Eastern Sierra Snowpack Summary - 2016-01-30 07:15

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THIS ADVISORY EXPIRED ON February 1, 2016 @ 7:15 am
Avalanche Advisory published on January 30, 2016 @ 7:15 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.

Sensitive wind slabs will continue to form throughout the day today as snowfall and high winds out of the southwest and west continue in the 50-70mph at upper elevations, with gusts over 100mph over ridge-tops.  Natural avalanches will be likely and human triggered avalanches very likely in wind loaded terrain >35 degrees at mid to upper elevations on NW-N-NE-E-SE facing slopes.  Be on the lookout for these fresh dense wind slabs especially below ridgelines and cross-loaded slopes.  These wind slabs will be fairly obvious, denser, hollow sounding snow.  While exposed slopes will be of most obvious concern, high winds can also deposit snow into deeper soft wind slabs that could be sensitive to human triggering in open forested terrain.  These deposits in more protected terrain may only be slightly more dense than snow in un-wind affected areas.  Shooting cracks are an obvious warning sign that these softer wind deposits exist and may be unstable.  These avalanches will likely range from small slabs that could knock someone off balance, to carrying a person into undesirable terrain, to much larger avalanches that could bury, inujure and kill a person.

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.

Heavy dense new snow fall today will result in storm slab formation in wind protected areas on all aspects.  Mid to upper elevations will be of greater concern, where snowfall amounts will be greater.  As snowline drops throughout the day, this concern will increase to lower elevations as well.  Caution should be taken on all slopes greater than 35 degrees.  Be on the lookout for red flag indicators of instability including whoomphing, shoting cracks, and recent avalanche activity.          

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.

The concern over triggering failures in the weak layers deeper in the snowpack, most notably the basal facets, increases during periods of rapid new snow load.  While it is unlikely, there remains a possibility that a smaller avalanche in the upper new snow could produce enough force in a sweet spot that could result in a deeper much larger slope-wide failure.  This problem is more of a consideration in areas with a shallower snowpack (<1.5 meters) north and south of the mammoth area.  The warm moist weather along with increased winds likely destroyed all of the recent surface hoar formation that took place on Tuesday.  There has still been recent reports of finding isolated pockets of buried surface hoar 2-3' below the surface.  While no avalanche activity has been noted to have occured as a result of a failure in this weakness to date, it is still something to keep an eye for in your snowpits and to consider when making decisions about what slopes you travel on.  

Snowpack Discussion

The weekly pattern of a few days of winter storms followed by a few days of CA sunshine that we’ve been experiencing for the past month continues.  Heavy snowfall, both in terms of water content and intensity, began early this morning at the 9000’ level on Mammoth Mountain.  Snow level was barely above 8000’ at 4am, and should drop down to 5000’ as the day continues.  We could expect over a foot of snow at mid to upper elevations, with much more in wind deposited areas, before snowfall intensities tapper off this afternoon.    

Avalanche danger will increase quickly this morning as snowfall continues, snow levels drop, and high winds out of the west and southwest continue in the 50-70 mph range, with ridge top gusts exceeding 100 mph.  Natural avalanches will be likely and human triggered avalanches very likely especially in wind loaded terrain >35 degrees at mid to upper elevations on NW-N-NE-E-SE facing slopes.  Be on the lookout for fresh dense wind slabs especially below ridgelines, cross-loaded gullies and open slopes.  This dense snow will also form storm slabs in terrain protected from the wind on all aspects that will likely be sensitive to human triggering on slopes steeper than 35 degrees.  Resulting avalanches today and tomorrow could likely be large enough to bury, injure or kill a person.  Use good judgment in recognizing avalanche terrain and choosing safe travel routes. 

The concern over triggering failures in the weak layers deeper in the snowpack, most notably the basal facets, increases during periods of rapid new snow load.  While it is unlikely, there remains a possibility that a smaller avalanche in the upper new snow could produce enough force in a sweet spot that could result in a deeper much larger slope-wide failure.  This problem is more of a consideration in areas with a shallower snowpack (<1.5 meters) north and south of the mammoth area.  The warm moist weather along with increased winds likely destroyed all of the recent surface hoar formation that took place on Tuesday.

recent observations

Recent observations in Mammoth, June, Convict and McGee have shown an increasingly stable underlying snow pack, with the older windslabs that formed earlier this week bonding to the underlying snow.  

Observations made in the Sherwins in Mammoth Rock Bowl (1/29) revealed some wet snow instability in the form of surface rollerballs as a result of very warm temperatures and moist air mass.  Drizzle reached 9,300' in elevation, before turning to light snow.  Exrtended column tests did not lead to any propagating fractures.  

Observations made in Solitude Canyon (1/26) reveal the persistence of basal facets.  Extended column tests did not lead to porpagating fractures.

An observer on Earthquake Dome (1/26) did identify buried surface hoar 55cm down in a snowpit at 9000' elev. on a NNE aspect. 

 

weather

Another “atmospheric river” event (a zonal flow bringing heavy moisture and back to back storms from the pacific) has led to the start of precipitation in our region early this morning.  Very warm temperatures (snowline above 9000’ during yesterday morning’s light precipitation), will cool throughout the day today (Saturday) and tonight, as a cold front sags south through our area, bringing snowline to below 5000’ by evening.  Heaviest snowfall is expected this morning, with intensities tapering off through the afternoon.  Total snowfall today could reach a foot+ in upper elevations.  High winds out of the southwest and west in the 50-70mph range at upper elevations with gusts over 100mph over ridge tops are expected to continue through the morning, and are expected to gradually decrease in the afternoon and evening.

Light snowfall is expected overnight, with another wave of increased intensity beginning early Sunday morning and continuing into Sunday afternoon, potentially bringing another foot+ of snow to upper elevations.  Moderate southwest to west winds will continue.  Light snowfall is expected Sunday night, with a couple more inches of snow accumulation, before high pressure and clear weather move in Monday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY, SNOW CLOUDY, SNOW CLOUDY, SNOW SHOWERS
Temperatures: HIGHS IN THE LOW TO UPPER 30S, COOLING THROUGHOUT THE DAY deg. F. LOWS INTHE MID TEENS TO 20 deg. F. HIGHS IN THE LOW UPPER 20S deg. F.
Wind direction: WEST WEST WEST
Wind speed: 55 TO 60 MPH WITH GUSTS TO 90 MPH DECREASING TO 45 TO 50 MPH WITH GUSTS TO 75 MPH IN THE AFTERNOON 30 TO 35 MPH WITH GUSTS TO 50 MPH DECREASING TO 10 TO 15 MPH WITH GUSTS TO 25 MPH AFTER MIDNIGHT 25 TO 30 MPH WITH GUSTS TO 40 MPH DECREASING TO 15 TO 20 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON
Expected snowfall: 4-8 in. 1-4 in. 3-7 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY, SNOW CLOUDY, SNOW CLOUDY, SNOW SHOWERS
Temperatures: HIGHS IN THE UPPER 20S TO LOW 30S, COOLING THROUGHOUT THE DAY deg. F. LOWS IN MID SINGLE DIGITS TO MID TEENS deg. F. HIGHS IN THE MD TEENS TO LOW 20S deg. F.
Wind direction: WEST WEST SOUTHWEST
Wind speed: 70 TO 75 MPH WITH GUSTS TO 115 MPH DECREASING TO 60 TO 65 MPH WITH GUSTS TO 100 MPH IN THE AFTERNOON 45 TO 50 MPH WITH GUSTS TO 75 MPH DECREASING TO 25 TO 30 MPH WITH GUSTS TO 40 MPH AFTER MIDNIGHT 30 TO 35 MPH WITH GUSTS TO 55 MPH DECREASING TO 15 TO 20 MPH WITH GUSTS TO 30 MPH IN THE AFTERNOON
Expected snowfall: 4-8 in. 1-4 in. 4-7 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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