Avalanche Forecast

After a benign start to Monday, an incoming frontal system will quickly spread light to moderate rain and snow over the Cascades with precipitation beginning around mid-day for the north Cascades and Monday afternoon for the south and central Cascades. Above treeline winds will quickly ramp up in the afternoon. Expect stormy conditions Monday night with increasing avalanche danger. 

Fresh wind slab should begin to develop Monday afternoon near and especially above treeline. Wind slab will likely be the most sensitive in areas where it poorly bonds to an underlying crust.   

Shallow storm slabs may develop in the Mt. Baker area by Monday afternoon as the storm intensifies. 

The latest tests of the December 17th PWL in the Cascades don't seem to indicate a regionally reactive layer. However, since this layer is still showing the ability to propagate in snowpack tests throughout much of the range, we still advise observing the snowpack structure in your local area and skiing or riding on lower angled slopes until there is more certainty that this layer is no longer a problem. While triggering this layer seems unlikely, remember that persistent weak layers are generally involved in larger avalanches.

The surface crust formed last week is strong and hard enough in some areas of the west slopes to present an out of control fall danger. Avoid steep slopes where the slide for life hazard is present. 

Weather and Snowpack

Strong westerly flow directed two Pacific frontal systems across the Northwest last Sunday night and again Monday night (12/18-12/19) with generally 1 to 2 inches of water accumulating along the west slopes through early Tuesday morning. A period of rain 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. In most areas the crust is strong enough to support a skier's weight.

After a fair weather period midweek another front crossed the Northwest on Thursday followed by an upper trough on Friday with low snow levels. About 20 inches of snow accumulated at Mt. Baker over this period with about 2-8 inches accumulating elsewhere along the west slopes including the Passes. 

Scattered snow showers, sunbreaks and generally light winds summed up the weather on Saturday with fair and cold weather seen on Christmas Day. 

Recent Observations

Observations received over the last several days from across the west slopes of the Cascades regarding the 12/17 persistent weak layer (PWL) showed no avalanches or direct signs of deeper instability. However, the 12/17 PWL still shows some propensity for propagation in snowpack tests. 

The Alpental pro-patrol on Friday and Saturday reported shallow wind slab on wind loaded areas which was poorly bonded to the crust formed last week. Elsewhere new snow was not cohesive and was sluffing on the crust. Sensitive and shallow wind slab was also reported in the Silver Basin area of Crystal Friday. 

Pro-observer Ian Nicholson was at Mt. Snoqualmie Saturday morning. Ian identified shallow wind slab as his greatest potential concern on steeper wind loaded slopes where it had bonded poorly to a graupel layer overlying the uppermost crust. The 12/17 PWL was showing signs of propagation in snowpit tests, but was requiring more load and showed signs of rounding versus earlier in the week.  

Dallas Glass was in the below tree-line band of the Snoqualmie Pass area on Christmas Day and observed 6" (15 cm) of weak snow poorly bonded to the crust, setting up a likely weak snow and bed surface combo heading into the upcoming storm cycle. 

Crusts in the Snoqualmie Pass and Paradise area have been reported as especially stout with ski crampons helpful on steeper slopes.