Avalanche Forecast

A frontal system overnight should enhance and cause increasing winds and precipitation rates, before changing to showers with cooling Friday.   

Avalanche problems Friday should be associated with new storm and wind slab layers. New and recent snow will be deposited on lee NW and E aspects, except extending to all west aspects in the Stevens and Snoqualmie Pass areas. Most avalanches should be confined to the new storm snow since Thursday, but a few may step down to older storm layers or crusts in isolated locations.  

In areas below treeline, the warming and rain may have caused natural avalanches on some steep slopes. In addition cooling should begin to form a new crust layer below the elevation that experienced rain or wet, heavy snow, likely about 5500 feet away from the passes. This should limit any avalanches to mainly shallow new snow.  

However, near and above treeline, increasing snowfall and winds are expected overnight Thursday and early Friday. This should cause sensitive storm layers. Therefore, best to avoid steeper slopes and those showing signs of recent wind loading.  

Terrain anchors are still causing significant anchoring at the lowest elevations. Use caution near creeks which are still open in some areas.

Reports from field observers and pro patrol on control missions Thursday in the Alpental valley, indicated increasingly sensitive slab releases, both controlled and natural my midday Thursday. Heavy wind loading was occurring on a wide variety of aspects as the winds were channelled and redirected by local terrain features. The reported slides were confined to the new storm snow, but were propagating wide and running fast, mostly up to about 1 foot in crown height at that time, but building. 

An atmospheric river around Dec 9th pushed heavy rain up to at least 6000 feet in the north Cascades and above 7000 feet in the south. Below these elevations liquid water percolated down below the early December crust (date when the crust was buried) and is expected to have eliminated or subdued persistent weak layers formed mainly in late November in the Stevens and Snoqualmie area. 

An active and cool weather pattern over the last week has produced about 2 to 3 plus feet of new snowfall is well bonded to the 12/9 crust or previously wet snow. The below treeline zone snowdepth gaining in snowdepth nicely, but still contains many barely hidden hazards like rocks, streams or snags. The skiing in non-wind affected areas with enough snow cover has been good! 

Last weekend, most avalanches released within storm layers, but a few released down to the recent rain crust. Storm instabilities have had time to heal and no new avalanches were reported Tuesday or Wednesday. The NWAC staff along with other Stevens Pass professionals had field training Monday and Tuesday in the Stevens Pass area and found a reactive but strengthening graupel or lower density stellar crystal layer at 35-40 cm below the surface. Tests on this did not indicate propagation was likely. Moderate west-northwest winds in the above treeline zone along with light amounts of new snow received Tuesday night through Wednesday morning have likely redistributed recent and new snow onto lee easterly slopes in the near and above treeline zones.  

A strong frontal system is producing moderate to heavy rain or snowfall at rising freezing levels Thursday.  Cooling east flow is maintaining precipitation as snowfall in the passes, however light freezing rain Thursday afternoon was reported at Snoqualmie Pass, likely to form a new thin crust layer. 

The heavy wet snow or rain should cause a significant increase in danger as the weak underlying snow becomes heavily loaded.