weather

The series of storms will continue through the weekend, with oscillating periods of very windy conditions and scattered light snowfall from now through Friday afternoon.  Up to 4” of snow is possible Thursday night, and up to 2” Friday.  Winds will be predominately out of the West and Southwest, blowing in the 30-60mph range, with gusts over 1000mph over high ridges and peaks.  High temperatures are expected in the upper 20s to mid 30s, with lows at night in the mid teens to mid 20s. 

A Brief break Friday afternoon is expected, before another weak system comes in Saturday with more wind and light snowfall.  There is talk of an “atmospheric river” setting up, a zonal flow bringing back to back storms with heavy moisture from the pacific, which could potentially hit our area Sunday night through Tuesday.  As of now, however, models are currently showing this zonal flow favored to be centered on Northern California and Oregon, but this may shift south… we hope!

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: MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW IN THE AFTERNOON CLOUDY. SNOW LIKELY CLOUDY THEN BECOMING PARTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING
Temperatures: HIGHS IN THE MID TO UPPER 30S deg. F. LOWS IN THE UPER TEENS TO MID 20S deg. F. HIGHS IN THE MID TO UPPER 30S deg. F.
Wind direction: WEST WEST WEST
Wind speed: 30 TO 35 MPH WITH GUSTS TO 50 MPH BECOMING SOUTHWEST AND INCREASING TO 35 TO 45 MPH WITH GUSTS TO 75 MPH IN THE AFTERNOON 40 TO 50 MPH WITH GUSTS TO 85 MPH WEST 45 TO 50 MPH WITH GUSTS TO 75 MPH DECREASING TO 35 TO 50MPH WITH GUSTS TO 60 MPH IN THE AFTERNOON
Expected snowfall: 0 in. UP TO 4 in. UP TO 2 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW IN THE AFTERNOON CLOUDY. SNOW LIKELY IN THE EVENING. SNOW AFTER MIDNIGHT CLOUDY THEN BECOMING PARTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE MORNING
Temperatures: HIGHS IN UPPER 20S TO LOW 30S deg. F. LOWS IN THE MID TEENS TO LOW 20S deg. F. HIGHS IN THE UPPER 20S TO LOW 30S deg. F.
Wind direction: WEST WEST WEST
Wind speed: 35 TO 40 MPH WITH GUSTS TO 60 MPH INCREASING TO 55 TO 60 MPH WITH GUSTS TO 85 MPH IN THE AFTERNOON 65 TO 75 MPH WITH GUSTS TO 115 MPH 50 TO 55 MPH WITH GUSTS TO 85 MPH DECREASING TO 40 TO 45 MPH WITH GUSTS TO 70 MPH IN THE \AFTERNOON
Expected snowfall: 0 in. UP TO 4 in. UP TO 2 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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