Eastern Sierra Snowpack Summary - 2016-03-10 07:11

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THIS ADVISORY EXPIRED ON March 12, 2016 @ 7:11 am
Avalanche Advisory published on March 10, 2016 @ 7:11 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 winds are once again blowing.  Moderate to strong winds from many different directions have continued for most of the week, with brief periods of calm interspersed.  This wind-transported snow has created a mosaic of wind slabs on exposed slopes facing all directions.  Windslabs are most sensitive as they form, and gradually bond and strengthen as they are given time to sit.  However, with a period of continuing strong winds like we’ve had, it is hard to tell which windslabs are old and more stable and which are newer and more sensitive just by looking.  It is best to assume any dense, smooth, hollow sounding snow that you come across may be relatively freshly formed.  Do your own localized stability assessments to determine for yourself how sensitive these slabs are, and recognize that this sensitivity could vary greatly even over relatively small distances across one slope.  Areas of greatest concern are slopes >35 degrees that are just below ridges, sidewalls of gullies and cross-loaded slopes that face mostly NW-N-NE-E-SE.  Natural windslab avalanches may be possible, and human triggered avalanches likely on isolated terrain features.  While these avalanches are not likely to be very large, they could still be large enough to bury a person especially given a terrain trap, and they can most certainly be large enough to carry a person violently through trees or rocks. 

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.

In exposed, high elevation terrain (>11,500') that faces N to NE some areas of very faceted shallow snowpacks have been found to exist prior to this past weekend's storm.  It is likely that some of these weak areas avalanched naturally during this past storm, and it is also likely that some still exist that are now covered with windslab.  This underlying weak faceted layer wont stabilize quickly.  While a stiff thick overlying windslab may bridge over this weak layer effectively at many some points, it is very possible that a human could trigger a failure where the slab is thinner.  It is possilbe that such a failure would propagate above and result in a dangerous avalanche that a human would have a hard time getting out of.  Be aware of this potential problem if you find yourself in this kind of high elevation terrain.           

Snowpack Discussion

Strong southwest winds are once again howling through the mountains after a short relatively calm period yesterday afternoon.  These strong winds in the 60+mph range, with gusts over ridge tops near 100mph, are expected to continue through today into tomorrow when they will shift to more directly out of the south.  These winds will keep the avalanche concern heightened for sensitive fresh new windslabs on exposed slopes >35 degrees that face mostly NW-N-NE-E –SE.  

The relatively warm temperatures combined with adequate time has given the storm slabs and windslabs that formed as a result of the 2-3’ of new snow and strong winds that occurred this past weekend a chance to settle.  A widespread natural avalanche cycle took place throughout the region during the storm, which is no longer a concern.  However, moderate to strong winds from many different directions have continued for most of the week since, with brief periods of calm interspersed.  This wind-transported snow has created a mosaic of wind slabs on exposed slopes facing all directions.  Windslabs are most sensitive as they form, and gradually bond and strengthen as they are given time to sit.  However, with a period of continuing strong winds like we’ve had, it is hard to tell which windslabs are old and more stable and which are newer and more sensitive just by looking.  It is best to assume any dense, smooth, hollow sounding snow that you come across may be relatively freshly formed.  Do your own localized stability assessments to determine for yourself how sensitive these slabs are, and recognize that this sensitivity could vary greatly even over relatively small distances across one slope.  Areas of greatest concern are slopes >35 degrees that are just below ridges, sidewalls of gullies and cross-loaded slopes that face mostly NW-N-NE-E-SE.  Natural windslab avalanches may be possible, and human triggered avalanches likely on isolated terrain features.  While these avalanches are not likely to be very large, they could still be large enough to bury a person especially given a terrain trap, and they can most certainly be large enough to carry a person violently through trees or rocks. 

recent observations

Recent observations around the mammoth area have shown that the storm snow from this past weekend’s storms has settled and bonded fairly well. 

Parties have reported periods of moderate to strong winds transporting snow throughout the region throughout this week. 

A party on Carson Peak on Tuesday encountered many fresh sensitive windslabs, one of which released above a skier while he was skiing a committing slope and resulted in carrying him violently downslope, smashing him into a rock wall, breaking his ankle, and nearly over potentially deadly cliffs.  Click here for full report. 

weather

As an upper level jet slowly dips south over the Sierra, winds will be on the increase once again.  This has already begun to bring strong southwest winds into the 60mph range, with gusts expected near 100mph over the higher ridge tops.  These winds are expected to shift to more directly out of the south by Friday, and decrease slightly by Friday afternoon. 

Today expect mostly cloudy skies and high temperatures in the 40s at mid-mountain elevations.  Tomorrow (Friday) expect slightly cooler temperatures, cloudy skies, and increasing chances of precipitation as the day progresses, with up to 2” of new snow possible by the end of the day.

Models are showing another atmospheric river event is setting up to impact our area by the end of the weekend, with colder temperatures and best chances of significant snowfall Sunday night into Monday morning, with moisture amounts looking to be in the 1-2” of water range at the moment.                

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 MOSTLY CLOUDY. ISOLATED SHOWERS IN THE EVENING. ISOLATED SNOW SHOWERS AFTER MIDNIGHT. MOSTLY CLOUDY THEN BECOMING CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING...THEN CHANCE OF SHOWERS IN THE AFTERNOON.
Temperatures: HIGHS IN THE MID 40S TO MID 50S deg. F. LOWS IN THE MID 20S TO MID 30S deg. F. HIGHS IN THE LOW TO UPPER 40S deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTH
Wind speed: 45 TO 55 MPH WITH GUSTS TO 75 MPH 40 TO 45 MPH WITH GUSTS TO 65 MPH INCREASING TO 50 TO 55 MPH WITH GUSTS TO 85 MPH AFTER MIDNIGHT 55 TO 60 MPH DECREASING TO 45 TO 50 MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. UP TO 2 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY CLOUDY MOSTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE EVENING MOSTLY CLOUDY THEN BECOMING CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING...THEN SNOW SHOWERS IN THE AFTERNOON
Temperatures: HIGHS IN THE UPPR 30S TO UPPER 40S deg. F. LOWS IN THE LOW TOPPER 20S deg. F. HIGHS IN THE MID 30S TO LOW 40S deg. F.
Wind direction: SOUTHWEST SOUTHWEST WEST
Wind speed: 60 TO 70 MPH WITH GUSTS TO 95 MPH 55 TO 60 MPH WITH GUSTS TO 85 MPH INCREASING TO 65 TO 70 MPH WITH GUSTS TO 100 MPH AFTER MIDNIGHT 65 TO 70 MPH WITH GUSTS TO 100 MPH DECREASING TO 55 TO 60 MPH WITH GUSTS TO 85 MPH IN THE AFTERNOON
Expected snowfall: 0 in. TRACE 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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