THIS SNOWPACK SUMMARY EXPIRED ON January 10, 2019 @ 9:49 am
Snowpack Summary published on January 8, 2019 @ 9:49 am
Issued by Ryan Lewthwaite -

bottom line:

A complex stratigraphy of precipitation layers & atmospheric pressures has come with the recent storm. More than unusual snow conditions exist in the BWRA areas & caution must be excersized if traveling here today. Rain on snow & gale winds have sculpted the mountains, leaving behind peaks with less snow than before the onslaught. Most snow seems to have collected near & below treeline, & rains have washed some of the snow away below 7000'. In conjunction with each avalanche problem lies some uncertainty, but the danger is still there, sometimes hidden. Analyze each area you intend to ride & plan to dig each other out of the heavy "manky" snow. Don't get stuck in slide paths!

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.

At 8770' a measurable 62cm of storm snow was gained & has an upside-down characteristic. Dense snow over loose is the scenario & it is not stable. Settlement cones & rain/wind plastered snow on trees eludes to the recent snow growing more dense. With new snow on the way & cooling temps the strom snow could act as solid unit with a decreasing shear strength. The winds have majorly effected the distribution of the storm snow or erradicated it completely in alpine locations.

Avalanche Character 2: 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.

Rain profusing through dry surface snow was very evident in our travels. To a depth of 3" on top of loose dry snow was a moist dense layer at 0C. This cohesive layer is peeling off the dryer density change interface in slopes steeper than 35 degrees. Pinwheels show that this layer is acting collectively. The rain weight could increase the potential for surface shearing to the density change.

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.

Although evidence was not readily available on seen slopes, a previously documented instability in old snow could present itself with the recent heavy load. Activity in stability tests has shown an interface in the old wind effected snow to hold a facet layer. This layer can vary in depth (+40cm) & is substantially more reactive in shallow snowpacks. If a traveler was to create this failure in the snowpack the size & destruction of a avalanche could be catastrophic.

Snowpack Discussion

Interesting snow dynamic yesterday with the exit of a historically windy storm by Sierra standards. Significant rain on the tail end of this blizzard was apparent as the surface snow was heavy & sticky. Below 7000' rains had deteriorated a lot of the newly fallen snow & turned it into something much less desirable. @ 8770' in our Sonora snow study plot our stormboard measured 29cm, making the storm total for that location 61cm. To complicate matters the dense/wet surface snow sits on dry intact stellars, needles, rime, & broken precip particles that have very low density & high pore space ratios. Tests were propigating 17cm deep with light force within this density change, perfect for pinwheels & push-a-lanches. Furthermore below the new storm snow lies several variable wind packed layers that harbor facets. Persistent in nature the facet layer is activated in stability tests with light to moderate forces 40cm deep in the pack. Elsewhere in the Eastern Sierra front this persistent slab instability is more widespread & obvious, but the evidence of it causing avalanches is inconclusive. Mid-storm avalanches were followed by harsh winds & the spackling of rain & visual clues were lost. Our travel was limited to areas where snow tractors had smushed us a supportable surface, walking was tiresome.

recent observations

Sometimes after observations & a day in the field there are many more questions, that is our reality here. Straight forward clues & invisible evidence of avalanches has our confidence low. We excersized caution with our travel & were limited visually yesterday. 

CURRENT CONDITIONS Weather observations from Sonora Pass
0600 temperature: 32 deg. F.
Max. temperature in the last 24 hours: 34 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: moderate mph
Maximum wind gust in the last 24 hours: strong mph
New snowfall in the last 24 hours: trace inches
Total snow depth: 44 inches

Cloudiness & sporatic snow showers are likely today. Later this afternoon the South winds will increase with possible gale force gusts to 90 mph tonight. Wet snow & rain mix may be seen up to 8000'. Wow, a report of 150 mph gust Sunday night, at ridgeline, blowing around the light snow would have been something to see.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy with a slight chance for snow showers later Snow Level 8000' Mostly cloudy with a chance of snow Snow Level 7000' Mostly cloudy & a chance of snow Snow Level 7000'
Temperatures: 35 to 40 deg. F. 24 to 29 deg. F. 34 to 40 deg. F.
Wind direction: South South South
Wind speed: 15 to 20 mph; gusts to 35 mph increasing to 50 mph later 15 to 20 mph increasing to 20 to 30 mph late; gusts to 75 mph 25 to 35 mph decreasing to 15 to 30 mph later; gusts to 75 mph
Expected snowfall: 0 in. 2 in. 2 to 4 in.
For above 10000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy with a slight chance of snow showers later Snow Level 8000' Cloudy with a chance for snow Snow Level 7500' decreasing to 7000' Cloudy with snow
Temperatures: 29 to 34 deg. F. 18 to 23 deg. F. 28 to 34 deg. F.
Wind direction: South South South becoming Southwest
Wind speed: 15 to 25 mph; gusts to 40 mph increasing to 25 to 35 mph; gusts to 65 mph later 30 to 45 mph; gusts to 70 mph increasing to 90 mph late 40 to 60 mph; gusts to 90 mph shifting SW & decreasing to 60 mph later
Expected snowfall: 0 in. 1 to 3 in. 2 to 4 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.