Analysis of the Feb 20 Avalanche Above Lower Leavitt Lake Road

Location Name: 
Lower Leavitt Lake Road Slide Path
Region: 
BWRA
Date and time of observation: 
Fri, 02/24/2017 - 13:00
Location Map: 


Red Flags: 
Recent loading by new snow, wind, or rain
Obvious avalanche path

Observation made by: Avalanche Specialist
Snowpit Observations
More detailed information about the snowpack: 

As previously documented by Ryan, a large and destructive avalanche released sometime on President's Day (Monday Feb 20th, 2017) during the middle of the storm.  This avalanche ran all the way down across Leavitt Lake Road, into the creek, and up the other side of the ravine, taking out a stand of mature trees in the process.  It was reported that there were two snowmobilers traveling along the road on Monday with others in the area.  Luckily, no one was in the path when the slide released.  Since Monday, the crownline and debris pile have mostly been covered up by subsequent snowfall and strong winds, but some debris can still be seen.  The likely culprit of this slide was a mid storm density change that put wet heavy snow on top of lighter snow, combined with wind loading from hurricane force winds.  3-4' of new snow fell during the storm on Monday/Tuesday and likely overloaded the entire starting zone above the road.

On Friday, I dug a pit just NE of the slide path, on an east facing 35 degree slope at about 9100'.  I found several density changes in the storm snow with wind slabs on top of softer still decomposing new snow.  These layers were somewhat reactive in my stability test and I was able to get them to propagate with some force.  This shows there is some lingering instability in the storm snow and may be reactive under the weight of a snowmobiler or additional wind loading.  While on the slope yesterday, strong SW winds were actively loading the same avalanche path and forming some cornices on convex rollovers.  At the same time, two snowmobilers were high marking the slope, just below the cornice.  This was a bad place to be riding and the clues were very obvious (active loading, previous slide, repeat offender).  This slope has avalanched naturally several times this season under the same conditions (i.e., wind loading on a clear day).  Luckily, the slope did not release but It is best to remain off these wind loaded areas until the winds calm down and the wind slabs have had a chance to strengthen.

Snowpack Photo Captions

1.  Looking up the slide path to the start zone

2. Looking down across the creek where the debris ran up the side of the ravine

3.  Two tracks under an active wind loading zone, same path where the President's Day avalanche ran

Snowpit or crown profile photo or graph: 
Snowpack photos: 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
25% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Strong
Precipitation: 
None
Air temperature trend: 
Warming
Wind Direction: 
Southwest
More detailed information about the weather: 

Strong ridgetop SW winds loading NE/E facing slopes with visible snow transport