THIS SNOWPACK SUMMARY EXPIRED ON January 9, 2017 @ 9:31 am
Snowpack Summary published on January 6, 2017 @ 9:31 am
Issued by Kyle Van Peursem - Bridgeport Avalanche Center

bottom line:

Potentially dangerous avalanche conditions still exist in the BWRA due to heavy snow and strong winds on Tuesday and Wednesday which loaded NW-N-NE-SE facing slopes. Though instabilities in the snowpack are gaining strength and stabilizing, it is still quite possible to trigger a large and destructive avalanche. For the next 24-36 hours, it is recommended to stay off any wind loaded slope steeper than 35 degrees.

Beginning Saturday afternoon through Monday, the avalanche danger which quickly rise with onset of a major atmospheric river event, which is forecasted to drop 6-12" of water and freezing level up to near 10k ft. Large and long running avalanches are almost certain during this event and it is advised to avoid any and all avalanche terrain.

Avalanche Character 1: 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.

Prior to the most recent storm, clear and cool weather persisted forming facets and surface hoar on the snow surface in sheltered northerly facing slopes. 4-7" of SWE with additional wind loading was applied on top of this snow surface and, as a result, several large and destructive avalanches were observed yesterday (Thursday) in the BWRA. Though this problem will be confined to somewhat sheltered northerly facing slopes, large and destructive human triggered slab avalanches are possible, which can be anywhere from 3-7 ft deep. These avalanche problems can be unpredictable with the potential to remotely trigger one from a connected slope. Previous sled tracks will not be an indicator of stability. The best mitigation strategy is to stay off of and to keep away from the bottom of slopes steeper than 35 degrees.

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 loaded northerly facing slopes and likely combined with weak surface layers to produce several large and long running avalanches. Wind slabs are gaining strength and should be less sensitive today and tomorrow but it is still possible to trigger these. Look for signs of wind loading including cornices and pillows, and avoid these features.

Avalanche Character 3: Wet Slab
Wet Slab avalanches occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoid avalanche terrain when you see pinwheels, roller balls, loose wet avalanches, or during rain-on-snow events.

Wet slab problems will not be an issue until Sunday, when several inches of rain are expected all the way up to 10k ft. Water percolating through the snowpack will quickly weaken the bonds between the snow grains, destabilizing the snowpack. Heavy rain on snow is notorious for producing massive, historic sized avalanches which can take out the entire season's snowpack and run into mature forests. With the forecast for this weekend, it is almost certain that we will see these types of avalanches in the BWRA. My advice, stay out of the BWRA until things cool down again on Monday.

Snowpack Discussion

The latest storm deposited between 4-7" of SWE in the BWRA on top of a mix of surface layers, including facets and surface hoar on sheltered northerly slopes, sun crusts on exposed southerly slopes, and hard wind board on more exposed leeward slopes. Additionally, the storm came in upside down, with denser snow falling on top of softer snow. All of these interfaces can act as a bed surface and failure point in the snowpack, leading to potentially dangerous avalanche conditions. Though wind slab and storm snow instabilities are gaining strength, the persistent slab problem with the buried weak layers prior to this past storm will provide instabilities for a while in the BWRA. The heavy rain forecasted will rapidly destabilize the snowpack, likely leading to large, historic type avalanches, but may be good in the long run as the persistent weak layers will likely be destroyed and the snowpack 'reset'.

recent observations

We rode up to Leavitt Lake and near the PCT crossing yesterday and observed several large avalanches, including one that took out PCT marker poles and another that ran an estimated 1000 vertical feet taking out trees in the process. Check out the recent observations for photos and more info.

The snow surface in most locations in the BWRA is heavily wind effected, which made the riding fairly enjoyable as the denser snow provided a supportable surface for your sled. The MWTC has groomed 108 from the People's gate to about half way between Leavitt Lake Rd junction and Sonora Pass. Leavitt Lake Road has also been packed down and groomed.

weather

Since Monday, the Sonora Pass SNOTEL station picked up about 4" of SWE with a snow depth increase of about 25", to 50" of total settled snow depth. The Leavitt Lake SNOTEL station picked up 7.1" of SWE with a snow depth increase of about 33", to 100" of total settled snow depth, though the snow depth was likely impacted by strong winds. Today, conditions will be nice with sunny skies and moderate winds. The highly advertised atmospheric river event will begin to impact our area by Saturday morning. Right now, it looks like precipitation will start out as snow at most elevations, then begin changing over to rain by Sat evening with rapidly rising snow levels, peaking at near 10k ft by late Sunday morning, then dropping back down below 8k ft by Monday morning. Here is what I am thinking for the BWRA:

For 8-9k ft:

Sat 4am - 10pm: 12" snow. Sat 10pm - Mon 4am: 6-8" rain. Mon 4am - 10pm: 11" snow

Total snow: 23", Total rain: 6-8", Total Precip: 8-10"

For 10k ft:

Sat 4am - Sun 10am: 25-30" snow. Sun 10am - Sun 4pm: 1-2" rain. Sun 4pm - Mon 10pm: 20-25" snow

Total snow: 45-55", Total rain: 1-2", Total Precip: 9-11"

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Friday Friday Night Saturday
Weather: SUNNY THEN BECOMING PARTLY CLOUDY CLOUDY. SLIGHT CHANCE OF SNOW AFTER MIDNIGHT CLOUDY. SNOW.
Temperatures: 27 TO 32 deg. F. 17-22 deg. F. 35 TO 40 deg. F.
Wind direction: West SW S
Wind speed: 10 TO 15 MPH IN THE MORNING BECOMING LIGHT. GUSTS UP TO 30 MPH 10 TO 15 MPH. GUSTS UP TO 30 MPH 15 TO 25 MPH. GUSTS UP TO 45 MPH
Expected snowfall: 0 in. 0 in. 4-7 in.
For above 10000 ft.
  Friday Friday Night Saturday
Weather: SUNNY THEN BECOMING PARTLY CLOUDY CLOUDY. SLIGHT CHANCE OF SNOW AFTER MIDNIGHT CLOUDY. SNOW.
Temperatures: 24 TO 30 deg. F. 14 TO 19 deg. F. 30 TO 36 deg. F.
Wind direction: W SW SW
Wind speed: 15 TO 20 MPH WITH GUSTS TO 35 MPH 20 TO 30 MPH. GUSTS UP TO 40 MPH INCREASING TO 50 MPH AFTER MIDNIGHT 25 TO 35 MPH WITH GUSTS TO 70 MPH INCREASING TO 30 TO 50 MPH WITH GUSTS TO 90 MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. 4-8 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.