Avalanche in Echo north bowl

Location Name: 
Bowl north of Echo Peak
Region: 
Desolation Wilderness Area (including Emerald Bay)
Date and time of avalanche (best estimate if unknown): 
Thu, 04/13/2017 - 16:00
Location Map: 
United States
38° 51' 27.144" N, 120° 4' 27.4224" W
See map: Google Maps
US


Red Flags: 
Recent loading by new snow, wind, or rain
Rapid warming
Obvious avalanche path
Observation made by: Public
Avalanche Observations
Avalanche Type: 
Slab
Slope: 
40degrees
Crown Height: 
1 ft
Aspect: 
East
Weak Layer: 
Other
Avalanche Width: 
250ft.
Terrain: 
Above Treeline
Elevation: 
8 720ft.
Bed Surface: 
Other - explain below
Avalanche Length: 
320ft.
More detailed information about the avalanche: 

About noon on Friday (4/14), from an elevation of about 8600 ft. on the ridge SSW of the bowl,  we observed avalanche debris from a trigger point just below the top of the bowl north of Echo Peak.  A crown was visible across most of the bowl, and the debris had run down to somewhere around the 8400 ft. elevation (the 320 ft avalanche length estimate above is of vertical feet; the running distance was longer).  The largest debris blocks were in the refrigerator class.  The debris path was more or less divided in two sections.  From the distance it was not possible to tell the exact height of the crown, which varied; the highest section was directly below large rocks protruding through the snow skier's right of the center of the bowl and appeared to be more than 1 ft. high; skier's left the crown was just below the base of the rock face/cliff on that side of the bowl and appeared to be less than 1 ft.  At its highest point, the crown was directly below and close to the center of the cornice which stretched all the way across the top of the bowl, which suggested, along with the large size of some of the debris, that a cornice fall caused the avalanche--but there was no (from our perspective) major gap in the cornice, and some of the large debris was in in the more northerly branch of the avalanche path where it was not directly below the cornice, so it's possible that substantial blocks of snow released from the rocks or that the crown was higher than it appeared to us.

New snow was not visible on top of the debris, and on our ascent we didn't hear or notice any sign of the avalanche in motion on our ascent, so it seems most likely that the avalanche was Friday morning before 11, or maybe Thursday afternoon (after the forecaster's observations that day), with subsequent snowfall not noticeable from our viewpoint.

We didn't descend to the avalanche path, but there was a definite bed surface.  With the likely redistribution of snow in the area, it's hard to say, but the bed could have been either a rain layer (detectable on our ascent) or the surface of snow from the last previous storm.

We'll try to post some pictures tomorrow. 

Weather Observations
More detailed information about the weather: 

Since we don't know when the avalanche occurred, we don't know what the weather was.