Avalanche Forecast

Storm slabs should have settled out by Wednesday, shifting the focus to lingering wind slab and our new persistent slab problem. 

We know in at least one area, Stevens Pass, that recently buried surface hoar from 12/17 was buried intact and sensitive in snowpit tests. Strengthening crusts in many areas near and below treeline may limit how sensitive this layer will be to human triggering. For elevations that did not see rain, wind loaded slopes could present the dangerous combination of a deeper slab overlying a persistent weak layer. We need more information about the sensitivity and spatial distribution of the 12/17 PWL throughout the west slopes, but based on limited observations and weather station data, persistent slabs are more likely an issue from Stevens Pass and north. 

We know wind loading occurred into the below treeline band due to the strong and persistent westerly winds during the last storm cycle. While wind slab will most likely be found lee easterly aspects, the gusty nature of this recent event may have built wind slab on a variety of aspects. Be suspicious of any wind loaded slope Wednesday, especially one that might harbor the combination of wind slab and a persistent weak layer. 

Choose conservative and low consequence terrain and evaluate the snowpack carefully Wednesday before committing to any avalanche terrain. Persistent weak layers lurk below the most recent storm snow and may be sensitive to human triggering. Observe local wind loading patterns and allow for wind slab even into the below treeline band. 

Weather and Snowpack

Clear and cold weather from Wednesday, December 14th to Friday, December 17th allowed widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts formed on steeper solar slopes during sunny periods. These persistent weak layers were buried intact December 17th during a period of light snow and light winds.

Strong westerly flow directed two Pacific frontal systems into the PNW Sunday night and again Monday night with generally 1 to 2+ inches of water accumulating along the west slopes through early Tuesday morning. Storm snow totals varied quite a bit due to a mix of precipitation types during the storm cycle. The Snoqualmie Pass area developed more than one crust due to freezing rain and rain Monday. The Monday night system was warmer and wetter as rain reached up to at least up 5500 ft in the south Washington Cascades (Paradise) and at least briefly to 5000 ft at Stevens and Snoqualmie. In the Baker area, rain was seen up to about 3800 feet. A sharp cooling trend Tuesday morning was followed by 2-5 inches of snow in post-frontal showers Tuesday. Many areas experienced a natural avalanche cycle involving either shallow loose wet or storm slabs Monday night or early Tuesday morning.  A strengthening rain crust was noted in many areas by later in the day Tuesday with the arrival of colder air.  

Westerly winds were especially strong with the 2nd system late Monday night and into Tuesday with gusty winds mixing down into usually more wind sheltered terrain. 

Recent Observations

Evidence of a natural loose wet avalanche cycle was noted below treeline by NPS rangers at Paradise Tuesday with a 1 cm thick rain crust near the surface.

Natural slab avalanches were noted on Shuskan Arm in the Baker area Tuesday as well as extremely sensitive and widespread storm slabs during avalanche control work in the ski area, with one larger avalanche propagating several hundred feet across a NE slope at 5200 feet. Easy skier triggered slides were noted during control work and also separately by backcountry skiers outside the ski area.  

In the Alpental area, pro-patrol reported that the new storm snow had layered upside down but was sandwiched between two rain crusts and generally not sensitive on Tuesday. 

NWAC observers Ian Nicholson and Dallas Glass were in the Skyline Ridge area of Stevens Pass Tuesday and found ongoing wind transport of new snow well into the below treeline band. In more wind sheltered areas, about 8-10" (20-25 cm) of snow from this storm cycle was found on all aspects and to be highly reactive in snowpack tests, failing on the 12/17 buried surface hoar. This layer produced cracking but was not especially sensitive on short test slopes, but the PWL's sensitivity kept them from venturing into any consequential terrain. Some evidence of a natural cycle from late Monday night was observed but visibility was generally poor.