Avalanche Forecast

Stormy conditions Monday night into Tuesday will cause an increasing avalanche danger through Tuesday.

Storm slabs will continue to build over a variety of weak surface snow conditions, or to exposed crusts with increased load. Natural or triggered storm slabs may break down to deeper persistent layers where present, making larger and more dangerous avalanches possible Tuesday, especially the Crystal Mountain and Stevens Pass areas.  

Fresh wind slabs should continue to build Monday night and Tuesday near and above treeline. Wind slabs will likely be the most sensitive in areas where it poorly bonds to an underlying crust.   

The persistent slab problem still warrants attention in the Cascades, especially in areas void of a stout recent crust layer, most notably outside the Paradise, Snoqualmie and Mt Baker area. Significant loading by Tuesday will make this layer more sensitive to trigger where present. Remember that persistent weak layers are generally involved in larger avalanches and cautious route-finding and conservative decision making will be essential for safe travel Tuesday.  Err on the side of caution Tuesday by avoiding avalanche terrain of consequence.  

Weather and Snowpack

Strong storms a week ago Sunday and Monday deposited generally 1 to 2 inches of water equivalent along the west slopes through early Tuesday morning. 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. In most areas the crust is strong enough to support a skier's weight. The crust is very thin or non existent in the Crystal backcountry. 

A front Thursday and upper trough on Friday with low snow levels deposited about 20 inches of snow 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 and early Monday, before a strong front moved into the region Monday afternoon.

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 slabs in exposed areas were poorly bonded to the crust formed last week. Elsewhere, the new snow was not cohesive and was sluffing on the crust. Sensitive and shallow wind slab were 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 weak surface snow and slick bed surface combination heading into the upcoming storm cycle, now arriving late Monday. These weak surface snow conditions, and poorly bonded old snow to a smooth underlying crust also exist in the Paradise area, as reported over the past few days by NPS rangers. 

Dallas Glass was back in the Crystal area backcountry Monday 12/26 and reports the 12/17 persistent layer is very much in play in that region, buried about 1 foot below the surface. Large column tests continued to show propensity to propagate in multiple tests.