THIS SNOWPACK SUMMARY EXPIRED ON January 29, 2021 @ 8:57 am
Snowpack Summary published on January 28, 2021 @ 8:57 am
Issued by Jason Mozol - Bridgeport Avalanche Center

bottom line:

Very dangerous avalanche conditions exist today due to a powerful snowstorm impacting the region. Natural and human triggered avalanches are certain and will be widespread. These avalanches will be very large and could run far into low angle terrain you normally consider safe. Avoid travelling in, near, or below all avalanche terrain today.

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.

Moderate to strong southerly winds will continue to deposit this storm’s unrelenting snowfall (3-6 feet) into deep wind slabs on almost all aspects and elevations, especially leeward terrain. These avalanches will be touchy and prone to occur naturally. They could be several feet deep and run into low angle slopes you normally consider safe. Avoid all avalanche terrain today where avalanches are certain and will be very large and destructive.

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.

3 to 6 feet of new snow has fallen in the last 30 hours and the heavy snowfall is continuing into tonight. These impressive accumulations have created deep storm slabs on all aspects and elevations. Both natural and human triggered avalanches are certain. These avalanches will be very large today. Again, even lower angled slopes that are exposed to steeper slopes above are not safe today.

Avalanche Character 3: 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.

We have received a remarkable amount of new snow (3-6 feet, 3-7” Snow Water Equivalent) over the last 30 hours. This rapid snowfall is adding a significant load to the weak faceted snow that exists at the bottom of our snowpack on northerly slopes. This is a big red flag that the weak layers may be getting overloaded and could produce very large persistent slab avalanches. There is a lot of uncertainty surrounding these persistent slab avalanches. It is hard to predict how much snow will be required to reactivate them, but we have reached a threshold of snow where these avalanches have become likely. Today, you need to be aware that if persistent slab avalanches occur they will be very large and destructive. They could propagate widely and be remotely triggered from below or adjacent to the slope. They could also be triggered by a smaller avalanche that steps down. They will likely run far into low angle terrain you normally consider safe. It is not worth placing yourself in the potential path of one of these monsters; avoid all avalanche terrain today.

Snowpack Discussion

We have received 3 to 6 feet of new snow in the mountains from this storm already and it is forecasted to continue heavily through tomorrow morning. This is a major red flag for significant dangerous avalanche activity. We must continue to take a step back today. We are all excited to finally receive a big snowstorm, but we need to acknowledge that it has brought very dangerous avalanche conditions in the backcountry. We can’t let our powder-starved excitement get in the way of good decision making. 

Last night an avalanche in Walker Canyon pushed a Caltrans plow into the Walker River. This storm has produced exceptional conditions; expect exceptional avalanches in places that typically do not avalanche. Widespread avalanches are certain today and they could easily be large and destructive enough to run far into low angle slopes. Avoid travelling in, near, or below any and all avalanche terrain today. Avalanche terrain means any slope steeper than 30 degrees including low angle slopes below. Today avalanche terrain even includes steep roofs where enough snow could slide off to bury you, especially young ones and pets. Be aware of these hazards today even when out shoveling or playing in the snow. Today would be a great day to catch up on some movie-watching; there will be plenty of time to get out once these dangerous avalanche conditions subside.

recent observations

Last 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. 

3-6 feet of snow and 3 to 7” SWE has accumulated from the storm thus far and the heavy snowfall and strong to extreme winds will continue today. 

Winds have been blowing in the strong to extreme category out of the south during this storm. 

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-N-E aspects. 

CURRENT CONDITIONS Weather observations from Sonora Pass
0600 temperature: 21 deg. F.
Max. temperature in the last 24 hours: 24 deg. F.
Average wind direction during the last 24 hours: S
Average wind speed during the last 24 hours: 15-30 mph
Maximum wind gust in the last 24 hours: 68 mph
New snowfall in the last 24 hours: 24-48 inches
Total snow depth: 83 inches
weather

Blizzard warning until 4 AM Friday. A powerful atmospheric river event will continue to bring us heavy snowfall and strong to extreme winds today. The intensity of the storm will lessen through the night and taper off tomorrow morning. Snowfall has already been measured in feet and we can expect an additional 1.5 to 3 feet today and tonight. Storm total accumulations will likely surpass 6 feet in the mountains.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Thursday Thursday Night Friday
Weather: Cloudy. Snow. Cloudy. Snow in the evening, then chance of snow after midnight. Mostly cloudy. Chance of snow.
Temperatures: 24 to 30 deg. F. 11 to 16 deg. F. 21 to 27 deg. F.
Wind direction: South South Southwest
Wind speed: 15 to 30 mph with gusts to 50 mph. Around 15 mph with gusts to 30 mph. Light winds becoming southwest around 15 mph in the afternoon. Gusts up to 25 mph.
Expected snowfall: 70% probability of 13 to 20 inches. 30% probability of 20 to 25 inches. in. 70% probability of 6 to 12 inches. 30% probability of 3 to 6 inches. in. 80% probability up to 2 inches. 20% probability no accumulation. in.
For above 10000 ft.
  Thursday Thursday Night Friday
Weather: Cloudy. Snow. Cloudy. Snow in the evening, then snow likely after midnight. Mostly cloudy. Snow likely.
Temperatures: 17 to 23 deg. F. 6 to 11 deg. F. 13 to 19 deg. F.
Wind direction: South Southwest Southwest
Wind speed: 30 to 45 mph with gusts to 55 mph decreasing to 25 to 35 mph with gusts to 45 mph in the afternoon. 15 to 25 mph. Around 15 mph with gusts to 25 mph.
Expected snowfall: 80% probability of 17 to 25 inches. 20% probability of 20 to 30 inches. in. 70% probability of 6 to 12 inches. 30% probability of 4 to 8 inches. in. 80% probability up to 2 inches. 20% probability no accumulation. in.
Disclaimer

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.