THIS SNOWPACK SUMMARY EXPIRED ON January 31, 2021 @ 9:41 am
Snowpack Summary published on January 29, 2021 @ 9:41 am
Issued by Jason Mozol - Bridgeport Avalanche Center

bottom line:

Dangerous avalanche conditions will persist even as snowfall tapers off.  4-7+ feet of new snow and winds gusting to 70 mph during the storm have created a very unstable snowpack at all elevations and aspects. Expect wind slabs, storm slabs, and widespread areas of wind drifted snow that will be very sensitive to human triggering. The strength of buried weak layers is being tested and may result in very large avalanches if triggered. Travel in or below avalanche terrain is not recommended today. 


The BWRA is OPEN to snowmobiles! US 395 remains closed this morning and travel by any means will be challenging today. If you head out, use good assessment skills and give yourself a wide safety margin as human triggered avalanches remain very likely today. Meadows and low angle terrain will have plenty of powder and recreating there greatly reduces risks to avalanche problems. Please wait until roads open before travelling to trailheads. Be sure to have a partner and be on the same page about which terrain to avoid.


Avalanche Character 1: 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.

4-7+ feet of new snow has fallen in the last 48+ hours. The new snow fell on a few inches of low density snow over variable crust layers. This new/old interface and temperature variations during the storm have left weakness within the layer of new snow. Avalanches may occur within this layer or include the entire layer. Sloughing will occur while descending any steep slope and could trigger a slab avalanche. The potential for storm slabs will be widespread, both June Mountain and Mammoth Mountian triggered mdium to large avalanches in the storm snow during control routes this morning ,use extreme caution and good terrain assessment. Avoiding avalanche terrain is the best way to steer clear of storm slabs today.


Avalanche Character 2: 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 winds came in from the south with gusts up to 70mph as the storm began. This has resulted in wind slabs on nearly all aspects, especially leeward terrain. Be particularly alert in near and above treeline terrain. These wind slabs will be large to very large and easy to trigger. Use good terrain assessment today to avoid traveling  in, below or adjacent to avalanche terrain.

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 strength of buried depth hoar at the ground on W-NW-N-NE-E-SE aspects is now being tested with new loading. Failure of this layer will result in large to very large avalanches that include practically  the entire season’s snowpack. We updated the persistent slab problem to a deep persistent slab problem today. These weak layers are now deeply buried in the snowpack and may not provide any signs of their instability. Just because you don’t see cracking or collapsing doesn’t mean that you can’t trigger one of these very large avalanches. Triggering of a wind or storm slab may allow for avalanches to step down to this layer. Deep persistent slabs may also be remotely triggered from areas adjacent or below avalanche terrain.  These avalanches could run far into low angle terrain you normally consider safe. Traveling in or below avalanche terrain is not recommended today.

Snowpack Discussion

We have received 4-7+ feet of snow in the past three days. Intense loading, up to 5” of snowfall per hour and winds gusting up to 70 mph, are strong indicators that dangerous avalanche conditions exist in the mountains. As the storm finally gives way, wind slab, storm slab and deep persistent slabs are all possible to nearly certain today. Due to inaccessibility and a lack of observations the last few days, there is significant uncertainty as to exactly where and how large avalanches will be. Avalanche control results at June Mountain and Mammoth Mountain show many medium to large avalanches have been triggered in the storm snow. 


We do not know how sensitive the deeply buried weak snow at the bottom of the snowpack is; but we do know that if you trigger one of these deep persistent slab avalanches it will be very large and destructive. The only way to mitigate deep slab instability is to avoid convex rolls and stick to lower angle slopes less than 30 degrees that don’t have steep terrain above. 


Give yourself plenty of time to travel today and be prepared for a wide safety margin as we assess how the snowpack is handling the new load. Meadows and low angle terrain will have plenty of powder and recreating there will greatly reduce risks to avalanche problems.  


Please wait until roads are open before travelling to trailheads. Be prepared for difficult travel by sled and ski. Deep, unsupportive snow will make getting stuck on a sled easy and breaking trail by ski exhausting. 


Many roof avalanches were observed yesterday. Most were big enough to cause injury or bury a person, certainly a child or pet. Even when shoveling, keep an eye on overhead hazards. In the backcountry, cornices present an increased hazard. Many are new or have grown in size in recent days. Cornice stability is unreliable and can break behind the ridges they extend from. Give cornices a large berth.

recent observations

Tuesday night an avalanche in Walker Canyon pushed a Caltrans plow into the Walker River. 


Observations were limited yesterday due to visibility and travel conditions but observers noted cracking and collapsing in the new snow and some storm and wind slab avalanches. Trail breaking is very difficult in the deep, low density new snow. 


Roof avalanches were observed throughout the region. Most were big enough to cause injury or bury a person, certainly a child or pet.


4-7+ feet of snow and 4 to 9” of SWE has accumulated from the storm thus far, snowfall and winds finally settled today. 


Winds have been blowing in the strong to extreme category out of the south during this storm, gusting up to 70mph. 


Before this storm 3-5 inches of low density snow rested on a highly variable old snow surface with stout crusts on southerlies and a mixed bag of breakable crusts, near surface facets, and wind board on northerlies


Depth hoar exists at the ground on W-NW-N-NE-E-SE aspects.

CURRENT CONDITIONS Weather observations from Sonora Pass
0600 temperature: 18 deg. F.
Max. temperature in the last 24 hours: 24 deg. F.
Average wind direction during the last 24 hours: SW-S
Average wind speed during the last 24 hours: 5-20 mph
Maximum wind gust in the last 24 hours: 49 mph
New snowfall in the last 24 hours: 15+ inches
Total snow depth: 98 inches

Snow will continue this morning with decreasing intensity. Winds remain light 15mph from the S to W, gusting up to 25mph. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Friday Friday Night Saturday
Weather: Mostly cloudy, chance of snow Cloudy becoming partly cloudy Sunny
Temperatures: 28 deg. F. 7 deg. F. 33 deg. F.
Wind direction: W SW
Wind speed: 15mph, gusts to 25mph 15mph, gusts to 25mph Light
Expected snowfall: Up to 3" in. 0 in. 0 in.
For above 10000 ft.
  Friday Friday Night Saturday
Weather: Cloudy, chance of snow Partly cloudy Partly cloudy becoming sunny
Temperatures: 20 deg. F. 3 deg. F. 27 deg. F.
Wind direction: SW SW W
Wind speed: 15mph, gusts to 25 mph 15mph, gusts to 25 mph 15mph, gusts to 25 mph
Expected snowfall: Up to 4" in. 0 in. 0 in.

This snowpack summary applies only to backcountry areas in the Bridgeport Winter Recreation Area. Click here for a map of the area. This snowpack summary describes general avalanche conditions and local variations always occur. This snowpack summary expires in 48 hours unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.