THIS SNOWPACK SUMMARY EXPIRED ON January 20, 2019 @ 7:41 am
Snowpack Summary published on January 18, 2019 @ 7:41 am
Issued by Ryan Lewthwaite -

bottom line:

The snowpack is unstable due to the recent storm which deposited over 30" of new & wet snow. The retreat of an Atmospheric River event is underway, leaving us with an upside-down snowpack from a heavy period of rains to almost 9000' in elevation. Leavitt Snotel is reporting around 7" of Snow-Water-Equivalent, think of that as water weight pulling down on the snowpack in some areas. The new Storm & Wind Slabs are dicey, with an ability to break-up easily when disturbed. If avalanches occur today they could have the ability to entrain enough snow to bury a human & destroy a home. Time to find a meadow with low consequences..

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.

Use caution on aspects NW-N-E-SE especially near & above treeline as wind loading has deposited dangerous amounts of new snow, & will continue to do so. Wind features are sharp, ominus, & scuplted by rain. Include Cornices in this wind slab problem as their failure could cause a step-down avalanche which includes lower layers of the snowpack. Look to the trees & see evidence of Gale winds collecting snow in the windward sides of trunks.

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.

The amount of storm snow (+30"), coupled with the effects of rain & wind has us with dynamic surface snow conditions. The storm slab is variable in thickness & density & is acting as a cohesive layer. It is easy to propagate a fracture through this layer when initiated. An upside-down effect has happened to the new snow with an intense duration of mid-storm rains up to almost 9000'. Unsupportable rain crusts also exist below 9K.

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.

If a deep slab avalanche were to occur the survival of a human victim would be very low. The deeply buried Persistent avalanche problem has morphed into a darker issue as it gains depth within the pack. Very hard to predict, this hazardous scenario can be remotely triggered & done so at slope angles shallower that 30 degrees. Dig a test pit & look for "sugary" facets below a firm layer that can loosly fall from the pit wall.

Snowpack Discussion

A complex & relatively uncertain snowpack is on the mountains today. Over 30" of new snow coupled with rain has created a dramatically snowy & sculpted mountainscape. The force in which the new snow was blown around was of Gale strength, & a spackling can be seen on the trunks of tall trees. Rain was visible in the upper layer of the pack to nearly 9000' elevation. The appearance of very large Deep Slab avalanches should be concerning, as the Persistent issue has been buried by a few feet. The upside-down snowpack at lower elevations is tough for traveling, & the crust around 8000' is not supportable. A contrived cornice test showed that the new snow has not bonded well to the old snow & avalanches are likely to occur with the entire new load of snow. Storm & wind slabs are still growing as transport of snow is readily available. Propagations of cracks horizontally through dense snow slabs can cause avalanches that entrain huge amounts of snow & mass, D3- D4. Rain wieght will also casue stress on the pack, & at Leavitt Snotel we have seen around 7" of SWE!

recent observations

Snowmobiling in yesterday's Snow-Globe conditions was tiring & exciting. Winds were irratic & gusting at gale force with +30" of airborne snowflakes. The now deeply buried Persistent layer is still easy to find & in some areas is the base of the snowpack to the ground. A controlled Cornice avalanche was triggered by a duo of snow-rangers who found it takes very little force to cause collapse & propagate a fracture. The storm & wind slab avalanche problems have created soft surface slabs which have a high propensity to cracking. Rain found its way up to almost 9000' as the Atmospheric River produced a typical "goooey" centered storm. The upside-down snowpack turns into a breakable crust as you gain elevation, then above 9K it's soft.

CURRENT CONDITIONS Weather observations from Sonora Pass
0600 temperature: 25 deg. F.
Max. temperature in the last 24 hours: 28 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: Strong mph
Maximum wind gust in the last 24 hours: Gale mph
New snowfall in the last 24 hours: 10 inches
Total snow depth: 64 inches

Snow tapers off today as warmer air moves into our mountains. Winds will be Light from the West, with some moderate ridgetop gusts. The mostly cloudy skies will dissipate tomorrow & we will have sun. Sunday ushers in a another storm with high mountain snow & rain possibly up to 8000'.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Friday Friday Night Saturday
Weather: Cloudy with a chance of snow Snow Level Below 7000' Mostly cloudy with a chance for snow showers Snow Level 8000' Partly cloudy then becoming sunny Snow Level 8000'
Temperatures: 30 to 40 deg. F. 24 to 29 deg. F. 35 to 45 deg. F.
Wind direction: West West Light winds
Wind speed: 10 to 20 mph; gusts to 40 mph 10 to 20 mph; gusts to 40 mph Light winds
Expected snowfall: 1 in. 0 in. 0 in.
For above 10000 ft.
  Friday Friday Night Saturday
Weather: Cloudy with a slight chance for snow Snow Level 7000' Mostly cloudy then becoming partly cloudy Snow Level 8000' Partly cloudy then becoming sunny Snow level 8000'
Temperatures: 30 to 35 deg. F. 19 to 24 deg. F. 31 to 39 deg. F.
Wind direction: West Northwest West
Wind speed: 20 to 35 mph; gusts to 60 mph 15 to 30 mph; gusts to 50 mph 15 to 20 mph; gusts to 35 mph
Expected snowfall: 1 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.