Avalanche Forecast

A passing front overnight should move south of the Cascades by early New Year's Day. Generally light to moderate amounts of new snow accumulation are expected by Sunday, however strong and shifting winds should redistribute any available surface snow to a variety of aspects.

Watch for isolated areas where the observed surface hoar before the current storm may have been buried intact.

New or lingering wind slabs should be the primary avalanche problem and mainly found near and above treeline New Year's Day. Strong and gusty winds and very low density snow may deposit snow further downslope than usual and on a variety of aspects.

Newly formed cornices have been reported sensitive in the Alpental backcountry, with one cornice fall, triggering a large avalanche down to the Solstice crust Friday afternoon. Give cornices a large berth as they often break back further than expected and don't travel below the slopes with large overhanging cornices.

The 12/17 PWL persistent slab problem is becoming more deeply buried and less sensitive in snowpack tests and has been removed from the avalanche problem set. Continue to identify and test this layer in snowpit tests. 

Weather and Snowpack

Strong storms around the Solstice deposited generally 1 to 2 inches of water equivalent along the west slopes. A period of rain or freezing rain (Snoqualmie) during this storm cycle allowed crust layers to form in the Baker area from 4000-4500 feet, the Passes up to around 5000 feet (Stevens) to 6000 feet (Snoqualmie) and 6000-7000 feet in the Paradise area. The crust(s) were especially stout in the Snoqualmie and Paradise areas and very thin in the Crystal backcountry. 

A strong front and strong west flow aloft was seen over the Olympics and Cascades on Monday and Tuesday. The White Pass avalanche fatality involving a skier triggered storm slab failing on the Solstice crust occurred on Tuesday afternoon. NWAC stations along the west slopes and crest had strong west winds Monday and Tuesday with 1-3 feet of new snow for the 48 hours ending Wednesday morning with a cooling trend.

A warm front kept light snow and strong winds going through much of the day in the Mt. Baker area Thursday before pushing south Thursday night.  Up to 12 inches of snow fell around Mt. Baker, Stevens Pass and above 5000 feet in the Snoqualmie Pass area. Less snow was seen further at lower elevations at Snoqualmie Pass due to rain at the onset of precipitation, which formed a thin breakable crust below the new storm snow. A thin rain crust was also found up to 6000 feet above Paradise Friday morning with 4" snow on top.

Clearing Friday night allowed surface hoar to form in some areas, reported Saturday morning, 12/31 above Paradise on Mt Rainier. Sunshine Saturday may have destroyed   

Recent Observations

NWAC pro-observer Ian Nicholson was at Stevens Pass on Wednesday and reported no signs of instability on east slopes to 5800 feet. Ian found the Solstice crust at 65-70 cm. The 12/17 PWL was strengthening and unlikely to be triggered. Additional reports in the Stevens Pass area Friday confirms these observations. We are removing the persistent slab from the set of avalanche problems as we have good confidence that the 12/17 PWL is not a reactive layer along the west slopes. 

The weather system that affected the area Thursday night generally layered new snow right side up and avalanche control results from Stevens, Alpental and Mt. Baker Friday morning only observed shallow pockets of storm slab generally in the 6-12" range. One natural avalanche likely involving the new storm snow had occurred Thursday night off the Shuskan Arm in the Baker backcountry. 

However, at Alpental on Friday, newly formed cornices were weak with natural cornice fall observed in the afternoon. One cornice break in the Back Bowls of Alpental triggered a large slab up to 4' deep, 100' wide, 300' vertical, that broke down to the Solstice crust. 

NWAC observer Lee Lazzara was in the Mt. Baker backcountry Friday. Lee found wind affected snow surfaces well into the below treeline band. Wind slab was stubborn on test slopes, but reactive in test pits. The uneven distribution of large wind slab on varied aspects dictated more conservative terrain choices Friday.  

NPS rangers at Paradise found the 4 inches of new snow well bonded near treeline with a breakable crust formed during the recent warm up below. Strong winds higher on the mountain early Saturday made ongoing wind transport visible on the upper volcano. Also reported above Paradise Saturday 12/31, a non supportable crust (image below) buried 10 cm below the surface, as well as surface hoar surviving on shaded slopes. 

 

 

Thin near surface crust seen in a snowpit above Paradise 12/31. Photo, Seth Waterfall, NPS