Avalanche Forecast

Shallow 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 found at lower elevations and in the southeast Cascades that experienced more warming 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 east slopes, but based on limited observations and weather station data, persistent slabs are more likely an issue for the central and northeast Cascades. The 12/8 layer should have received a decent test during this last storm cycle and unless we receive information that this layer is reactive, we will focus on PWLs closer to the surface and more likely to be human triggered. 

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 half to 1 inch of water accumulating along the east slopes through early Tuesday morning. Storm totals generally ranged from 6 - 12 inches during this cycle. A brief warming trend peaked mid-morning for many east slope stations before a sharp cooling trend ensued by mid-day. A period of rain and freezing rain were likely seen at lower elevations along the east slopes prior to the sharp cooling trend Tuesday. 

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. West winds at Mission Ridge gusted over 100 mph for several consecutive hours Tuesday afternoon!

Recent Observations

NWAC pro-observer Tom Curtis found weak faceted snow above the ground in a still fairly shallow overall snowpack in the Blewett Pass area Thursday. No observed avalanches were noted, but snowpack tests indicate the potential. See a recent NWAC instagram post about the results of a PST test.

On Friday the NCMG at Harts Pass had strong N-NW winds causing wind transport, shooting cracks and wind slab layer collapsing.

Also on Friday the NCMG had strong E-NE winds causing major snow transport seen in the Washington Pass area and off all major peaks south to Bonanza Peak. But near Rainy Pass a generally right side up snowpack was found with a good bond was seen at the December 8th interface.

On Tuesday the Mission Ridge pro-patrol reported wind slab building on all aspects except for west aspects near and above treeline. The new snow had bonded poorly to the weak old snow surface but was lacking a cohesive slab structure in non-wind affected terrain.